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The evolution of star-forming galaxies at high redshifts is very sensitive to the strength and nature of stellar feedback. Using two sets of cosmological, zoom-in simulations from the VELA suite, we compare the effects of two different models of feedback: with and without kinetic feedback from the expansion of supernovae shells and stellar winds. At a fixed halo mass and redshift, the stellar mass is reduced by a factor of ∼1–3 in the models with stronger feedback, so the stellar mass–halo mass relation is in better agreement with abundance matching results. On the other hand, the three-dimensional shape of low-mass galaxies is elongated along a major axis in both models. At a fixed stellar mass, M * < 1010 M⊙, galaxies are more elongated in the strong-feedback case. More massive, star-forming discs with high surface densities form giant clumps. However, the population of round, compact, old (agec > 300 Myr), quenched, stellar (or gas-poor) clumps is absent in the model with strong feedback. On the other hand, giant star-forming clumps with intermediate ages (agec = 100–300 Myr) can survive for several disc dynamical times, independently of feedback strength. The evolution through compaction followed by quenching in the plane of central surface density and specific star formation rate is similar under the two feedback models. [ABSTRACT FROM AUTHOR]

We study the survival of giant clumps in high-redshift disc galaxies, short-lived (S) versus long-lived (L), and two L subtypes, via analytic modelling and simulations. We develop a criterion for clump survival, with/without gas, based on a survivability parameter S. It compares the energy sources by supernova feedback and gravitational contraction to the clump binding energy and losses by outflows and turbulence dissipation. The clump properties are derived from Toomre instability, approaching virial/Jeans equilibrium, and the supernova energy deposit uses an up-to-date bubble analysis. For moderate feedback, we find L clumps with circular velocities |${\sim}50\, {\rm km}\, {\rm s}^{-1}$| and masses ≥108 M⊙. They favour galaxies with circular velocities |${\ge}200\, {\rm km\,s}^{-1}$|⁠ , consistent at z ∼ 2 with the typical disc stellar mass, ≥109.3 M⊙. L clumps favour disc gas fractions ≥0.3, low-mass bulges, and z ∼ 2. They disfavour more effective feedback due to, e.g. supernova clustering, very strong radiative feedback, top-heavy stellar mass function, or particularly high star-formation-rate (SFR) efficiency. A subtype of L clumps (LS), which lose their gas in several free-fall times but retain bound stellar components, may be explained by less contraction and stronger gravitational effects, where clump mergers increase the SFR efficiency. These may give rise to globular clusters. The more massive L clumps (LL) retain most of their baryons for tens of free-fall times with a roughly constant star-formation rate. [ABSTRACT FROM AUTHOR]

We introduce gizmo-simba , a new suite of galaxy cluster simulations within The Three Hundred project. The Three Hundred consists of zoom re-simulations of 324 clusters with |$M_{200}\gtrsim 10^{14.8}\, \mathrm{M}_\odot$| drawn from the MultiDark-Planck N -body simulation, run using several hydrodynamic and semi-analytical codes. The gizmo-simba suite adds a state-of-the-art galaxy formation model based on the highly successful Simba simulation, mildly re-calibrated to match |$z$|  = 0 cluster stellar properties. Comparing to The Three Hundred zooms run with gadget-x , we find intrinsic differences in the evolution of the stellar and gas mass fractions, BCG ages, and galaxy colour–magnitude diagrams, with gizmo-simba generally providing a good match to available data at |$z$|  ≈ 0. gizmo-simba 's unique black hole growth and feedback model yields agreement with the observed BH scaling relations at the intermediate-mass range and predicts a slightly different slope at high masses where few observations currently lie. Gizmo-Simba provides a new and novel platform to elucidate the co-evolution of galaxies, gas, and black holes within the densest cosmic environments. [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]

Emission line galaxies provide a crucial tool for the study of galaxy formation and evolution, providing a means to trace a galaxy's star formation history or metal enrichment, and to identify galaxies at a range of stellar masses. In this paper, we present a study of emission line galaxies in the Survey for High- z Absorption Red and Dead Sources (SHARDS) Frontier Fields (FF) medium-band survey. Through detailed flux calibrations we combine the first results of the SHARDS-FF survey with existing Hubble Frontier Field data to select 1098 candidate emission line galaxies from the Hubble Frontier Filed clusters Abell 370 and MACS J1149.5+2223. Furthermore, we implement this deep medium-band imaging to update photometric redshift estimates and stellar population parameters and discover 38 predominantly low-mass Hα emitters at redshifts 0.24 < z < 0.46. Overall, 27 of these sources have corresponding ultraviolet (UV) data from the Hubble Space Telescope that allow us to distinguish these sources and investigate the burstiness of their star formation histories. We find that more than 50 per cent of our sample shows an enhancement in Hα over UV, suggesting recent bursts in star formation on time-scales of a few to tens of Myr. We investigate these sources and find that they are typically low-mass discy galaxies with normal sizes. Their structures and star formation suggest that they are not undergoing mergers but are bursting due to alternative causes, such as gas accretion. [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 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 model the projected b / a –log  a distributions of CANDELS star-forming main-sequence galaxies, where a (b) is the half-light semimajor (semiminor) axis of the galaxy images measured by galfit. We find that smaller a galaxies are rounder at all stellar masses M * and redshifts, so we include a when analysing b / a distributions. Approximating intrinsic shapes of the galaxies as triaxial ellipsoids and assuming a multivariate normal distribution of galaxy size and two shape parameters, we construct their intrinsic shape and size distributions to obtain the fractions of elongated (prolate), discy (oblate), and spheroidal galaxies in each redshift and mass bin. We find that galaxies tend to be prolate at low M * and high redshifts, and discy at high M * and low redshifts, qualitatively consistent with van der Wel et al. implying that galaxies tend to evolve from prolate to discy. These results are consistent with the predictions from simulations that the transition from prolate to oblate is caused by a compaction event at a characteristic mass range, making the galaxy centre baryon dominated. We give probabilities of a galaxy's being elongated, discy, or spheroidal as a function of its M *, redshift, and projected b / a and a, which can facilitate target selections of galaxies with specific shapes at high redshifts. [ABSTRACT FROM AUTHOR]

Using the FirstLight data base of 300 zoom-in cosmological simulations we provide rest-frame UV-optical spectral energy distributions of galaxies with complex star-formation histories that are coupled to the non-uniform gas accretion history of galactic haloes during cosmic dawn. The population at any redshift is very diverse ranging from starbursts to quiescent galaxies even at a fixed stellar mass. This drives a redshift-dependent relation between UV luminosity and stellar mass with a large scatter, driven by the specific star formation rate. The UV slope and the production efficiency of Lyman continuum photons have high values, consistent with dust-corrected observations. This indicates young stellar populations with low metallicities. The FirstLight simulations make predictions on the rest-frame UV-optical absolute magnitudes, colours, and optical emission lines of galaxies at |$z$|  = 6–12 that will be observed for the first time with James Webb Space Telescope and the next generation of telescopes in the coming decade. [ABSTRACT FROM AUTHOR]

In the FirstLight project, we have used ∼300 cosmological, zoom-in simulations to determine the star formation histories of distinct first galaxies with stellar masses between M * = 106 and |$3 \times 10^9\, {\rm M}_{\odot}$| during cosmic dawn (⁠|$z$| = 5–15). The evolution of the star formation rate (SFR) in each galaxy is complex and diverse, characterized by bursts of star formation. Overall, first galaxies spend 70 per cent of their time in star formation bursts. A sample of 1000 of these bursts indicates that the typical burst at |$z$| ≃ 6 has a specific SFR (sSFR) maximum of |$5\hbox{--}15 \, {\rm Gyr}^{-1}$| with an effective width of ∼100 Myr, one-tenth of the age of the Universe at that redshift. A quarter of the bursts populate a tail with very high sSFR maxima of |$20\hbox{--}30 \, {\rm Gyr}^{-1}$| and significantly shorter time-scales of ∼40–80 Myr. This diversity of bursts sets the mean and the mass-dependent scatter of the star-forming main sequence. This scatter is driven by a population of low-mass, |$M_*\le 10^8\, {\rm M}_{\odot}$|⁠, quiescent galaxies. The mean sSFR and the burst maximum at fixed mass increase with redshift, with the later always being a factor of ∼2 higher than the former. This implies sSFR maxima of |${\sim } 20\hbox{--}60 \, {\rm Gyr}^{-1}$| at |$z$| = 9–10. The SFR histories are publicly available at the FirstLight website. [ABSTRACT FROM AUTHOR]

We introduce the FirstLight project,which aims to generate a large data base of high-resolution, zoom-in simulations of galaxy formation around the epoch of reionization (z≥6). The first results of this programme agree well with recent observational constraints at z=6-8, including the ultraviolet (UV) luminosity function and galaxy stellarmass function, as well as the scaling relationships between halo mass, stellar mass and UV magnitude. The UV luminosity function starts to flatten below MUV>-14 due to stellar feedback in haloes with maximum circular velocities of V=30-40 km s-1. The power-law slope of the luminosity function evolves rapidly with redshift, reaching a value of α≃2.5 at z=10. On the other hand, the galaxy stellar mass function evolves slowly with time between z=8 and 10, in particular, at the low-mass end. [ABSTRACT FROM AUTHOR]

We present results of a high-resolution zoom cosmological simulation of the evolution of a low-mass galaxy with a maximum velocity of V ≃ 100 km s-1 at z ≃ 0, using the initial conditions from the AGORA project (Kim et al.). The final disc-dominated galaxy is consistent with local disc scaling relations, such as the stellar-halo mass relation and the baryonic Tully-Fisher. The galaxy evolves from a compact, dispersion-dominated galaxy into a rotation-dominated but dynamically hot disc in about 0.5 Gyr (from z = 1.4 to 1.2). The disc dynamically cools down for the following 7 Gyr, as the gas velocity dispersion decreases over time, in agreement with observations. The primary cause of this slow evolution of velocity dispersion in this low-mass galaxy is stellar feedback. It is related to the decline in gas fraction, and to the associated gravitational disc instability, as the disc slowly settles from a global Toomre Q > 1 turbulent disc to a marginally unstable disc (Q ≃ 1). [ABSTRACT FROM AUTHOR]

We study the evolution and properties of giant clumps in high-z disc galaxies using adaptive mesh refinement cosmological simulations at redshifts z ~ 6-1. Our sample consists of 34 galaxies, of halo masses 1011-1012 M at z = 2, run with and without radiation pressure (RP) feedback from young stars. While RP has little effect on the sizes and global stability of discs, it reduces the amount of star-forming gas by a factor of ~2, leading to a similar decrease in stellar mass by z ~ 2. Both samples undergo extended periods of violent disc instability continuously 7 9 forming giant clumps of masses 107-109 M&#8857 at a similar rate, though RP significantly reduces the number of long-lived clumps (LLCs). When RP is (not) included, clumps with circular velocity ≲ 40 (20) km s-1, baryonic surface density ≲ 200 (100) M&#8857 pc-2 and baryonic mass ≲ 108.2 (107.3) M&#8857 are short-lived, disrupted in a few free-fall times. More massive and dense clumps survive and migrate towards the disc centre over a few disc orbital times. In the RP simulations, the distribution of clump masses and star formation rates (SFRs) normalized to their host disc is similar at all redshifts, exhibiting a truncated power law with a slope slightly shallower than -2. The specific SFR (sSFR) of the LLCs declines with age as they migrate towards the disc centre, producing gradients in mass, stellar age, gas fraction, sSFR and metallicity that distinguish them from the short-lived clumps which tend to populate the outer disc. Ex situ mergers comprise ~37 per cent of the mass in clumps and ~29 per cent of the SFR. They are more massive and with older stellar ages than the in situ clumps, especially near the disc edge. Roughly half the galaxies at redshifts z = 4-1 are clumpy, with ~3-30 per cent of their SFR and ~0.1-3 per cent of their stellar mass in clumps. [ABSTRACT FROM AUTHOR]

The velocity structure imprinted in the Hα emission line profiles contains valuable information about galactic outflows. Using a set of high-resolution zoom-in cosmological simulations of galaxies at z ≃ 2, we generate Hα emission line profiles, taking into account the temperaturedependent Hα emissivity, as well as dust extinction. The Hα line can be described as a sum of two Gaussians, as typically done with observations. In general, its properties are in good agreement with those observed in local isolated galaxies with similar masses and star formation rates, assuming a spatially constant clumping factor of c ≃ 24. Blueshifted outflows are very common in the sample. They extend several kpc above the galaxy discs. They are also spread over the full extent of the discs. However, at small radii, the material with high velocities tends to remain confined within a thick disc, as part of galactic fountains or a turbulent medium, most probably due to the deeper gravitational potential at the galaxy centre. [ABSTRACT FROM AUTHOR]

We study the evolution of global shapes of galaxies using cosmological simulations. The shapes refer to the components of dark matter (DM), stars and gas at the stellar halfmass radius. Most galaxies undergo a characteristic compaction event into a blue nugget at z ~ 2-4, which marks the transition from a DM-dominated central body to a self-gravitating baryonic core. We find that in the high-z, DM-dominated phase, the stellar and DM systems tend to be triaxial, preferentially prolate and mutually aligned. The elongation is supported by an anisotropic velocity dispersion that originates from the assembly of the galaxy along a dominant large-scale filament. We estimate that torques by the dominant halo are capable of inducing the elongation of the stellar system and its alignment with the halo. Then, in association with the transition to self-gravity, small-pericentre orbits puff up and the DM and stellar systems evolve into a more spherical and oblate configuration, aligned with the gas disc and associated with rotation. This transition typically occurs when the stellar mass is ~109 M☉ and the escape velocity in the core is #126;100 km s-1, indicating that supernova feedback may be effective in keeping the core DM dominated and the system prolate. The early elongated phase itself may be responsible for the compaction event, and the transition to the oblate phase may be associated with the subsequent quenching in the core. [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]

We present star formation histories (SFHs) for a sample of 104 massive (stellar mass M > 1010 M⊙) quiescent galaxies (MQGs) at z = 1.0-1.5 from the analysis of spectrophotometric data from the Survey for High-z Absorption Red and Dead Sources (SHARDS) and HST/WFC3 G102 and G141 surveys of the GOODS-North field, jointly with broad-band observations from ultraviolet (UV) to far-infrared (far-IR). The sample is constructed on the basis of rest-frame UVJ colours and specific star formation rates (sSFRs = SFR/Mass). The spectral energy distributions (SEDs) of each galaxy are compared to models assuming a delayed exponentially declining SFH. A Monte Carlo algorithm characterizes the degeneracies, which we are able to break taking advantage of the SHARDS data resolution, by measuring indices such as MgUV and D4000. The population of MQGs shows a duality in their properties. The sample is dominated (85 per cent) by galaxies with young mass-weighted ages, tM < 2 Gyr, short star formation time-scales, <τ> ~ 60-200 Myr, and masses log(M/M⊙) ~ 10.5. There is an older population (15 per cent) with tM = 2-4 Gyr, longer star formation time-scales, <τ> ~ 400 Myr, and larger masses, log(M/M⊙) ~ 10.7. The SFHs of our MQGs are consistent with the slope and the location of the main sequence of star-forming galaxies at z > 1.0, when our galaxies were 0.5-1.0 Gyr old. According to these SFHs, all the MQGs experienced a luminous infrared galaxy phase that lasts for ~500 Myr, and half of them an ultraluminous infrared galaxy phase for ~100 Myr. We find that the MQG population is almost assembled at z ~ 1, and continues evolving passively with few additions to the population. [ABSTRACT FROM AUTHOR]

Gas inflow feeds galaxies with low-metallicity gas from the cosmic web, sustaining star formation across the Hubble time.We make a connection between these inflows and metallicity inhomogeneities in star-forming galaxies, by using synthetic narrow-band images of the Hα emission line from zoom-in AMR cosmological simulations of galaxies with stellar masses of M* ≃ 109 M☉ at redshifts z = 2-7. In ∼50 per cent of the cases at redshifts lower than 4, the gas inflow gives rise to star-forming, Hα-bright, off-centre clumps. Most of these clumps have gas metallicities, weighted by Hα luminosity, lower than the metallicity in the surrounding interstellar medium by ∼0.3 dex, consistent with observations of chemical inhomogeneities at high and low redshifts. Due to metal mixing by shear and turbulence, these metallicity drops are dissolved in a few disc dynamical times. Therefore, they can be considered as evidence for rapid gas accretion coming from cosmological inflow of pristine gas. [ABSTRACT FROM AUTHOR]

We report the identification of elongated (triaxial or prolate) galaxies in cosmological simulations at z ≃ 2. These are preferentially low-mass galaxies (M* ≤ 109.5 M⊙), residing in dark matter (DM) haloes with strongly elongated inner parts, a common feature of high-redshift DM haloes in the A cold dark matter cosmology. Feedback slows formation of stars at the centres of these haloes, so that a dominant and prolate DM distribution gives rise to galaxies elongated along the DM major axis. As galaxies grow in stellar mass, stars dominate the total mass within the galaxy half-mass radius, making stars and DM rounder and more oblate. A large population of elongated galaxies produces a very asymmetric distribution of projected axis ratios, as observed in high-z galaxy surveys. This indicates that the majority of the galaxies at high redshifts are not discs or spheroids but rather galaxies with elongated morphologies. [ABSTRACT FROM AUTHOR]

From mock Hubble Space Telescope images, we quantify non-parametric statistics of galaxy morphology, thereby predicting the emergence of relationships among stellar mass, star formation, and observed rest-frame optical structure at 1 < z < 3. We measure automated diagnostics of galaxy morphology in cosmological simulations of the formation of 22 central galaxies with 9.3 < log10M*/M⊙ < 10.7. These high-spatial-resolution zoom-in calculations enable accurate modelling of the rest-frame UV and optical morphology. Even with small numbers of galaxies, we find that structural evolution is neither universal nor monotonic: galaxy interactions can trigger either bulge or disc formation, and optically bulge-dominated galaxies at this mass may not remain so forever. Simulated galaxies with M* > 1010M⊙ contain relatively more disc-dominated light profiles than those with lower mass, reflecting significant disc brightening in some haloes at 1 < z < 2. By this epoch, simulated galaxies with specific star formation rates below 10-9.7 yr-1 are more likely than normal star-formers to have a broader mix of structural types, especially at M* > 1010M⊙. We analyse a cosmological major merger at z ~ 1.5 and find that the newly proposed Multimode-Intensity-Deviation (MID) morphology diagnostics trace later merger stages while Gini-M20 trace earlier ones. MID is sensitive also to clumpy star-forming discs. The observability time of typical MID-enhanced events in our simulation sample is <100 Myr. A larger sample of cosmological assembly histories may be required to calibrate such diagnostics in the face of their sensitivity to viewing angle, segmentation algorithm, and various phenomena such as clumpy star formation and minor mergers. [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 properties of giant clumps and their radial gradients in high-z disc galaxies using AMR cosmological simulations. Our sample consists of 770 snapshots in the redshift range z = 4-1 from 29 galaxies that at z = 2 span the stellar mass range (0.2–3) × 1011 M⊙. Extended gas discs exist in 83 per cent of the snapshots. Clumps are identified by gas density in 3D and their stellar and dark matter components are considered thereafter. While most of the overdensities are diffuse and elongated, 91 per cent of their mass and 83 per cent of their star formation rate (SFR) are in compact round clumps. Nearly all galaxies have a central, massive bulge clump, while 70 per cent of the discs show off-centre clumps, 3–4 per galaxy. The fraction of clumpy discs peaks at intermediate disc masses. Clumps are divided based on dark matter content into in situ and ex situ originating from violent disc instability (VDI) and minor mergers, respectively. 60 per cent of the discs are in a VDI phase showing off-centre in situ clumps, which contribute 1–7 per cent of the disc mass and 5–45 per cent of its SFR. The in situ clumps constitute 75 per cent of the off-centre clumps in terms of number and SFR but only half the mass, each clump containing on average 1 per cent of the disc mass and 6 per cent of its SFR. They have young stellar ages, 100–400 Myr, and high specific SFR (sSFR), 1–10 Gyr−1. They exhibit gradients resulting from inward clump migration, where the inner clumps are somewhat more massive and older, with lower gas fraction and sSFR and higher metallicity. Similar observed gradients indicate that clumps survive outflows. The ex situ clumps have stellar ages 0.5–3 Gyr and sSFR ∼0.1–2 Gyr−1, and they exhibit weaker gradients. Massive clumps of old stars at large radii are likely ex situ mergers, though half of them share the disc rotation. [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]