ABSTRACT
We use the simba cosmological galaxy formation simulation to investigate the relationship between major mergers ($\lesssim$4:1), starbursts, and galaxy quenching. Mergers are identified via ...sudden jumps in stellar mass M* well above that expected from in situ star formation, while quenching is defined as going from specific star formation rate (sSFR) $\gt t_{\rm H}^{-1}$ to $\lt 0.2t_{\rm H}^{-1}$, where tH is the Hubble time. At z ≈ 0–3, mergers show ∼2–3× higher SFR than a mass-matched sample of star-forming galaxies, but globally represent $\lesssim 1{{\ \rm per\ cent}}$ of the cosmic SF budget. At low masses, the increase in SFR in mergers is mostly attributed to an increase in the H2 content, but for $M_*\gtrsim 10^{10.5} \,\mathrm{ M}_{\odot }$ mergers also show an elevated star formation efficiency suggesting denser gas within merging galaxies. The merger rate for star-forming galaxies shows a rapid increase with redshift, ∝(1 + z)3.5, but the quenching rate evolves much more slowly, ∝(1 + z)0.9; there are insufficient mergers to explain the quenching rate at $z\lesssim 1.5$. simba first quenches galaxies at $z\gtrsim 3$, with a number density in good agreement with observations. The quenching time-scales τq are strongly bimodal, with ‘slow’ quenchings (τq ∼ 0.1tH) dominating overall, but ‘fast’ quenchings (τq ∼ 0.01tH) dominating in M* ∼ 1010–1010.5 M$\odot$ galaxies, likely induced by simba’s jet-mode black hole feedback. The delay time distribution between mergers and quenching events suggests no physical connection to either fast or slow quenching. Hence, simba predicts that major mergers induce starbursts, but are unrelated to quenching in either fast or slow mode.
We present the evolution in the number density and stellar mass functions of photometrically selected post-starburst galaxies in the UKIDSS Ultra Deep Survey, with redshifts of 0.5 < z < 2 and ...stellar masses log (M/M⊙) >10. We find that this transitionary species of galaxy is rare at all redshifts, contributing ∼5 per cent of the total population at z ∼ 2, to <1 per cent by z ∼ 0.5. By comparing the mass functions of quiescent galaxies to post-starburst galaxies at three cosmic epochs, we show that rapid quenching of star formation can account for 100 per cent of quiescent galaxy formation, if the post-starburst spectral features are visible for ∼250 Myr. The flattening of the low-mass end of the quiescent galaxy stellar mass function seen at z ∼ 1 can be entirely explained by the addition of rapidly quenched galaxies. Only if a significant fraction of post-starburst galaxies have features that are visible for longer than 250 Myr, or they acquire new gas and return to the star-forming sequence, can there be significant growth of the red sequence from a slower quenching route. The shape of the mass function of these transitory post-starburst galaxies resembles that of quiescent galaxies at z ∼ 2, with a preferred stellar mass of log (M/M⊙) ∼10.6, but evolves steadily to resemble that of star-forming galaxies at z < 1. This leads us to propose a dual origin for post-starburst galaxies: (1) at z ≳ 2 they are exclusively massive galaxies that have formed the bulk of their stars during a rapid assembly period, followed by complete quenching of further star formation; (2) at z ≲ 1 they are caused by the rapid quenching of gas-rich star-forming galaxies, independent of stellar mass, possibly due to environment and/or gas-rich major mergers.
We present a systematic study of the shape of the dust attenuation curve in star-forming galaxies from the far-ultraviolet (far-UV) to the near-infrared (NIR; ∼0.15-2 μ m), as a function of specific ...star formation rate (ψS) and axial ratio (b/a), for galaxies with and without a significant bulge. Our sample comprises 23 000 (15 000) galaxies with a median redshift of 0.07, with photometric entries in the Sloan Digital Sky Survey (SDSS), UKIRT Infrared Deep Sky Survey-Large Area Survey and Galaxy Evolution Explorer-All-Sky Imaging Survey catalogues and emission-line measurements from the SDSS spectroscopic survey. We develop a new pair-matching technique to isolate the dust attenuation curves from the stellar continuum emission. The main results are: (i) the slope of the attenuation curve in the optical varies weakly with ψS, strongly with b/a, and is significantly steeper than the Milky Way extinction law in bulge-dominated galaxies; (ii) the NIR slope is constant and matches the slope of the Milky Way extinction law; (iii) the UV has a slope change consistent with a dust bump at 2175 Å which is evident in all samples and varies strongly in strength with b/a in the bulge-dominated sample; (iv) there is a strong increase in emission-line-to-continuum dust attenuation (τ
V, line/τ
V, cont) with both decreasing ψS and increasing b/a; and (v) radial gradients in dust attenuation increase strongly with increasing ψS, and the presence of a bulge does not alter the strength of the gradients. These results are consistent with the picture in which young stars are surrounded by dense 'birth clouds' with low covering factor which disperse on time-scales of ∼107 yr and the diffuse interstellar dust is distributed in a centrally concentrated disc with a smaller scaleheight than the older stars that contribute the majority of the red and NIR light. Within this model, the path-length of diffuse dust, but not of birth-cloud dust, increases with increasing inclination and the apparent optical attenuation curve is steepened by the differential effect of larger dust opacity towards younger stars than towards older stars. Additionally, our findings suggest that: (i) galaxies with higher star formation rates per unit stellar mass have a higher fraction of diffuse dust, which is more centrally concentrated; (ii) the observed strength of the 2175-Å dust feature is affected predominantly by global geometry; and (iii) only highly inclined discs are optically thick. We provide new empirically derived attenuation curves for correcting the light from star-forming galaxies for dust attenuation.
Abstract
We investigate the relationship between the quenching of star formation and the structural transformation of massive galaxies, using a large sample of photometrically selected post-starburst ...galaxies in the UKIDSS Ultra-Deep Survey field. We find that post-starburst galaxies at high redshift (z > 1) show high Sérsic indices, significantly higher than those of active star-forming galaxies, but with a distribution that is indistinguishable from the old quiescent population. We conclude that the morphological transformation occurs before (or during) the quenching of star formation. Recently quenched galaxies are also the most compact; we find evidence that massive post-starburst galaxies (M* > 1010.5 M⊙) at high redshift (z > 1) are on average smaller than comparable quiescent galaxies at the same epoch. Our findings are consistent with a scenario in which massive passive galaxies are formed from three distinct phases: (1) gas-rich dissipative collapse to very high densities, forming the proto-spheroid, (2) rapid quenching of star formation to create the ‘red nugget’ with post-starburst features and (3) a gradual growth in size as the population ages, perhaps as a result of minor mergers.
We present a study exploring the impact of a starburst on the properties of the surrounding circumgalactic medium (CGM): gas located beyond the galaxy's stellar body and extending out to the virial ...radius (~200 kpc). We obtained ultraviolet spectroscopic data from the Cosmic Origins Spectrograph (COS) probing the CGM of 20 low-redshift foreground galaxies using background QSOs. Our sample consists of starburst and control galaxies. The latter comprises normal star-forming and passive galaxies with similar stellar masses and impact parameters as the starbursts. We used optical spectra from the Sloan Digital Sky Survey to estimate the properties of the starbursts, inferring average ages of ~200 Myr and burst fractions involving ~10% of their stellar mass. The COS data reveal highly ionized gas traced by C IV in 80%(4/5) of the starburst and in 17%(2/12) of the control sample. The two control galaxies with C IV absorbers differed from the four starbursts in showing multiple low-ionization transitions and strong saturated Ly alpha lines. They therefore appear to be physically different systems. We show that the C IV absorbers in the starburst CGM represent a significant baryon repository. The high detection rate of this highly ionized material in the starbursts suggests that starburst-driven winds can affect the CGM out to radii as large as 200 kpc. This is plausible given the inferred properties of the starbursts and the known properties of starburst-driven winds. This would represent the first direct observational evidence of local starbursts impacting the bulk of their gaseous halos, and as such provides new evidence of the importance of this kind of feedback in the evolution of galaxies.
We present a new method to classify the broad-band optical-near-infrared spectral energy distributions (SEDs) of galaxies using three shape parameters (super-colours) based on a principal component ...analysis of model SEDs. As well as providing a compact representation of the wide variety of SED shapes, the method allows for easy visualization of information loss and biases caused by the incomplete sampling of the rest-frame SED as a function of redshift. We apply the method to galaxies in the United Kingdom Infrared Telescope Infrared Deep Sky Survey Ultra Deep Survey with 0.9 < z < 1.2, and confirm our classifications by stacking rest-frame optical spectra for a fraction of objects in each class. As well as cleanly separating a tight red sequence from star-forming galaxies, three unusual populations are identifiable by their unique colours: very dusty star-forming galaxies with high metallicity and old mean stellar age; post-starburst galaxies which have formed 10 per cent of their mass in a recent unsustained starburst event; and metal-poor quiescent dwarf galaxies. We find that quiescent galaxies account for 45 per cent of galaxies with log M*/M > 11, declining steadily to 13 per cent at log M*/M = 10. The properties and mass function of the post-starburst galaxies are consistent with a scenario in which gas-rich mergers contribute to the growth of the low- and intermediate-mass range of the red sequence.
Abstract
Post-starburst galaxies are typically considered to be a transition population, en route to the red sequence after a recent quenching event. Despite this, recent observations have shown that ...these objects typically have large reservoirs of cold molecular gas. In this paper we study the star-forming gas properties of a large sample of post-starburst galaxies selected from the cosmological, hydrodynamical EAGLE simulations. These objects resemble observed high-mass post-starburst galaxies both spectroscopically and in terms of their space density, stellar mass distribution, and sizes. We find that the vast majority of simulated post-starburst galaxies have significant gas reservoirs, with star-forming gas masses ≈109 M⊙, in good agreement with those seen in observational samples. The simulation reproduces the observed time evolution of the gas fraction of the post-starburst galaxy population, with the average galaxy losing ≈90 per cent of its star-forming interstellar medium in only ≈600 Myr. A variety of gas consumption/loss processes are responsible for this rapid evolution, including mergers and environmental effects, while active galactic nuclei play only a secondary role. The fast evolution in the gas fraction of post-starburst galaxies is accompanied by a clear decrease in the efficiency of star formation due to a decrease in the dense gas fraction. We predict that forthcoming ALMA observations of the gas reservoirs of low-redshift post-starburst galaxies will show that the molecular gas is typically compact and has disturbed kinematics, reflecting the disruptive nature of many of the evolutionary pathways that build up the post-starburst galaxy population.
We investigate the recent and current star formation activity of galaxies as function of distance from the cluster centre in a sample of 521 Sloan Digital Sky Survey clusters at z < 0.1. We ...characterize the recent star formation history (SFH) by the strength of the 4000 Å break and the strength of the Balmer absorption lines, and thus probe the SFH over the last ∼2 Gyr. We show that when the brightest cluster galaxies are excluded from the galaxy sample, there is no evidence for mass segregation in the clusters, so that differences in cluster and field populations cannot simply be attributed to different mass functions. We find a marked star formation–radius relation in that almost all galaxies in the cluster core are quiescent, i.e. have terminated star formation a few Gyr ago. This star formation–radius relation is most pronounced for low-mass galaxies and is very weak or absent beyond the virial radius. The typical star formation rate (SFR) of non-quiescent galaxies declines by approximately a factor of 2 towards the cluster centre. However, the fraction of galaxies with young stellar populations indicating a recently completed starburst or a truncation of star formation does not vary significantly with radius. These results favour a scenario in which star formation is quenched slowly, on time-scales similar to the cluster crossing time, i.e. a few Gyr. The fraction of star-forming galaxies which host a powerful optical active galactic nucleus (AGN) is also independent of clustercentric radius, indicating that the link between star formation and AGN in these galaxies operates independent of environment. The fraction of red galaxies which host a weak optical AGN decreases, however, towards the cluster centre, with a similar time-scale as the decline of star-forming galaxies. Our results can be fully explained by a gradual decline of SFR upon infall into the cluster, and rule out significant contributions from more violent processes, at least beyond cluster radii ≳0.1R200.
Submillimetre galaxies (SMGs) are among the most luminous dusty galaxies in the Universe, but their true nature remains unclear; are SMGs the progenitors of the massive elliptical galaxies we see in ...the local Universe, or are they just a short-lived phase among more typical star-forming galaxies? To explore this problem further, we investigate the clustering of SMGs identified in the SCUBA-2 Cosmology Legacy Survey. We use a catalogue of submillimetre (850 mu m) source identifications derived using a combination of radio counterparts and colour/infrared selection to analyse a sample of 610 SMG counterparts in the United Kingdom Infrared Telescope (UKIRT) Infrared Deep Survey (UKIDSS) Ultra Deep Survey (UDS), making this the largest high-redshift sample of these galaxies to date. Using angular cross-correlation techniques, we estimate the halo masses for this large sample of SMGs and compare them with passive and star-forming galaxies selected in the same field. We find that SMGs, on average, occupy high-mass dark matter haloes ... at redshifts z > 2.5, consistent with being the progenitors of massive quiescent galaxies in present-day galaxy clusters. We also find evidence of downsizing, in which SMG activity shifts to lower mass haloes at lower redshifts. In terms of their clustering and halo masses, SMGs appear to be consistent with other star-forming galaxies at a given redshift. (ProQuest: ... denotes formulae/symbols omitted.)
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