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ABSTRACT Using analytic modelling and simulations, we address the origin of an abundance of star-forming clumpy extended gas rings about massive central bodies in massive galaxies at z < 4. Rings form by high-angular-momentum streams and survive in galaxies of Mstar > 109.5–10 M⊙ where merger-driven spin flips and supernova feedback are ineffective. The rings survive after events of compaction to central nuggets. Ring longevity was unexpected based on inward mass transport driven by torques from violent disc instability. However, evaluating the torques from a tightly wound spiral structure, we find that the time-scale for transport per orbital time is long and $\propto \! \delta _{\rm d}^{-3}$, with δd the cold-to-total mass ratio interior to the ring. A long-lived ring forms when the ring transport is slower than its replenishment by accretion and the interior depletion by star formation rate, both valid for δd < 0.3. The central mass that lowers δd is a compaction-driven bulge and/or dark matter, aided by the lower gas fraction at z < 4, provided that it is not too low. The ring is Toomre unstable for clump and star formation. The high-z dynamic rings are not likely to arise form secular resonances or collisions. Active galactic nucleus feedback is not expected to affect the rings. Mock images of simulated rings through dust indicate qualitative consistency with observed rings about bulges in massive z ∼ 0.5–3 galaxies, in H α and deep HST imaging. ALMA mock images indicate that z ∼ 0.5–1 rings should be detectable. We quote expected observable properties of rings and their central nuggets.

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]

Different star formation models at Cosmic Dawn produce detectable signatures in the observables of upcoming 21-cm experiments. In this work, we consider the physical scenario of feedback-free starbursts (FFB), according to which the star formation efficiency (SFE) is enhanced in sufficiently massive haloes at early enough times, thus explaining the indication from the JWST for an excess of bright galaxies at |$z \ge 10$|⁠. We model the contribution of FFBs to popII SFE and compute the impact these have on the 21-cm global signal and power spectrum. We show that FFBs affect the evolution of the brightness temperature and the 21-cm power spectrum, but they only have a limited effect on the neutral hydrogen fraction. We investigate how the observables are affected by changes in the underlying star formation model and by contribution from popIII stars. Finally, we forecast the capability of next-generation Hydrogen Epoch of Reionization Array (HERA) to detect the existence of FFB galaxies via power spectrum measurements. Our results show the possibility of a significant detection, provided that popII stars are the main drivers of lowering the spin temperature. Efficient popIII star formation will make the detection more challenging. [ABSTRACT FROM AUTHOR]

The recent decade has seen an exponential growth in spatially resolved metallicity measurements in the interstellar medium (ISM) of galaxies. To first order, these measurements are characterized by the slope of the radial metallicity profile, known as the metallicity gradient. In this work, we model the relative role of star formation feedback, gas transport, cosmic gas accretion, and galactic winds in driving radial metallicity profiles and setting the mass–metallicity gradient relation (MZGR). We include a comprehensive treatment of these processes by including them as sources that supply mass, metals, and energy to marginally unstable galactic discs in pressure and energy balance. We show that both feedback and accretion that can drive turbulence and enhance metal-mixing via diffusion are crucial to reproduce the observed MZGR in local galaxies. Metal transport also contributes to setting metallicity profiles, but it is sensitive to the strength of radial gas flows in galaxies. While the mass loading of galactic winds is important to reproduce the mass–metallicity relation (MZR), we find that metal mass loading is more important to reproducing the MZGR. Specifically, our model predicts preferential metal enrichment of galactic winds in low-mass galaxies. This conclusion is robust against our adopted scaling of the wind mass-loading factor, uncertainties in measured wind metallicities, and systematics due to metallicity calibrations. Overall, we find that at z ∼ 0, galactic winds and metal transport are more important in setting metallicity gradients in low-mass galaxies whereas star formation feedback and gas accretion dominate setting metallicity gradients in massive galaxies. [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]

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 study the evolution of L * elliptical galaxies in the colour–magnitude diagram in terms of their star formation history and environment, in an attempt to learn about their quenching process. We have visually extracted 1109 L * galaxies from a sample of 36 500 galaxies that were spectroscopically selected from Stripe82 of the Sloan Digital Sky Survey (SDSS). From this sample, we have selected 51 ellipticals based on their surface-brightness profile being well-fitted by a single S |$\acute{e}$| rsic profile with S |$\acute{e}$| rsic indices 3 < n < 6. Our sample consists of 12 blue-cloud L * ellipticals (BLEs), 11 green-valley L * ellipticals (GLEs), and 28 red-sequence L * ellipticals (RLEs). We find that most of the RLEs and GLEs have been quenched only recently, or are still forming stars, based on their [O  iii ] and H α emission, while the BLEs are forming stars vigorously. The star formation in BLEs is found to be extended over the galaxy and not confined to their central region. In about 40 per cent of the L * ellipticals (10 BLEs, 4 GLEs, and 5 RLEs), star formation quenching seems to have started only recently, based on the lower [O  iii ] emission compared to the [O  ii ] and H α emission, at a given metallicity. We also find that the galaxy colour is correlated with the cosmic-web environment, with the BLEs tending to reside in lower density regions, the RLEs preferring denser, clustered regions, and the GLEs found in either. One possible scenario is that as the star-forming ellipticals migrate into the clusters, their star formation is suffocated by the hot intracluster medium. [ABSTRACT FROM AUTHOR]

Shell galaxies make a class of tidally distorted galaxies, characterized by wide concentric arc(s), extending out to large galactocentric distances with sharp outer edges. Recent observations of young massive star clusters in the prominent outer shell of NGC 474 suggest that such systems host extreme conditions of star formation. In this paper, we present a hydrodynamic simulation of a galaxy merger and its transformation into a shell galaxy. We analyse how the star formation activity evolves with time, location-wise within the system, and what are the physical conditions for star formation. During the interaction, an excess of dense gas appears, triggering a starburst, i.e. an enhanced star formation rate and a reduced depletion time. Star formation coincides with regions of high-molecular gas fraction, such as the galactic nucleus, spiral arms, and occasionally the tidal debris during the early stages of the merger. Tidal interactions scatter stars into a stellar spheroid, while the gas cools down and reforms a disc. The morphological transformation after coalescence stabilizes the gas and thus quenches star formation, without the need for feedback from an active galactic nucleus. This evolution shows similarities with a compaction scenario for compact quenched spheroids at high-redshift, yet without a long red nugget phase. Shells appear after coalescence, during the quenched phase, implying that they do not host the conditions necessary for in situ star formation. The results suggest that shell-forming mergers might be part of the process of turning blue late-type galaxies into red and dead early-types. [ABSTRACT FROM AUTHOR]

We address the puzzling observational indications for very 'cold' galactic discs at redshifts z ≳ 3, an epoch when discs are expected to be highly perturbed. Using a high-resolution cosmological zoom-in simulation, we identify such a cold disc at z ∼ 3.5, with a rotation velocity to velocity dispersion ratio of v ϕ/σ r ≃ 5 for the total gas. It forms as a result of a period of intense accretion of co-planar, co-rotating gas via cold cosmic-web streams. This thin disc survives for ∼5 orbital periods, after which it is disrupted by mergers and counter-rotating streams, longer but consistent with our estimate that a galaxy of this mass (⁠|$M_\star \sim 10^{10}\, \mathrm{M_\odot }$|⁠) typically survives merger-driven spin flips for ∼2–3 orbital periods. We find that v ϕ/σ r is highly sensitive to the tracer used to perform the kinematic analysis. While it is v ϕ/σ r ≃ 3.5 for atomic H  i gas, it is v ϕ/σ r ≃ 8 for molecular CO and H2. This reflects the confinement of molecular gas to cold, dense clouds that reside near the disc mid-plane, while the atomic gas is spread into a turbulent and more extended thicker disc. The proposed mechanisms is a theoretical proposal that has not been validated yet with proper statistical measurements and it remains unclear whether it occurs frequently enough to explain the multiple discoveries of cold gas discs in high-z galaxies. [ABSTRACT FROM AUTHOR]

We use deep learning to explore the nature of observed giant clumps in high-redshift disc galaxies, based on their identification and classification in cosmological simulations. Simulated clumps are detected using the 3D gas and stellar densities in the VELA zoom-in cosmological simulation suite, with |${\sim}25\ \rm {pc}$| maximum resolution, targeting main-sequence galaxies at 1 < z < 3. The clumps are classified as long-lived clumps (LLCs) or short-lived clumps (SLCs) based on their longevity in the simulations. We then train neural networks to detect and classify the simulated clumps in mock, multicolour, dusty, and noisy HST -like images. The clumps are detected using an encoder–decoder convolutional neural network (CNN), and are classified according to their longevity using a vanilla CNN. Tests using the simulations show our detector and classifier to be |${\sim}80{{\ \rm per\ cent}}$| complete and |${\sim}80{{\ \rm per\ cent}}$| pure for clumps more massive than ∼107.5 M⊙. When applied to observed galaxies in the CANDELS/GOODS S+N fields, we find both types of clumps to appear in similar abundances in the simulations and the observations. LLCs are, on average, more massive than SLCs by ∼0.5 dex, and they dominate the clump population above M c ≳ 107.6 M⊙. LLCs tend to be found closer to the galactic centre, indicating clump migration to the centre or preferential formation at smaller radii. The LLCs are found to reside in high-mass galaxies, indicating better clump survivability under supernova feedback there, due to clumps being more massive in these galaxies. We find the clump masses and radial positions in the simulations and the observations to agree within a factor of 2. [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]