Compaction and quenching of high-z galaxies in cosmological simulations: blue and red nuggets

We present a comparative study of the galactic and small scale environments of gamma-ray bursts (GRB) and core collapse supernovae (CCSN). We use a sample of 34 GRB hosts at z<1.2, and a comparison sample of 58 supernova hosts located within the Great Observatories Origins Deep Survey footprint. We fit template spectra to the available photometric data, which span the range 0.45-24 micron, and extract absolute magnitudes, stellar masses and star formation rates from the resulting fits. Our results broadly corroborate previous findings, but offer significant enhancements in spectral coverage and a factor 2-3 increase in sample size. Specifically, we find that CCSN occur frequently in massive spirals (spiral fraction ~50%). In contrast GRBs occur in small, relatively low mass galaxies with high specific and surface star formation rates, and have a spiral fraction of only ~10%. A comparison of the rest frame absolute magnitudes of the GRB and CCSN sample is less conclusive than found in previous work, suggesting that while GRB hosts are typically both smaller and bluer than those of CCSN their total blue light luminosities are only slightly lower. We suggest this is likely due to rapid periods of intensified star formation activity, as indicated by the high specific star formation rates, which both create the GRB progenitors and briefly significantly enhance the host galaxy blue luminosity. Finally, our analysis of local environments of GRBs and CCSN shows that GRBs are highly concentrated on their host light, and further occur in regions of higher absolute surface luminosity than CCSN.

The primary goal of this study is to explore the dependence of the clustering properties of galaxies on star formation rate (SFR) and specific star formation rate (SSFR). From the main galaxy sample of the Sloan Digital Sky Survey Data Release 7, we construct two volume-limited samples with absolute magnitudes above and below [Formula: see text], and then divide each volume-limited main galaxy sample into two subsamples with low SFRs and high SFRs or low SSFRs and high SSFRs. A strong dependence of the clustering properties on SFR and SSFR is found: high SFR and SSFR galaxies are preferentially isolated or found in close pairs and small groups, whereas low SFR and SSFR galaxies preferentially inhabit dense groups and clusters.

We present a z = 0 census of nuggets—compact galaxies that form via gas-rich violent disk instability—within the luminosity- and volume-limited RESOLVED Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. We use random forest (RF) models to predict near-ultraviolet (NUV) magnitudes for ECO galaxies that lack high-quality NUV magnitudes, thereby doubling the number of ECO galaxies with reliable extinction-corrected star formation rates (SFRs) and red/green/blue classifications based on specific SFRs (sSFRs). The resulting RF-enhanced RESOLVE+ECO nugget sample allows us to analyze rare subpopulations—green nuggets and nuggets with active galactic nuclei (AGN)—likely associated with quenching. Green nuggets are more similar to red nuggets than to blue nuggets in halo mass (M halo) distribution, with both red and green nuggets being found mainly at M halo ≥ 1011.4 M ⊙, where permanent halo quenching is predicted. At these masses, the AGN frequency for green nuggets is higher (48.2% − 5.3 % 5.3 % ) than for either blue (39.2% − 2.8 % 2.9 % ) or red (29.3% − 2.8 % 3.0 % ) nuggets. Between M halo = 1011.4–1012 M ⊙, at the onset of permanent quenching, the AGN frequency for green nuggets is nearly double the frequency for blue or red nuggets, implying AGN are associated with this transition. At M halo &lt; 1011.4 M ⊙, where temporary cyclic quenching is expected, the AGN frequency for blue nuggets (7.5% − 1.2 % 1.4 % ) is lower than for either green (31.3% − 7.5 % 8.7 % ) or red (18.8% − 7.8 % 11.5 % ) nuggets. At all masses, nuggets with AGN have reduced sSFRs and likely also atomic gas content compared to nuggets without AGN, but the quenching is more extreme below M halo = 1011.4 M ⊙.

We compare the galaxy population of a smoothed particle hydrodynamics (SPH) simulation to those predicted by the GalICS (Galaxies In Cosmological Simulations) N-body + semi-analytic model and a stripped down version of GalICS that omits the effects of supernova and active galactic nucleus (AGN) feedback. The SPH simulation and the no-feedback GalICS model make similar predictions for the baryonic mass functions of galaxies and for the dependence of these mass functions on environment and redshift. The two methods also make similar predictions for the galaxy content of dark matter haloes as a function of halo mass and for the gas accretion history of galaxies. There is a fairly good correspondence between the 'cold' and 'hot' accretion modes of the SPH simulation and the rapid and slow cooling regimes of the GalICS calculation. Both the SPH and no-feedback GalICS models predict a bimodal galaxy population at z = 0. The 'red' sequence of gas poor, old galaxies is populated mainly by satellite systems, which are starved of fresh gas after they begin orbiting in larger haloes, while, contrary to observations, the central galaxies of massive haloes lie on the 'blue' star-forming sequence as a result of continuing hot gas accretion at late times. Furthermore, both models overpredict the observed baryonic mass function, especially at the high-mass end. In the full GalICS model, supernova-driven outflows reduce the masses of low and intermediate mass galaxies by about a factor of 2. AGN feedback suppresses gas cooling in large haloes, producing a sharp cut-off in the baryonic mass function and moving the central galaxies of these massive haloes to the red sequence. Our results imply that the observational failings of the SPH simulation and the no-feedback GalICS model are a consequence of missing input physics rather than computational inaccuracies. Truncating the accretion of gas in satellite galaxies automatically produces a bimodal distribution with a quenched population, but explaining the star formation shutdown in the most massive galaxies requires a mechanism like AGN feedback that suppresses the accretion of gas on to central galaxies in large haloes.

Sources of ionization of diffuse gas at different altitudes in 436 edg^on disk galaxies are explored. The MaNGA data together with theoretical 3MdB models are used. It is demonstrated that models of the gas photoionization in a combination of young OB-stars and hot low-mass evolved stars adequately describe the gas ionization state in the galaxies of all types. Moreover, the fraction of the ionizing flux from OB-stars and the ionization parameter da crease with the altitude, while the role of the ionization by the hot low-mass evolved stars increases. The d^erence in the contribution from these types of ionizing sources correlates with the specific star formation rate and with stellar masses of galaxies. The hot low-mass evolved stars are the principal gas ionization sources in massive galaxies with low specific star formation rate, while OB-stars dominate the gas ionization in low-mass galaxies with high specific star formation rate.

We study the specific star formation rate (SSFR) and its evolution at z ≳ 4, in models of galaxy formation, where the star formation is driven by cold accretion flows. We show that constant star formation and feedback efficiencies cannot reproduce the observed trend of SSFR with stellar mass and its observed lack of evolution at z &amp;gt; 4. Model galaxies with log (M*) ≲ 9.5 M⊙ show systematically lower SSFRs by orders of magnitudes, while massive galaxies with M* ≳ 5 × 1010 M⊙ have up to an order of magnitude larger SSFRs, compared to recent observations by Stark et al. To recover these observations we apply an empirical star formation efficiency in galaxies that scales with the host halo velocity dispersion as ∝ 1/σ3 during galaxy mergers. We find that this modification needs to be of stochastic nature to reproduce the observations, i.e. only applied during mergers and not during accretion driven star formation phases. Our choice of star formation efficiency during mergers allows us to capture both, the boost in star formation at low masses and the quenching at high masses, and at the same time produce a constant SSFR–stellar mass relation at z ≳ 4 under the assumption that most of the observed galaxies are in a merger-triggered star formation phase. Our results suggest that observed high-z low-mass galaxies with high SSFRs are likely to be frequently interacting systems, which experienced bursts in their star formation rate and efficiency (mode 1), in contrast to low redshift z ≲ 3 galaxies which are cold accretion-regulated star forming systems with lower star formation efficiencies (mode 2).

We introduce a collection of primarily centrally star-forming galaxies that are selected by disk color to have truncated disk star formation. We show that common explanations for centrally concentrated star formation—low stellar mass, bars, and high-density environments—do not universally apply to this sample. To gain insight into our sample, we compare these galaxies to a parent sample of strongly star-forming galaxies and to a parent sample of galaxies with low specific star formation rates. We find that in star formation and color space from ultraviolet to the infrared these galaxies either fall between the two samples or agree more closely with galaxies with high specific star formation rates. Their morphological characteristics also lie between high and low specific star formation rate galaxies, although their Petrosian radii agree well with that of the low specific star formation rate parent sample. We discuss whether this sample is likely to be quenching or showing an unusual star formation distribution while continuing to grow through star formation. Future detailed studies of these galaxies will give us insights into how the local conditions within a galaxy balance environmental influence to govern the distribution of star formation. In this first paper in a series, we describe the global properties that identify this sample as separate from more average spiral galaxies, and we identify paths forward to explore the underlying causes of their differences.

We study the interplay between gas phase metallicity (Z), specific star formation rate (SSFR) and neutral hydrogen gas (H i) for galaxies of different stellar masses. Our study uses spectroscopic data from Galaxy and Mass Assembly and Sloan Digital Sky Survey (SDSS) star-forming galaxies, as well as H i detection from the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) and Galex Arecibo SDSS Survey (GASS) public catalogues. We present a model based on the Z–SSFR relation that shows that at a given stellar mass, depending on the amount of gas, galaxies will follow opposite behaviours. Low-mass galaxies with a large amount of gas will show high SSFR and low metallicities, while low-mass galaxies with small amounts of gas will show lower SSFR and high metallicities. In contrast, massive galaxies with a large amount of gas will show moderate SSFR and high metallicities, while massive galaxies with small amounts of gas will show low SSFR and low metallicities. Using ALFALFA and GASS counterparts, we find that the amount of gas is related to those drastic differences in Z and SSFR for galaxies of a similar stellar mass.

We study the origin of high-redshift, compact, quenched spheroids (red nuggets) through the dissipative shrinkage of gaseous discs into compact star-forming systems (blue nuggets). The discs, fed by cold streams, undergo violent disc instability (VDI) that drives gas into the centre (along with mergers). The inflow is dissipative when its timescale is shorter than the star formation timescale. This implies a threshold of 0.28 in the cold-to-total mass ratio within the disc radius. For the typical gas fraction 0.5 at z=2, this threshold is traced back to a maximum spin parameter of 0.05, implying that half the star-forming galaxies contract to blue nuggets, while the rest form extended stellar discs. Thus, the surface density of blue galaxies is expected to be bimodal about 10^9 Msun/kpc^2, slightly increasing with mass. Blue nuggets are expected to be rare at low z when the gas fraction is low. The blue nuggets quench to red nuggets by complementary internal and external mechanisms. Internal quenching by a compact bulge, operating as a fast mode and especially at high z, may involve starbursts, stellar and AGN feedback, or Q-quenching. Quenching due to hot-medium haloes above 10^{12} Msun provides maintenance and a slower mode at low redshift. These predictions are confirmed in simulations and are consistent with observations at z=0-3.

We study emission line profiles of 21 nearby low-mass (M * = 104–107 M ⊙) galaxies in deep medium-high resolution spectra taken with Magellan/MagE. These low-mass galaxies are actively star-forming systems with high specific star formation rates of ∼100–1000 Gyr−1 that are well above the star formation main sequence and its extrapolation. We identify broad-line components of Hα and [O iii]λ5007 emission in 14 out of the 21 galaxies that cannot be explained by the MagE instrumental profile or the natural broadening of line emission. We conduct double-Gaussian profile fitting to the emission of the 14 galaxies, and find that the broad-line components have line widths significantly larger than those of the narrow-line components, indicative of galactic outflows. The broad-line components have moderately large line widths of ∼100 km s−1. We estimate the maximum outflow velocities v max and obtain values of ≃60–200 km s−1, which are found to be comparable to or slightly larger than the escape velocities. Positive correlations of v max with star formation rates, stellar masses, and circular velocities extend down into this low-mass regime. Broad- to narrow-line flux ratios (BNRs) are generally found to be smaller than those of massive galaxies. The small v max and BNRs suggest that the mass-loading factors η can be as small as 0.1–1 or below, in contrast to the large η of energy-driven outflows predicted by numerical simulations.

We study the evolution of giant clumps in high-z disc galaxies using AMR cosmological simulations at redshifts z=6-1. Our sample consists of 34 galaxies, of halo masses 10^{11}-10^{12}M_s 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 decrease in stellar mass by a similar factor by z~2. Both samples undergo violent disc instability (VDI) and form giant clumps of masses 10^7-10^9M_s at a similar rate, though RP significantly reduces the number of long-lived clumps. When RP is (not) included, clumps with circular velocity <40(20)km/s, baryonic surface density <200(100)M_s/pc^2 and baryonic mass <10^{8.2}(10^{7.3})M_s are short-lived, disrupted in a few free-fall times. The more massive and dense clumps survive and migrate toward 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 very similar at all redshifts. They exhibit a truncated power-law with a slope slightly shallower than -2. Short-lived clumps preferentially have young stellar ages, low masses, high gas fractions and specific SFRs (sSFR), and they tend to populate the outer disc. The sSFR of massive, long-lived clumps 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 short-lived clumps. Ex situ mergers make up ~37% of the mass in clumps and ~29% 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 over a wide range of stellar mass, with clumps accounting for ~3-30% of the SFR but ~0.1-3% of the stellar mass.

Magnetic fields are ubiquitous in the Universe. Recently, cosmological simulations of galaxies have successfully begun to incorporate magnetic fields and their evolution in galaxies and their haloes. However, so far they have mostly focused on Milky Way-like galaxies. Here, we analyse a sample of high-resolution cosmological zoom simulations of disc galaxies in haloes with mass ${M}_\rm {200c}$ from $10^{10}$ to $10^{13}\, \rm {M}_\odot$, simulated with the Auriga galaxy formation model. We show that with sufficient numerical resolution the magnetic field amplification and saturation is converged. The magnetic field strength reaches equipartition with turbulent energy density for galaxies in haloes with ${M}_\rm {200c}\gtrsim 10^{11.5}\, \mathrm{M_\odot }$. For galaxies in less massive haloes, the magnetic field strength saturates at a fraction of equipartition that decreases with decreasing halo mass. For our lowest mass haloes, the magnetic field saturates significantly below 10 per cent of equipartition. We quantify the resolution we need to obtain converged magnetic field strengths and discuss our resolution requirements also in the context of the IllustrisTNG cosmological box simulations. We show that, at z = 0, rotation-dominated galaxies in our sample exhibit for the most part an ordered large-scale magnetic field, with fewer field reversals in more massive galaxies. Finally, we compare the magnetic fields in our cosmological galaxies at z = 0 with simulations of isolated galaxies in a collapsing halo set-up. Our results pave the way for detailed studies of cosmic rays and other physical processes in similar cosmological galaxy simulations that crucially depend on the strength and structure of magnetic fields.

We utilize high-resolution cosmological simulations to reveal that high-redshift galaxies tend to undergo a robust ‘wet compaction’ event when near a ‘golden’ stellar mass of $\sim \!\!10^{10}\, \rm M_\odot$ . This is a gaseous shrinkage to a compact star-forming phase, a ‘blue nugget’ (BN), followed by central quenching of star formation to a compact passive stellar bulge, a ‘red nugget’ (RN), and a buildup of an extended gaseous disc and ring. Such nuggets are observed at cosmic noon and seed today’s early-type galaxies. The compaction is triggered by a drastic loss of angular momentum due to, e.g. wet mergers, counter-rotating cold streams, or violent disc instability. The BN phase marks drastic transitions in the galaxy structural, compositional, and kinematic properties. The transitions are from star forming to quenched inside-out, from diffuse to compact with an extended disc or ring and a stellar envelope, from dark matter to baryon central dominance, from prolate to oblate stellar shape, from pressure to rotation support, from low to high metallicity, and from supernova to AGN feedback. The central black hole growth, first suppressed by supernova feedback when below the golden mass, is boosted by the compaction, and the black hole keeps growing once the halo is massive enough to lock in the supernova ejecta.

We present an analysis of galaxies in the CDF-South. We find a tight relation to z = 3 between color and size at a given mass, with red galaxies being small, and blue galaxies being large. We show that the relation is driven by stellar surface density or inferred velocity dispersion: galaxies with high surface density are red and have low specific star formation rates, and galaxies with low surface density are blue and have high specific star formation rates. Surface density and inferred velocity dispersion are better correlated with specific star formation rate and color than stellar mass. Hence stellar mass by itself is not a good predictor of the star formation history of galaxies. In general, galaxies at a given surface density have higher specific star formation rates at higher redshift. Specifically, galaxies with a surface density of (1-3) × 109 M⊙ kpc-2 are "red and dead" at low redshift, approximately 50% are forming stars at z = 1, and almost all are forming stars by z = 2. This provides direct additional evidence for the late evolution of galaxies onto the red sequence. The sizes of galaxies at a given mass evolve like 1/(1 + Z)0.59±0.10, Hence galaxies undergo significant upsizing in their history. The size evolution is fastest for the highest mass galaxies and quiescent galaxies. The persistence of the structural relations from z = 0 to z = 2.5, and the upsizing of galaxies imply that a relation analogous to the Hubble sequence exists out to z = 2.5, and possibly beyond. The star-forming galaxies at z ≥ 1.5 are quite different from star-forming galaxies at z = 0, as they have likely very high gas fractions, and star formation timescales comparable to the orbital time.

Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended timescales. We expand the census of star formation in compact group galaxies by \citet{tzanavaris10} and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 \micron\ photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFR$_{\mathrm{UV}}$) using the UVOT uvw2photometry. Similarly, we use the MIPS 24 \micron\ photometry to estimate the component of the SFR that is obscured by dust (SFR$_{\mathrm{IR}}$). We find that galaxies which are MIR-active (MIR-"red"), also have bluer UV colours, higher specific star formation rates, and tend to lie in H~{\sc i}-rich groups, while galaxies that are MIR-inactive (MIR-"blue") have redder UV colours, lower specific star formation rates, and tend to lie in H~{\sc i}-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M$_{\odot}$ yr$^{-1}$, indicating this might be the most common value in compact groups. In contrast, the specific star formation rate distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific star formation rate is the best tracer of gas depletion and galaxy evolution in compact groups.