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Aims. We extend the analysis of a physical model within the standard cosmology that robustly predicts a high star-formation efficiency (SFE) in massive galaxies at cosmic dawn due to feedback-free starbursts (FFBs). This model implies an excess of bright galaxies at z​ ≳ ​10 compared to the standard models based on the low SFE at later epochs, an excess that is indicated by JWST observations. Methods. Here we provide observable predictions of galaxy properties based on the analytic FFB scenario. These can be compared with simulations and JWST observations. We use the model to approximate the SFE as a function of redshift and mass, assuming a maximum SFE of ϵmax​ = ​0.2 − 1 in the FFB regime. From this, we derive the evolution of the galaxy mass and luminosity functions as well as the cosmological evolution of stellar and star-formation densities. We then predict the star-formation history (SFH), galaxy sizes, outflows, gas fractions, metallicities, and dust attenuation, all as functions of mass and redshift in the FFB regime. Results. The major distinguishing feature of the model is the occurrence of FFBs above a mass threshold that declines with redshift. The luminosities and star formation rates in bright galaxies are predicted to be in excess of extrapolations of standard empirical models and standard cosmological simulations, an excess that grows from z​ ∼ ​9 to higher redshifts. The FFB phase of ∼100 Myr is predicted to show a characteristic SFH that fluctuates on a timescale of ∼10 Myr. The stellar systems are compact (Re​ ∼ ​0.3 kpc at z​ ∼ ​10 and declining with z). The galactic gas consists of a steady wind driven by supernovae from earlier generations, with high outflow velocities (FWHM​ ∼ ​1400 − 6700 km s−1), low gas fractions (< 0.1), low metallicities (≲0.1 Z⊙), and low dust attenuation (AUV​ ∼ ​0.5 at z​ ∼ ​10 and declining with z). We make tentative comparisons with current JWST observations for initial insights, anticipating more complete and reliable datasets for detailed quantitative comparisons in the future. The FFB predictions are also offered in digital form. [ABSTRACT FROM AUTHOR]

We explore the evolution of cold streams from the cosmic web that feed galaxies through their shock-heated circumgalactic medium (CGM) at cosmic noon, |$z\simeq 1-5$|⁠. In addition to the hydrodynamical instabilities and radiative cooling that we have incorporated in earlier works, we embed the stream and the hot CGM in the gravitational potential of the host dark matter halo, deriving equilibrium profiles for both. Self-gravity within the stream is tentatively ignored. We find that the cold streams gradually entrain a large mass of initially hot CGM gas that cools in the mixing layer and condenses onto the stream. This entrainment, combined with the acceleration down the gravitational potential well, typically triples the inward cold inflow rate into the central galaxy, compared to the original rate at the virial radius, which makes the entrained gas the dominant source of gas supply to the galaxy. The potential sources for the hot gas to be entrained are recycled enriched gas that has been previously ejected from the galaxy, and fresh virial-shock-heated gas that has accumulated in the CGM. This can naturally elevate the star formation rate in the galaxy by a factor of |$\sim 3$| compared to the gas accretion rate onto the halo, thus explaining the otherwise puzzling observed excess of star formation at cosmic noon. When accounting for self-shielding of dense gas from the ultraviolet background, we find that the energy radiated from the streams, originating predominantly from the cooling of the entrained gas, is consistent with observed Lyman- |$\alpha$| blobs around galaxies. [ABSTRACT FROM AUTHOR]

Nearby dwarf galaxies display a variety of effective radii (sizes) at a given stellar mass, suggesting different evolution scenarios according to their final "stellar" size. The TNG hydrodynamical simulations present a bimodality in the z = 0 size–mass relation (SMRz0) of dwarf galaxies, at r1/2, ⋆ ∼ 450 pc. Using the TNG50 simulation, we explored the evolution of the most massive progenitors of dwarf galaxies (z = 0 log(M⋆/M⊙) between 8.4 and 9.2) that end up as central galaxies of their groups. We split these dwarfs into three classes of the SMRz0: "Normals" from the central spine of the main branch, and "Compacts" from the secondary branch as well as the lower envelope of the main branch. Both classes of Compacts see their stellar sizes decrease from z ∼ 1 onwards in contrast to Normals, while the sizes of the gas and dark matter (DM) components continue to increase (as for Normals). A detailed analysis reveals that Compacts live in poorer environments, and thus suffer fewer major mergers from z = 0.8 onwards, which otherwise would pump angular momentum into the gas, allowing strong gas inflows, producing inner star formation, and thus leading to the buildup of a stellar core. Compacts are predicted to be rounder and to have bluer cores. Compact dwarfs of similar sizes are observed in the GAMA survey, but the bimodality in size is less evident and the most compact dwarfs tend to be passive rather than star forming, as in TNG50. Our conclusions should therefore be confirmed with future cosmological hydrodynamical simulations. [ABSTRACT FROM AUTHOR]

We analyse the internal structure and dynamics of cosmic-web filaments connecting massive high- z haloes. Our analysis is based on a high-resolution arepo cosmological simulation zooming-in on three Mpc-scale filaments feeding three massive haloes of |$\sim 10^{12}\, \text{M}_\odot$| at z ∼ 4, embedded in a large-scale sheet. Each filament is surrounded by a cylindrical accretion shock of radius |$r_{\rm shock} \sim 50 \, {\rm kpc}$|⁠. The post-shock gas is in virial equilibrium within the potential well set by an isothermal dark-matter filament. The filament line-mass is |$\sim 9\times 10^8\, \text{M}_\odot \, {\rm kpc}^{-1}$|⁠ , the gas fraction within r shock is the universal baryon fraction, and the virial temperature is ∼7 × 105 K. These all match expectations from analytical models for filament properties as a function of halo mass and redshift. The filament cross-section has three radial zones. In the outer 'thermal' (T) zone, |$r \ge 0.65 \, r_{\rm shock}$|⁠ , inward gravity, and ram-pressure forces are overbalanced by outward thermal pressure forces, decelerating the inflowing gas and expanding the shock outwards. In the intermediate 'vortex' (V) zone, 0.25 ≤ r / r shock ≤ 0.65, the velocity field is dominated by a quadrupolar vortex structure due to offset inflow along the sheet through the post-shock gas. The outward force is dominated by centrifugal forces associated with these vortices, with additional contributions from global rotation and thermal pressure. Shear and turbulent forces associated with the vortices act inwards. The inner 'stream' (S) zone, |$r \lt 0.25 \, r_{\rm shock}$|⁠ , is a dense isothermal core, |$T\sim 3 \times 10^4 \, {\rm K}$| and |$n_{\rm H}\sim 0.01 \, {\rm cm^{-3}}$|⁠ , defining the cold streams that feed galaxies. The core is formed by an isobaric cooling flow and is associated with a decrease in outward forces, though exhibiting both inflows and outflows. [ABSTRACT FROM AUTHOR]

The recent reported discovery of a low-mass z  = 5.2 and an intermediate-mass z  = 7.3 quenched galaxy with JWST/NIRSpec is the first evidence of halted star formation above z ≈ 5. Here, we show how bursty star formation at z  = 4–8 gives rise to temporarily quenched, or mini-quenched galaxies in the mass range M ⋆ = 107–109 M⊙ using four models of galaxy formation: the periodic box simulation IllustrisTNG , the zoom-in simulations vela and FirstLight and an empirical halo model. The main causes for mini-quenching are stellar feedback, lack of gas accretion onto galaxies, and galaxy–galaxy interactions. The abundance of (mini-)quenched galaxies agrees across the models: the population first appears below z ≈ 8, after which their proportion increases with cosmic time, from ∼0.5–1.0 per cent at z  = 7 to ∼2–4 per cent at z  = 4, corresponding to comoving number densities of ∼10−5 and ∼10−3 Mpc−3, respectively. These numbers are consistent with star formation rate duty cycles inferred for vela and FirstLight galaxies. Their star formation histories (SFHs) suggest that mini-quenching at z  = 4–8 is short-lived with a duration of ∼20–40 Myr, which is close to the free-fall time-scale of the inner halo. However, mock spectral energy distributions of mini-quenched galaxies in IllustrisTNG and vela do not match JADES-GS-z7-01-QU photometry, unless their SFHs are artificially altered to be more bursty on time-scales of ∼40 Myr. Studying mini-quenched galaxies might aid in calibrating sub-grid models governing galaxy formation, as these may not generate sufficient burstiness at high redshift to explain the SFH inferred for JADES-GS-z7-01-QU. [ABSTRACT FROM AUTHOR]

Giant star-forming clumps are a prominent feature of star-forming galaxies (SFGs) and contain important clues on galaxy formation and evolution. However, the basic demographics of clumps and their host galaxies remain uncertain. Using the Hubble Space Telescope/Wide Field Camera 3 F275W images from the Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we detect and analyze giant star-forming clumps in galaxies at 0.5 ≤ z ≤ 1, connecting two epochs when clumps are common (at cosmic high noon, z ∼ 2) and rare (in the local Universe). We construct a clump sample whose rest-frame 1600 Å luminosity is 3 times higher than the most luminous local H ii regions (M UV ≤ −16 AB). In our sample, 35% ± 3% of low-mass galaxies (log[ M ∗/ M ⊙] < 10) are clumpy (i.e., containing at least one off-center clump). This fraction changes to 22% ± 3% and 22% ± 4% for intermediate (10 ≤ log[ M ∗/ M ⊙] ≤ 10.5) and high-mass (log[ M ∗/ M ⊙] > 10.5) galaxies, in agreement with previous studies. When compared to similar-mass nonclumpy SFGs, low- and intermediate-mass clumpy SFGs tend to have higher star formation rates (SFRs) and bluer rest-frame U − V colors, while high-mass clumpy SFGs tend to be larger than nonclumpy SFGs. However, clumpy and nonclumpy SFGs have similar Sérsic index, indicating a similar underlying density profile. Furthermore, we investigate how the UV luminosity of star-forming regions correlates with the physical properties of host galaxies. On average, more luminous star-forming regions reside in more luminous, smaller, and/or higher specific SFR galaxies and are found closer to their hosts' galactic centers. [ABSTRACT FROM AUTHOR]

JWST observations indicate a surprising excess of luminous galaxies at z ∼ 10 and above, consistent with efficient conversion of the accreted gas into stars, unlike the suppression of star formation by feedback at later times. We show that the high densities and low metallicities at this epoch guarantee  a high star formation efficiency (SFE) in the most massive dark-matter haloes. Feedback-free starbursts (FFBs) occur when the free-fall time is shorter than ∼ |$1\, {\rm Myr}$|⁠ , below the time for low-metallicity massive stars to develop winds and supernovae. This corresponds to a characteristic density of ∼ |$3\!\times \!10^3\, {\rm cm}^{-3}$|⁠. A comparable threshold density permits a starburst by allowing cooling to star-forming temperatures in a free-fall time. The galaxies within ∼1011 M⊙ haloes at z ∼ 10 are expected to have FFB densities. The halo masses allow efficient gas supply by cold streams in a halo crossing time ∼ |$80\, {\rm Myr}$|⁠. The FFBs gradually turn all the accreted gas into stars in clusters of ∼104–7 M⊙ within galaxies that are rotating discs or shells. The starbursting clouds are insensitive to radiative feedback and are shielded against feedback from earlier stars. We predict high SFE above thresholds in redshift and halo mass, where the density is |$10^{3\!-\!4}\, {\rm cm}^{-3}$|⁠. The z ∼ 10 haloes of ∼1010.8 M⊙ are predicted to host galaxies of ∼1010 M⊙ with star formation rate ∼ |$65\,\mathrm{ M}_\odot \, {\rm yr}^{-1}$|⁠ , blue colours, and sub-kpc sizes. The metallicity is ≤0.1 Z⊙ with little dust, gas, outflows, and hot circumgalactic gas, allowing a top-heavy initial mass function but not requiring it. The compact galaxies with thousands of young FFB clusters may have implications on reionization, black hole growth, and globular clusters. [ABSTRACT FROM AUTHOR]

We propose an analytic model, CuspCore II, for the response of dark matter (DM) haloes to central gas ejection, as a mechanism for generating DM-deficient cores in dwarfs and high- z massive galaxies. We test this model and three other methods using idealized N -body simulations. The current model is physically justified and provides more accurate predictions than the earlier version, CuspCore I (Freundlich et al. 2020a). The CuspCore model assumes an instantaneous change of potential, followed by a relaxation to a new Jeans equilibrium. The relaxation turns out to be violent relaxation during the first orbital period, followed by phase mixing. By tracing the energy diffusion d E  = d U  (r), iteratively, the model reproduces the simulated DM profiles with ∼10 per cent accuracy or better. A method based on adiabatic invariants shows similar precision for moderate mass change, but underestimates the DM expansion for strong gas ejection. A method based on a simple empirical relation between DM and total mass ratios makes slightly inferior predictions. The crude assumption used in CuspCore I, of energy conservation for shells that encompass a fixed DM mass, turns out to underestimate the DM response, which can be partially remedied by introducing an alternative 'energy' definition. Our model is being generalized to address the differential response of a multicomponent system of stars and DM in the formation of DM-deficient galaxies. [ABSTRACT FROM AUTHOR]

We address the nature of the giant clumps in high- z galaxies that undergo violent disc instability, distinguishing between long-lived and short-lived clumps. We study the evolution of long-lived clumps during migration through the disc via an analytical model tested by simulations and confront theory with CANDELS- HST observations. The clump 'bathtub' model, which considers gas and stellar gain and loss, involves four parameters: the accretion efficiency α, the star formation rate (SFR) efficiency ϵd, and the outflow mass-loading factors for gas and stars, η and ηs. The corresponding time-scales are comparable to the migration time, two-three orbital times. The accretion-rate dependence on clump mass, gas, and stars, allows an analytical solution involving exponential growing and decaying modes. For the fiducial parameter values there is a main evolution phase where the SFR and gas mass are constant and the stellar mass is rising linearly with time. This makes the inverse specific SFR an observable proxy for clump age. When η or ϵd are high, or α is low, the decaying mode induces a decline of SFR and gas mass till the migration ends. Later, the masses and SFR approach an hypothetical exponential growth with a constant specific SFR. The model matches simulations with different, moderate feedbacks, both in isolated and cosmological settings. The observed clumps agree with our predictions, indicating that the massive clumps are long-lived and migrating. A challenge is to model feedback that is non-disruptive in massive clumps but suppresses SFR to match the galactic stellar-to-halo mass ratio. [ABSTRACT FROM AUTHOR]

We present a suite of high-resolution cosmological zoom-in simulations to z = 4 of a 1012M⊙ halo at z = 0, obtained using seven contemporary astrophysical simulation codes (A rt-I , E nzo , R amses , C hanga , G adget-3 , G ear , and G izmo) widely used in the numerical galaxy formation community. The physics prescriptions for gas cooling and heating and star formation are the same as the ones used in our previous Assembling Galaxies of Resolved Anatomy (AGORA) disk comparison but now account for the effects of cosmological processes such as the expansion of the universe, intergalactic gas inflow, and the cosmic ultraviolet background radiation emitted by massive stars and quasars. In this work, we introduce the most careful comparison yet of galaxy formation simulations run by different code groups, together with a series of four calibration steps each of which is designed to reduce the number of tunable simulation parameters adopted in the final run. In the first two steps, we methodically calibrate the gas physics, such as cooling and heating, in simulations without star formation. In the third step, we seek agreement on the total stellar mass produced with the common star formation prescription used in the AGORA disk comparison, in stellar-feedback-free simulations. In the last calibration step, we activate stellar feedback, where each code group is asked to set the feedback prescription to as close to the most widely used one in its code community as possible, while aiming for convergence in the stellar mass at z = 4 to the values predicted by semiempirical models. After all the participating code groups successfully complete the calibration steps, we achieve a suite of cosmological simulations with similar mass assembly histories down to z = 4. With numerical accuracy that resolves the internal structure of a target halo (≲100 physical pc at z = 4), we find that the codes overall agree well with one another, e.g., in gas and stellar properties, but also show differences, e.g., in circumgalactic medium (CGM) properties. We argue that, if adequately tested in accordance with our proposed calibration steps and common parameters, high-resolution cosmological zoom-in simulations can have robust and reproducible results. New code groups are invited to join and enrich this comparison by generating equivalent models or to test the code's compatibility on their own, by adopting the common initial conditions, the common easy-to-implement physics package, and the proposed calibration steps. Further analyses of the zoom-in simulations presented here will be presented in forthcoming reports from the AGORA Collaboration, including studies of the CGM, simulations by additional codes, and results at lower redshift. [ABSTRACT FROM AUTHOR]

We use the Cosmic Assembly Deep Near-infrared Extragalactic Legacy Survey data to study the relationship between quenching and the stellar mass surface density within the central radius of 1 kpc (Σ1) of low-mass galaxies (stellar mass M* ≲ 109.5M⊙) at 0.5 ≤ z < 1.5. Our sample is mass complete down to ∼109M⊙ at 0.5 ≤ z < 1.0. We compare the mean Σ1 of star-forming galaxies (SFGs) and quenched galaxies (QGs) at the same redshift and M*. We find that low-mass QGs have a higher Σ1 than low-mass SFGs, similar to galaxies above 1010M⊙. The difference of Σ1 between QGs and SFGs increases slightly with M* at M* ≲ 1010M⊙ and decreases with M* at M* ≳ 1010M⊙. The turnover mass is consistent with the mass where quenching mechanisms transition from internal to environmental quenching. At 0.5 ≤ z < 1.0, we find that Σ1 of galaxies increases by about 0.25 dex in the green valley (i.e., the transition region from star forming to fully quenched), regardless of their M*. Using the observed specific star formation rate gradient in the literature as a constraint, we estimate that the quenching timescale (i.e., time spent in the transition) of low-mass galaxies is a few (∼4) Gyr at 0.5 ≤ z < 1.0. The mechanisms responsible for quenching need to gradually quench star formation in an outside-in way, i.e., preferentially ceasing star formation in outskirts of galaxies while maintaining their central star formation to increase Σ1. An interesting and intriguing result is the similarity of the growth of Σ1 in the green valley between low-mass and massive galaxies, which suggests that the role of internal processes in quenching low-mass galaxies is a question worthy of further investigation. [ABSTRACT FROM AUTHOR]

We present mock catalogues created to support the interpretation of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). We extract haloes along past light-cones from the Bolshoi Planck dissipationless N-body simulations and populate these haloes with galaxies using two different independently developed semi-analytic models of galaxy formation and the empirical model universemachine. Our mock catalogues have geometries that encompass the footprints of observations associated with the five CANDELS fields. In order to allow field-to-field variance to be explored, we have created eight realizations of each field. In this paper, we present comparisons with observable global galaxy properties, including counts in observed frame bands, luminosity functions, colour–magnitude distributions and colour–colour distributions. We additionally present comparisons with physical galaxy parameters derived from SED fitting for the CANDELS observations, such as stellar masses and star formation rates. We find relatively good agreement between the model predictions and CANDELS observations for luminosity and stellar mass functions. We find poorer agreement for colours and star formation rate distributions. All of the mock light-cones as well as curated 'theory friendly' versions of the observational CANDELS catalogues are made available through a web-based data hub. [ABSTRACT FROM AUTHOR]

The phenomenological study of evolving galaxy populations in Peng et al. has shown that star forming galaxies can be quenched by two distinct processes: mass quenching and environment quenching. To explore the mass quenching process in local galaxies, we study the massive central disk galaxies with stellar mass above the Schechter characteristic mass. In Zhang et al., we showed that during the quenching of the massive central disk galaxies as their star formation rate decreases, their molecular gas mass and star formation efficiency drop rapidly but their H i gas mass remains surprisingly constant. To identify the underlying physical mechanisms, in this work we analyze the change during quenching of various structure parameters, bar frequency, and active galactic nucleus (AGN) activity. We find three closely related facts. On average, as star formation rate decreases in these galaxies: (1) they become progressively more compact, indicated by their significantly increasing concentration index, bulge-to-total mass ratio, and central velocity dispersion, which are mainly driven by the growth and compaction of their bulge component; (2) the frequency of barred galaxies increases dramatically, and at a given concentration index the barred galaxies have a significantly higher quiescent fraction than unbarred galaxies, implying that the galactic bar may play an important role in mass quenching; and (3) the "AGN" frequency increases dramatically from 10% on the main sequence to almost 100% for the most quiescent galaxies, which is mainly driven by the sharp increase of LINERs. These observational results lead to a self-consistent picture of how mass quenching operates. [ABSTRACT FROM AUTHOR]

We explore a function with two shape parameters for the dark-matter halo density profile subject to baryonic effects, which is a special case of the general Zhao family of models applied to simulated dark-matter haloes by Dekel et al. This profile has variable inner slope and concentration parameter, and analytic expressions for the gravitational potential, velocity dispersion, and lensing properties. Using the Numerical Investigation of a Hundred Astrophysical Objects cosmological simulations, we find that it provides better fits than the Einasto profile and the generalized NFW profile with variable inner slope, in particular towards the halo centres. We show that the profile parameters are correlated with the stellar-to-halo mass ratio M star/ M vir. This defines a mass-dependent density profile describing the average dark-matter profiles in all galaxies, which can be directly applied to observed rotation curves of galaxies, gravitational lenses, and semi-analytic models of galaxy formation or satellite–galaxy evolution. The effect of baryons manifests itself by a significant flattening of the inner density slope and a 20 per cent decrease of the concentration parameter for M star/ M vir = 10−3.5–10−2, corresponding to |$M_{\rm star} \!\sim \! 10^{7-10}\, \mathrm{ M}_\odot$|⁠. The accuracy by which this profile fits simulated galaxies is similar to certain multiparameter mass-dependent profiles, but its fewer parameters and analytic nature make it most desirable for many purposes. [ABSTRACT FROM AUTHOR]

Existing models of galaxy formation have not yet explained striking correlations between structure and star formation activity in galaxies, notably the sloped and moving boundaries that divide star-forming from quenched galaxies in key structural diagrams. This paper uses these and other relations to "reverse engineer" the quenching process for central galaxies. The basic idea is that star-forming galaxies with larger radii (at a given stellar mass) have lower black hole (BH) masses due to lower central densities. Galaxies cross into the green valley when the cumulative effective energy radiated by their BH equals ∼4× their halo gas-binding energy. Because larger-radii galaxies have smaller BHs, one finds that they must evolve to higher stellar masses in order to meet this halo energy criterion, which explains the sloping boundaries. A possible cause of radii differences among star-forming galaxies is halo concentration. The evolutionary tracks of star-forming galaxies are nearly parallel to the green-valley boundaries, and it is mainly the sideways motions of these boundaries with cosmic time that cause galaxies to quench. BH scaling laws for star-forming, quenched, and green-valley galaxies are different, and most BH mass growth takes place in the green valley. Implications include the radii of star-forming galaxies are an important second parameter in shaping their BHs; BHs are connected to their halos but in different ways for star-forming, quenched, and green-valley galaxies; and the same BH–halo quenching mechanism has been in place since z ∼ 3. We conclude with a discussion of BH–galaxy coevolution and the origin and interpretation of BH scaling laws. [ABSTRACT FROM AUTHOR]

We study the effects of Kelvin–Helmholtz Instability (KHI) on the cold streams that feed massive haloes at high redshift, generalizing our earlier results to include the effects of radiative cooling and heating from a UV background, using analytic models and high resolution idealized simulations. We currently do not consider self-shielding, thermal conduction, or gravity. A key parameter in determining the fate of the streams is the ratio of the cooling time in the turbulent mixing layer which forms between the stream and the background following the onset of the instability, |$t_{\rm cool,\, mix}$|⁠ , to the time in which the mixing layer expands to the width of the stream in the non-radiative case, t shear. This can be converted into a critical stream radius, R s, crit, such that |$R_{\rm s}/R_{\rm s,crit}=t_{\rm shear}/t_{\rm cool,\, mix}$|⁠. If R s < R s, crit, the non-linear evolution proceeds similarly to the non-radiative case studied by Mandelker et al. If R s > R s,crit, which we find to almost always be the case for astrophysical cold streams, the stream is not disrupted by KHI. Rather, background mass cools and condenses on to the stream, and can increase the mass of cold gas by a factor of ∼3 within 10 stream sound crossing times. The mass entrainment induces thermal energy losses from the background and kinetic energy losses from the stream, which we model analytically. Roughly half of the dissipated energy is radiated away from gas with |$T \lt 5\times 10^4\, {\rm K}$|⁠ , suggesting much of it will be emitted in Ly α. [ABSTRACT FROM AUTHOR]

We predict, analytically and by simulations, that gas discs tend to survive only in haloes above a threshold mass ∼2 × 1011 M⊙ (stellar mass ∼109 M⊙), with only a weak redshift dependence. At lower masses, the disc spins typically flip in less than an orbital time due to mergers associated with a change in the pattern of the feeding cosmic-web streams. This threshold arises from the halo merger rate when accounting for the mass dependence of the ratio of galactic baryons and halo mass. Above the threshold, wet compactions lead to massive central nuggets that allow the longevity of extended clumpy gas rings. Supernova feedback has a major role in disrupting discs below the critical mass, by driving the stellar-to-halo mass ratio that affects the merger rate, by stirring up turbulence and suppressing high-angular-momentum gas supply, and by confining major compactions to the critical mass. Our predictions seem consistent with current observed fractions of gas discs, to be explored by future observations that will resolve galaxies below 109 M⊙ at high redshifts, e.g. by JWST. [ABSTRACT FROM AUTHOR]

We present a simple model for the response of a dissipationless spherical system to an instantaneous mass change at its centre, describing the formation of flat cores in dark matter haloes and ultra-diffuse galaxies (UDGs) from feedback-driven outflow episodes in a specific mass range. This model generalizes an earlier simplified analysis of an isolated shell into a system with continuous density, velocity, and potential profiles. The response is divided into an instantaneous change of potential at constant velocities due to a given mass-loss or mass-gain, followed by energy-conserving relaxation to a new Jeans equilibrium. The halo profile is modelled by a two-parameter function with a variable inner slope and an analytic potential profile, which enables determining the associated kinetic energy at equilibrium. The model is tested against NIHAO cosmological zoom-in simulations, where it successfully predicts the evolution of the inner dark matter profile between successive snapshots in about 75 per cent of the cases, failing mainly in merger situations. This model provides a simple understanding of the formation of dark matter halo cores and UDGs by supernova-driven outflows, and a useful analytic tool for studying such processes. [ABSTRACT FROM AUTHOR]

The similarity between the distributions of spins for galaxies (λgal) and for dark-matter haloes (λhalo), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via R e ≃ fj λhalo R vir, where R e is the half-mass radius of the galaxy, fj is the angular momentum retention factor, and R vir is the halo radius. Using two suites of zoom-in cosmological simulations, we find that λgal and the λhalo of its host halo are in fact barely correlated, especially at z ≥ 1, in line with previous indications. Since the spins of baryons and dark matter are correlated at accretion into R vir, the null correlation in the end reflects an anticorrelation between fj and λhalo, which can arise from mergers and a 'wet compaction' phase that many high-redshift galaxies undergo. It may also reflect that unrepresentative small fractions of baryons are tapped to the galaxies. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. While λhalo is not a useful predictor for R e, our simulations reproduce a general relation of the form of R e = AR vir, in agreement with observational estimates. We find that the relation becomes tighter with A  = 0.02(c /10)−0.7, where c is the halo concentration, which in turn introduces a dependence on mass and redshift. [ABSTRACT FROM AUTHOR]

We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We find that while field UDGs arise from dwarfs in a characteristic mass range by multiple episodes of supernova feedback (Di Cintio et al.), group UDGs may also form by tidal puffing up and they become quiescent by ram-pressure stripping. The field and group UDGs share similar properties, independent of distance from the group centre. Their dark-matter haloes have ordinary spin parameters and centrally dominant dark-matter cores. Their stellar components tend to have a prolate shape with a Sérsic index n ∼ 1 but no significant rotation. Ram pressure removes the gas from the group UDGs when they are at pericentre, quenching star formation in them and making them redder. This generates a colour/star-formation-rate gradient with distance from the centre of the dense environment, as observed in clusters. We find that ∼20 per cent of the field UDGs that fall into a massive halo survive as satellite UDGs. In addition, normal field dwarfs on highly eccentric orbits can become UDGs near pericentre due to tidal puffing up, contributing about half of the group-UDG population. We interpret our findings using simple toy models, showing that gas stripping is mostly due to ram pressure rather than tides. We estimate that the energy deposited by tides in the bound component of a satellite over one orbit can cause significant puffing up provided that the orbit is sufficiently eccentric. We caution that while the simulations produce UDGs that match the observations, they under-produce the more compact dwarfs in the same mass range, possibly because of the high threshold for star formation or the strong feedback. [ABSTRACT FROM AUTHOR]

We study the connection between active galactic nuclei (AGN) and their host galaxies through cosmic time in the large-scale cosmological IllustrisTNG simulations. We first compare BH properties, i.e. the hard X-ray BH luminosity function, AGN galaxy occupation fraction, and distribution of Eddington ratios, to available observational constraints. The simulations produce a population of BHs in good agreement with observations, but we note an excess of faint AGN in hard X-ray (⁠|$L_{\rm x}\sim 10^{43-44}\, \rm erg/s$|⁠), and a lower number of bright AGN (⁠|$L_{\rm x}\gt 10^{44} \, \rm erg/s$|⁠), a conclusion that varies quantitatively but not qualitatively with BH luminosity estimation method. The lower Eddington ratios of the |$10^{9}\, \rm M_{\odot }$| BHs compared to observations suggest that AGN feedback may be too efficient in this regime. We study galaxy star formation activity and structural properties, and design sample-dependent criteria to identify different galaxy types (star-forming/quiescent, extended/compact) that we apply both to the simulations and observations from the candels fields. We analyse how the simulated and observed galaxies populate the specific star formation rate – stellar mass surface density diagram. A large fraction of the |$z$|  = 0 |$M_{\star }\geqslant 10^{11}\, \rm M_{\odot }$| quiescent galaxies first experienced a compaction phase (i.e. reduction of galaxy size) while still forming stars, and then a quenching event. We measure the dependence of AGN fraction on galaxies' locations in this diagram. After correcting the simulations with a redshift and AGN luminosity-dependent model for AGN obscuration, we find good qualitative and quantitative agreement with observations. The AGN fraction is the highest among compact star-forming galaxies (⁠|$16-20{{\ \rm per\ cent}}$| at |$z$|  ∼ 1.5–2), and the lowest among compact quiescent galaxies (⁠|$6-10{{\ \rm per\ cent}}$| at |$z$|  ∼ 1.5–2). [ABSTRACT FROM AUTHOR]

We study the effects of Kelvin–Helmholtz instability (KHI) on the cold streams that feed high-redshift galaxies through their hot haloes, generalizing our earlier analyses of a 2D slab to a 3D cylinder, but still limiting our analysis to the adiabatic case with no gravity. We combine analytic modelling and numerical simulations in the linear and non-linear regimes. For subsonic or transonic streams with respect to the halo sound speed, the instability in 3D is qualitatively similar to 2D, but progresses at a faster pace. For supersonic streams, the instability grows much faster in 3D and can be qualitatively different due to azimuthal modes, which introduce a strong dependence on the initial width of the stream–background interface. Using analytic toy models and approximations supported by high-resolution simulations, we apply our idealized hydrodynamical analysis to the astrophysical scenario. The upper limit for the radius of a stream that disintegrates prior to reaching the central galaxy is |${\sim } 70{{\ \rm per\ cent}}$| larger than the 2D estimate; it is in the range |$0.5\hbox{--}5{{\ \rm per\ cent}}$| of the halo virial radius, decreasing with increasing stream density and velocity. Stream disruption generates a turbulent mixing zone around the stream with velocities at the level of |${\sim } 20{{\ \rm per\ cent}}$| of the initial stream velocity. KHI can cause significant stream deceleration and energy dissipation in 3D, contrary to 2D estimates. For typical streams, up to |$10\hbox{--}50{{\ \rm per\ cent}}$| of the gravitational energy gained by inflow down the dark matter halo potential can be dissipated, capable of powering Lyman α blobs if most of it is dissipated into radiation. [ABSTRACT FROM AUTHOR]

As part of our long-term campaign to understand how cold streams feed massive galaxies at high redshift, we study the Kelvin–Helmholtz instability (KHI) of a supersonic, cold, dense gas stream as it penetrates through a hot, dilute circumgalactic medium (CGM). A linear analysis (Paper I) showed that, for realistic conditions, KHI may produce non-linear perturbations to the stream during infall. Therefore, we proceed here to study the non-linear stage of KHI, still limited to a 2D slab with no radiative cooling or gravity. Using analytic models and numerical simulations, we examine stream break-up, deceleration, and heating via surface modes and body modes. The relevant parameters are the density contrast between stream and CGM (δ), the Mach number of the stream velocity with respect to the CGM (Mb) and the stream radius relative to the halo virial radius (Rs/Rv). We find that sufficiently thin streams disintegrate prior to reaching the central galaxy. The condition for break-up ranges from Rs < 0.03Rv for (Mb ∼ 0.75, δ ∼ 10) to Rs < 0.003Rv for (Mb ∼ 2.25, δ ∼ 100). However, due to the large stream inertia, KHI has only a small effect on the stream inflow rate and a small contribution to heating and subsequent Lyman-α cooling emission. [ABSTRACT FROM AUTHOR]

We explore observational and theoretical constraints on how galaxies might transition between the 'star-forming main sequence' (SFMS) and varying 'degrees of quiescence' out to z = 3. Our analysis is focused on galaxies with stellar mass M* > 1010M☉, and is enabled by GAMA and CANDELS observations, a semi-analytic model (SAM) of galaxy formation, and a cosmological hydrodynamical 'zoom in' simulation with momentum-driven AGN feedback. In both the observations and the SAM, transition galaxies tend to have intermediate Sérsic indices, half-light radii, and surface stellar mass densities compared to star-forming and quiescent galaxies out to z=3.We place an observational upper limit on the average population transition time-scale as a function of redshift, finding that the average high-redshift galaxy is on a 'fast track' for quenching whereas the average low-redshift galaxy is on a 'slow track' for quenching. We qualitatively identify four physical origin scenarios for transition galaxies in the SAM: oscillations on the SFMS, slow quenching, fast quenching, and rejuvenation. Quenching time-scales in both the SAM and the hydrodynamical simulation are not fast enough to reproduce the quiescent population that we observe at z ∼ 3. In the SAM, we do not find a clear-cut morphological dependence of quenching time-scales, but we do predict that the mean stellar ages, cold gas fractions, SMBH (supermassive black hole) masses and halo masses of transition galaxies tend to be intermediate relative to those of star-forming and quiescent galaxies at z < 3. [ABSTRACT FROM AUTHOR]

We address the origin of ultra-diffuse galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed L⋆ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project, we show that UDG analogues form naturally in dwarf-sized haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses 1010–11 M⊙, have stellar masses of 107–8.5 M⊙, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colours, an average Sérsic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible H i gas mass of 107 − 9 M⊙, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes which form UDGs: feedback-driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more H i gas than less extended dwarfs of similar M⋆. [ABSTRACT FROM AUTHOR]

We study the correlation of galaxy structural properties with their location relative to the SFR-M*, correlation, also known as the star formation 'star-forming main sequence' (SFMS), in the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and Galaxy and Mass Assembly Survey and in a semi-analytic model (SAM) of galaxy formation. We first study the distribution of median Sérsic index, effective radius, star formation rate (SFR) density and stellar mass density in the SFR-M* plane. We then define a redshift-dependent main sequence and examine the medians of these quantities as a function of distance from this main sequence, both above (higher SFRs) and below (lower SFRs). Finally, we examine the distributions of distance from the main sequence in bins of these quantities. We find strong correlations between all of these galaxy structural properties and the distance from the SFMS, such that as we move from galaxies above the SFMS to those below it, we see a nearly monotonic trend towards higher median Sérsic index, smaller radius, lower SFR density, and higher stellar density. In the SAM, bulge growth is driven by mergers and disc instabilities, and is accompanied by the growth of a supermassive black hole which can regulate or quench star formation via active galactic nucleus feedback. We find that our model qualitatively reproduces the trends described above, supporting a picture in which black holes and bulges co-evolve, and active galactic nucleus feedback plays a critical role in moving galaxies off of the SFMS. [ABSTRACT FROM AUTHOR]

Using cosmological simulations, we address the interplay between structure and star formation in high-redshift galaxies via the evolution of surface density profiles. Our sample consists of 26 galaxies evolving in the redshift range z = 7 - 1, spanning the stellar mass range (0.2-6.4) × 1010M⊙ at z = 2.We recover the main trends by stacking the profiles in accordance to their evolution phases. Following a wet compaction event that typically occurs when the stellar mass is ~109.5M⊙ at z ~ 2-4, the gas develops a cusp inside the effective radius, associated with a peak in star formation rate (SFR). The SFR peak and the associated feedback, in the absence of further gas inflow to the centre, marks the onset of gas depletion from the central 1 kpc, leading to quenching of the central SFR. An extended, star-forming ring that forms by fresh gas during the central quenching process shows as a rising specific SFR (sSFR) profile, which is interpreted as inside-out quenching. Before quenching, the stellar density profile grows self-similarly, maintaining its log-log shape because the sSFR is similar at all radii. During the quenching process, the stellar density saturates to a constant value, especially in the inner 1 kpc. The stellar mass and SFR profiles deduced from observations show very similar shapes, consistent with the scenario of wet compaction leading to inside-out quenching and the subsequent saturation of a dense stellar core. We predict a cuspy gas profile during the blue nugget phase, and a gas-depleted core, sometimes surrounded by a ring, in the post-blue nugget phase. [ABSTRACT FROM AUTHOR]

Using cosmological simulations, we address the properties of high-redshift star-forming galaxies (SFGs) across their main sequence (MS) in the plane of star formation rate (SFR) versus stellar mass. We relate them to the evolution of galaxies through phases of gas compaction, depletion, possible replenishment, and eventual quenching. We find that the high-SFR galaxies in the upper envelope of the MS are compact, with high gas fractions and short depletion times ('blue nuggets'), while the lower SFR galaxies in the lower envelope have lower central gas densities, lower gas fractions, and longer depletion times, consistent with observed gradients across the MS. Stellar-structure gradients are negligible. The SFGs oscillate about the MS ridge on time-scales ∼0.4tHubble (∼1 Gyr at z ∼ 3). The propagation upwards is due to gas compaction, triggered, e.g. by mergers, counter-rotating streams, and/or violent disc instabilities. The downturn at the upper envelope is due to central gas depletion by peak star formation and outflows while inflow from the shrunken gas disc is suppressed. An upturn at the lower envelope can occur once the extended disc has been replenished by fresh gas and a new compaction can be triggered, namely as long as the replenishment time is shorter than the depletion time. The mechanisms of gas compaction, depletion, and replenishment confine the SFGs to the narrow (±0.3 dex) MS. Full quenching occurs in massive haloes (Mvir > 1011.5M☉) and/or at low redshifts (z < 3), where the replenishment time is long compared to the depletion time, explaining the observed bending down of the MS at the massive end. [ABSTRACT FROM AUTHOR]

We utilize zoom-in cosmological simulations to study the nature of violent disc instability in clumpy galaxies at high redshift, z = 1-5. Our simulated galaxies are not in the ideal state assumed in Toomre instability, of linear fluctuations in an isolated, uniform, rotating disc. There, instability is characterized by a Q parameter below unity, and lower when the disc is thick. Instead, the high-redshift discs are highly perturbed. Over long periods they consist of non-linear perturbations, compact massive clumps and extended structures, with new clumps forming in interclump regions. This is while the galaxy is subject to frequent external perturbances. We compute the local, two-component Q parameter for gas and stars, smoothed on a ~1 kpc scale to capture clumps of 108-9M⊙. The Q < 1 regions are confined to collapsed clumps due to the high surface density there, while the interclump regions show Q significantly higher than unity. Tracing the clumps back to their relatively smooth Lagrangian patches, we find that Q prior to clump formation typically ranges from unity to a few. This is unlike the expectations from standard Toomre instability. We discuss possible mechanisms for high-Q clump formation, e.g. rapid turbulence decay leading to small clumps that grow by mergers, non-axisymmetric instability, or clump formation induced by non-linear perturbations in the disc. Alternatively, the high-Q non-linear VDI may be stimulated by the external perturbations such as mergers and counter-rotating streams. The high Q may represent excessive compressive modes of turbulence, possibly induced by tidal interactions. [ABSTRACT FROM AUTHOR]

We investigate the roles of two classes of quenching mechanisms for central and satellite galaxies in the Sloan Digital Sky Survey (z < 0.075): those involving the halo and those involving the formation of a compact centre. For central galaxies with inner compactness Σ1 kpc ∼ 109–9.4 M⊙ kpc−2, the quenched fraction fq is strongly correlated with Σ1 kpc with only weak halo mass Mh dependence. However, at higher and lower Σ1 kpc, specific star formation rate (sSFR) is a strong function of Mh and mostly independent of Σ1 kpc. In other words, Σ1 kpc ∼ 109–9.4 M⊙ kpc−2 divides galaxies into those with high sSFR below and low sSFR above this range. In both the upper and lower regimes, increasing Mh shifts the entire sSFR distribution to lower sSFR without a qualitative change in shape. This is true even at fixed M*, but varying M* at fixed Mh adds no quenching information. Most of the quenched centrals with Mh > 1011.8 M⊙ are dense (Σ1 kpc > 109 M⊙ kpc−2), suggesting compaction-related quenching maintained by halo-related quenching. However, 21 per cent are diffuse, indicating only halo quenching. For satellite galaxies in the outskirts of haloes, quenching is a strong function of compactness and a weak function of host Mh. In the inner halo, Mh dominates quenching, with ∼90 per cent of the satellites being quenched once Mh > 1013 M⊙. This regional effect is greatest for the least massive satellites. As demonstrated via semi-analytic modelling with simple prescriptions for quenching, the observed correlations can be explained if quenching due to central compactness is rapid while quenching due to halo mass is slow. [ABSTRACT FROM PUBLISHER]

We combine photometry from the Ultra Deep Survey (UDS), Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) UDS and CANDELS the Great Observatories Origins Deep Survey-South (GOODS-S) surveys to construct the galaxy stellar mass function probing both the low- and high-mass end accurately in the redshift range 0.3 < z < 3. The advantages of using a homogeneous concatenation of these data sets include meaningful measures of environment in the UDS, due to its large area (0.88 deg²), and the high-resolution deep imaging in CANDELS (H160 > 26.0), affording us robust measures of structural parameters. We construct stellar mass functions for the entire sample as parametrized by the Schechter function, and find that there is a decline in the values of ɸ and of α with higher redshifts, and a nearly constant M* up to z ~ 3. We divide the galaxy stellar mass function by colour, structure, and environment and explore the links between environmental overdensity, morphology, and the quenching of star formation. We find that a double Schechter function describes galaxies with high Sérsic index (n > 2.5), similar to galaxies which are red or passive. The low-mass end of the n > 2.5 stellar mass function is dominated by blue galaxies, whereas the high-mass end is dominated by red galaxies. This shows that there is a possible link between morphological evolution and star formation quenching in high mass galaxies, which is not seen in lower mass systems. This in turn suggests that there are strong mass-dependent quenching mechanisms. In addition, we find that the number density of high-mass systems is elevated in dense environments, suggesting that an environmental process is building up massive galaxies quicker in over densities than in lower densities. [ABSTRACT FROM AUTHOR]

We address the formation of massive stellar spheroids between redshifts z = 4 and 1 using a suite of adaptive mesh refinement hydrocosmological simulations. The spheroids form as bulges, and the spheroid mass growth is partly driven by violent disc instability (VDI) and partly by mergers. A kinematic decomposition to disc and spheroid yields that the mass fraction in the spheroid is between 50 and 90 per cent and is roughly constant in time, consistent with a cosmological steady state of VDI discs that are continuously fed from the cosmic web. The density profile of the spheroid is typically ‘classical’, with a Sérsic index n = 4.5 ± 1, independent of whether it grew by mergers or VDI and independent of the feedback strength. The disc is characterized by n = 1.5 ± 0.5, and the whole galaxy by n = 3 ± 1. The high-redshift spheroids are compact due to the dissipative inflow of gas and the high universal density. The stellar surface density within the effective radius of each galaxy as it evolves remains roughly constant in time after its first growth. For galaxies of a fixed stellar mass, the surface density is higher at higher redshifts. [ABSTRACT FROM AUTHOR]

We study the minimal ‘bathtub’ toy model as a tool for capturing key processes of galaxy evolution and identifying robust successes and challenges in reproducing high-z observations. The source and sink terms of the continuity equations for gas and stars are expressed in simple terms from first principles. The assumed dependence of star formation rate (SFR) on gas mass self-regulates the system into a unique asymptotic behaviour, which is approximated by an analytic quasi-steady-state (QSS) solution. We address the validity of the QSS at different epochs independent of earlier conditions. At high z, where the accretion is gaseous, the specific SFR (sSFR) is predicted to be sSFR ≃ [(1 + z)/3]5/2 Gyr−1, slightly above the cosmological specific accretion rate, as observed at z = 3–8. The gas fraction is expected to decline slowly, and the observations constrain the SFR efficiency per dynamical time to ϵ ≃ 0.02. The stellar-to-virial mass ratio fsv is predicted to be constant in time, and the observed value requires an outflow mass-loading factor η ≃ 1–3, depending on the penetration efficiency of gas into the galaxy. However, at z ∼ 2, where stars are also accreted through mergers, there is a conflict between model and observations. The model that maximizes the sSFR, with the outflows fully recycled, underestimates the sSFR by a factor of ∼3 and overestimates fsv. With strong outflows, the model can match the observed fsv but then it underestimates the sSFR by an order of magnitude. We discuss potential remedies including a bias due to the exclusion of quenched galaxies. [ABSTRACT FROM AUTHOR]

Cosmological simulations of galaxies have typically produced too many stars at early times. We study the global and morphological effects of radiation pressure (RP) in eight pairs of high-resolution cosmological galaxy formation simulations. We find that the additional feedback suppresses star formation globally by a factor of ∼2. Despite this reduction, the simulations still overproduce stars by a factor of ∼2 with respect to the predictions provided by abundance matching methods for haloes more massive than 5 × 1011 M⊙ h−1 . We also study the morphological impact of RP on our simulations. In simulations with RP the average number of low-mass clumps falls dramatically. Only clumps with stellar masses Mclump/Mdisc ≤ 5 per cent are impacted by the inclusion of RP, and RP and no-RP clump counts above this range are comparable. The inclusion of RP depresses the contrast ratios of clumps by factors of a few for clump masses less than 5 per cent of the disc masses. For more massive clumps, the differences between and RP and no-RP simulations diminish. We note, however, that the simulations analysed have disc stellar masses below about 2 × 1010 M⊙ h−1. By creating mock Hubble Space Telescope observations we find that the number of clumps is slightly reduced in simulations with RP. However, since massive clumps survive the inclusion of RP and are found in our mock observations, we do not find a disagreement between simulations of our clumpy galaxies and observations of clumpy galaxies. We demonstrate that clumps found in any single gas, stellar, or mock observation image are not necessarily clumps found in another map, and that there are few clumps common to multiple maps. [ABSTRACT FROM PUBLISHER]

Any successful model of galaxy formation needs to explain the low rate of star formation in the small progenitors of today's galaxies. This inefficiency is necessary for reproducing the low stellar-to-virial mass fractions, suggested by current abundance matching models. A possible driver of this low efficiency is the radiation pressure exerted by ionizing photons from massive stars. The effect of radiation pressure in cosmological, zoom-in galaxy formation simulations is modelled as a non-thermal pressure that acts only in dense and optically thick star-forming regions. We also include photoionization and photoheating by massive stars. The full photoionization of hydrogen reduces the radiative cooling in the 104-4.5 K regime. The main effect of radiation pressure is to regulate and limit the high values of gas density and the amount of gas available for star formation. This maintains a low star formation rate of ∼1 M⊙ yr−1 in haloes with masses about 1011 M⊙ at z ≃ 3. Infrared trapping and photoionization/photoheating processes are secondary effects in this mass range. The galaxies residing in these low-mass haloes contain only ∼0.6 per cent of the total virial mass in stars, roughly consistent with abundance matching. Radiative feedback maintains an extended galaxy with a rising circular velocity profile. [ABSTRACT FROM AUTHOR]

Cosmological hydrodynamic simulations predict the physical state of baryons in the circumgalactic medium (CGM), which can be directly tested via quasar absorption line observations. We use high-resolution “zoom-in” simulations of 21 galaxies to characterize the distribution of neutral hydrogen around halos in the mass range Mvir ∼ 2 × 1011 to 4 × 1012M☼ at z ∼ 2. We find that both the mass fraction of cool (T ⩽ 3 × 104 K) gas and the covering fraction of optically thick Lyman limit systems (LLSs) depend only weakly on halo mass, even around the critical value for the formation of stable virial shocks. The covering fraction of LLSs interior to the virial radius varies between fc ∼ 0.05-0.2, with significant scatter among halos. Our simulations of massive halos (Mvir ⩾ 1012M☼) underpredict the covering fraction of optically thick gas observed in the quasar CGM by a large factor. The reason for this discrepancy is unclear, but several possibilities are discussed. In the lower mass halos (Mvir ⩾ 5 × 1011M☼) hosting star-forming galaxies, the predicted covering factor agrees with observations; however, current samples of quasar-galaxy pairs are too small for a conclusive comparison. To overcome this limitation, we propose a new observable: the small-scale autocorrelation function of optically thick absorbers detected in the foreground of close quasar pairs. We show that this new observable can constrain the underlying dark halos hosting LLSs at z ∼ 2-3, as well as the characteristic size and covering factor of the CGM. [ABSTRACT FROM AUTHOR]

We predict the evolution of giant clumps undergoing star-driven outflows in high-z gravitationally unstable disc galaxies. We find that the mass-loss is expected to occur through a steady wind over many tens of free-fall times (tff ∼ 10 Myr) rather than by an explosive disruption in one or a few tff. Our analysis is based on the finding from simulations that radiation trapping is negligible because it destabilizes the wind (Krumholz & Thompson 2012, 2013). Each photon can therefore contribute to the wind momentum only once, so the radiative force is limited to L/c. When combining radiation, protostellar and main-sequence winds, and supernovae, we estimate the total direct injection rate of momentum into the outflow to be 2.5 L/c. The adiabatic phase of supernovae and main-sequence winds can double this rate. The resulting outflow mass-loading factor is of order unity, and if the clumps were to deplete their gas, the time-scale would have been a few disc orbital times, to end with half the original clump mass in stars. However, the clump migration time to the disc centre is of the order of an orbital time, about 250 Myr, so the clumps are expected to complete their migration prior to depletion. Furthermore, the clumps are expected to double their mass in a disc orbital time by accretion from the disc and clump–clump mergers, so their mass actually grows in time and with decreasing radius. From the six to seven giant clumps with observed outflows, five are consistent with these predictions, and one has a much higher mass-loading factor and momentum injection rate. The latter either indicates that the estimated outflow is an overestimate (within the 1σ error), that the star formation rate has dropped since the time when the outflow was launched or that the driving mechanism is different, e.g. supernova feedback in a cavity generated by the other feedbacks. [ABSTRACT FROM AUTHOR]

We combine high-resolution Hubble Space Telescope/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (M" > 1010M☉) galaxies at redshifts z = 1.4-3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates (SFRs) qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5-3. At z ≳ 2, cSFGs present SFR = 100-200M☉ yr-1, yet their specific star formation rates (sSFR ~ 10-9 yr-1) are typically half that of other massive SFGs at the same epoch, and host X-ray luminous active galactic nuclei (AGNs) 30 times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 108 yr). The cSFGs are continuously being formed at z = 2-3 and fade to cQGs down to z ~ 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary tracks of QG formation: an early (z ≳ 2), formation path of rapidly quenched cSFGs fading into cQGs that later enlarge within the quiescent phase, and a late-arrival (z ≳ 2) path in which larger SFGs form extended QGs without passing through a compact state. [ABSTRACT FROM AUTHOR]

ABSTRACT We analyse the evolution of turbulence and gravitational instability of a galactic disc in a quasi-steady state governed by cosmological inflow. We focus on the possibility that the coupling between the in-streaming gas and the disc is maximal, e.g. via dense clumps, and ask whether the streams could be the driver of turbulence in an unstable disc with a Toomre parameter Q ∼ 1. Our fiducial model assumes an efficiency of ∼0.5 per dynamical time for the decay of turbulence energy, and ∼0.02 for each of the processes that depletes the disc gas, i.e. star formation, outflow and inflow within the disc into a central bulge. In this case, the in-streaming drives a ratio of turbulent to rotation velocity σ/ V ∼ 0.2-0.3, which at z ∼ 2 induces an instability with Q ∼ 1, both as observed. However, in conflict with observations, this model predicts that σ/ V remains constant with time, independent of the cosmological accretion rate, because mass and turbulence have the same external source. Such strongly coupled cosmological inflow tends to stabilize the disc at low z, with Q ∼ a few, which may be consistent with observations. The instability could instead be maintained for longer, with a properly declining σ/ V, if it is self-regulated to oscillations about Q ≈ 1 by a duty cycle for disc depletion. However, the 'off' phases of this duty cycle become long at low z, which may be hard to reconcile with observations. Alternatively, the coupling between the in-streaming gas and the disc may weaken in time, reflecting an evolving nature of the accretion. If, instead, that coupling is weak at all times, the likely energy source for self-regulated stirring up of the turbulence is the inflow within the disc down the potential gradient (studied in a companion paper). [ABSTRACT FROM AUTHOR]

ABSTRACT We study the feeding of massive galaxies at high redshift through streams from the cosmic web using the Mare Nostrum hydrocosmological simulation. Our statistical sample consists of 350 dark matter haloes of ≃1012 M⊙ at z= 2.5. We find that ∼70 per cent of the influx into the virial radius Rv is in narrow streams covering 10 per cent of the virial shell. On average 64 per cent of the stream influx is in one stream, and 95 per cent is in three dominant streams. The streams that feed a massive halo tend to lie in a plane that extends from half to a few Rv, hereafter 'the stream plane' (SP). The streams are typically embedded in a thin sheet of low-entropy gas, a Zel'dovich pancake, which carries ∼20 per cent of the influx into Rv. The filaments-in-a-plane configuration about the massive haloes at the nodes of the cosmic web differs from the large-scale structure of the web where the filaments mark the intersections of slanted sheets. The SP is only weakly aligned with the angular momentum (AM) near Rv, consistent with the fact that typically 80 per cent of the AM is carried by one dominant stream. The galactic disc plane shows a weak tendency to be perpendicular to the large-scale SP, consistent with tidal-torque theory. Most interesting, the direction of the disc AM is only weakly correlated with the AM direction at Rv. This indicates a significant AM exchange at the interphase between streams and disc in the greater environment of the disc inside an 'AM sphere' of radius ∼0.3 Rv. The required large torques are expected based on the perturbed morphology and kinematics within this interaction sphere. This AM exchange may or may not require a major modification of the standard disc modelling based on AM conservation, depending on the extent to which the amplitude of the disc AM is affected, which is yet to be studied. [ABSTRACT FROM AUTHOR]

ABSTRACT The observational indications for a constant specific star formation rate (sSFR) in the redshift range z= 2-7 are puzzling in the context of current galaxy-formation models. Despite the tentative nature of the data, their marked conflict with theory motivates a study of the possible implications. The plateau at sSFR ∼ 2 Gyr−1 is hard to reproduce because (a) its level is low compared to the cosmological specific accretion rate at z≥ 6, (b) it is higher than the latter at z∼ 2, (c) the natural correlation between SFR and stellar mass makes it difficult to manipulate their ratio, and (d) a low SFR at a high z makes it hard to produce enough massive galaxies by z∼ 2. Using a flexible semi-analytic model, we explore ad hoc modifications to the standard physical recipes trying to obey the puzzling observational constraints. Successful models involve non-trivial modifications, such as (a) a suppressed SFR at z≥ 4 in galaxies of all masses, by enhanced feedback or reduced SFR efficiency, following an initial active phase at z > 7; (b) a delayed gas consumption into stars, allowing the gas that was prohibited from forming stars or ejected at high z to form stars later in more massive galaxies; and (c) enhanced growth of massive galaxies, in terms of either faster assembly or more efficient starbursts in mergers, or by efficient star formation in massive haloes. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the origin of the relations between stellar mass and optical circular velocity for early-type galaxies (ETGs) and late-type galaxies (LTGs) - the Faber-Jackson (FJ) and Tully-Fisher (TF) relations. We combine measurements of dark halo masses (from satellite kinematics and weak lensing), and the distribution of baryons in galaxies (from a new compilation of galaxy scaling relations), with constraints on dark halo structure from cosmological simulations. The principal unknowns are the halo response to galaxy formation and the stellar initial mass function (IMF). The slopes of the TF and FJ relations are naturally reproduced for a wide range of halo response and IMFs. However, models with a universal IMF and universal halo response cannot simultaneously reproduce the zero-points of both the TF and FJ relations. For a model with a universal Chabrier IMF, LTGs require halo expansion, while ETGs require halo contraction. A Salpeter IMF is permitted for high-mass (σ≳ 180 km s−1) ETGs, but is inconsistent for intermediate masses, unless Vcirc( Re)/σe≳ 1.6. If the IMF is universal and close to Chabrier, we speculate that the presence of a major merger may be responsible for the contraction in ETGs while clumpy accreting streams and/or feedback leads to expansion in LTGs. Alternatively, a recently proposed variation in the IMF disfavours halo contraction in both types of galaxies. Finally we show that our models naturally reproduce flat and featureless circular velocity profiles within the optical regions of galaxies without fine-tuning. [ABSTRACT FROM AUTHOR]

We derive analytic merger rates for dark matter haloes within the framework of the extended Press–Schechter (EPS) formalism. These rates become self-consistent within EPS once we realize that the typical merger in the limit of a small time-step involves more than two progenitors, contrary to the assumption of binary mergers adopted in earlier studies. We present a general method for computing merger rates that span the range of solutions permitted by the EPS conditional mass function, and focus on a specific solution that attempts to match the merger rates in N-body simulations. The corrected EPS merger rates are more accurate than the earlier estimates of Lacey & Cole by ∼20 per cent for major mergers and by up to a factor of ∼3 for minor mergers of mass ratio 1:104. Based on the revised merger rates, we provide a new algorithm for constructing Monte Carlo EPS merger trees, which could be useful in semi-analytic modelling. We provide analytic expressions and plot numerical results for several quantities that are very useful in studies of galaxy formation. This includes (i) the rate of mergers of a given mass ratio per given final halo, (ii) the fraction of mass added by mergers to a halo and (iii) the rate of mergers per given main progenitor. The creation and destruction rates of haloes serve for a self-consistency check. Our method for computing merger rates can be applied to conditional mass functions beyond EPS, such as those obtained by the ellipsoidal collapse model or extracted from N-body simulations. [ABSTRACT FROM AUTHOR]

Stellar-population analyses of today's galaxies show ‘downsizing’, where the stars in more massive galaxies tend to have formed earlier and over a shorter time-span. We show that this phenomenon is not necessarily ‘antihierarchical’ but rather has its natural roots in the bottom-up clustering process of dark-matter haloes. While the main progenitor does indeed show an opposite effect, the integrated mass in all the progenitors down to a given minimum mass shows a robust downsizing that is qualitatively similar to what has been observed. These results are derived analytically from the standard extended Press–Schechter (EPS) theory, and are confirmed by merger trees based on EPS or drawn from N-body simulations. The downsizing is valid for any minimum mass, as long as it is the same for all haloes at any given time, but the effect is weaker for smaller minimum mass. If efficient star formation is triggered by atomic cooling, then a minimum halo mass arises naturally from the minimum virial temperature for cooling, , though for such a small minimum mass the effect is weaker than observed. Baryonic feedback effects, which are expected to stretch the duration of star formation in small galaxies and shut it down in massive haloes at late epochs, are likely to play a subsequent role in shaping up the final downsizing behaviour. Other appearances of downsizing, such as the decline with time of the typical mass of star-forming galaxies, may not be attributed to the gravitational clustering process but rather arise from the gas processes. [ABSTRACT FROM AUTHOR]

We address the deviations of the scaling relations of elliptical galaxies from the expectations based on the virial theorem and homology, including the ‘tilt’ of the ‘Fundamental Plane’ and the steep decline of density with mass. We show that such tilts result from dissipative major mergers once the gas fraction available for dissipation declines with progenitor mass, and derive the scaling properties of the progenitors. We use hydrodynamical simulations to quantify the effects of major mergers with different gas fractions on the structural properties of galaxies. The tilts are driven by the differential shrinkage of the effective stellar radius as a function of dissipation in the merger, while the correlated smaller enhancements in internal velocity and stellar mass keep the slope of the velocity–stellar mass relation near . The progenitors match a straightforward model of disc formation in Λ cold dark matter haloes. Their total-to-stellar mass ratio within the effective radius varies as , consistent with the effect of supernova feedback. They are nearly homologous in the sense that the dark matter fractions within the effective and virial radii scale with mass in a similar way, with only a weak decline of density with mass. The progenitor radius is roughly , compatible with today's intermediate late-type galaxies. This may indicate that the latest dissipative mergers in the history of ellipticals involved relatively big discs. The dissipative gas-to-stellar mass ratio is predicted to decline , similar to the observed trend in today's blue-sequence galaxies. Such a trend should be observed in relatively massive gaseous galaxies at . The corresponding ‘baryonic’ relations are consistent with homology and spherical virial equilibrium, . [ABSTRACT FROM AUTHOR]

Studies in simple terms the role of feedback in establishing the scaling relations of low-surface-brightness and dwarf galaxies with certain range of stellar masses. Galaxies' demonstration of tight correlations of internal velocity, metallicity and surface brightness with M*; Galaxies' definition of fundamental line which distinguishes them from the brighter galaxies of high surfaces brightness and metallicity; Supernova remnants; Formation of galaxies.