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Register a new userGalaxies probing galaxies: cool halo gas from a z = 0.47 post-starburst galaxy
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Kate H. R. Rubin
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We study the cool gas around a galaxy at z = 0.4729 using Keck/LRIS spectroscopy of a bright (B = 21.7) background galaxy at z = 0.6942 at a transverse distance of 16.5/h_70 kpc. The background galaxy spectrum reveals strong FeII, MgII, MgI, and CaII absorption at the redshift of the foreground galaxy, with a MgII 2796 rest equivalent width of 3.93 +/- 0.08 Angstroms, indicative of a velocity width exceeding 400 km/s. Because the background galaxy is large (> 4/h_70 kpc), the high covering fraction of the absorbing gas suggests that it arises in a spatially extended complex of cool clouds with large velocity dispersion. Spectroscopy of the massive (log M_*/M_sun = 11.15 +/- 0.08) host galaxy reveals that it experienced a burst of star formation about 1 Gyr ago and that it harbors a weak AGN. We discuss the possible origins of the cool gas in its halo, including multiphase cooling of hot halo gas, cold inflow, tidal interactions, and galactic winds. We conclude the absorbing gas was most likely ejected or tidally stripped from the interstellar medium of the host galaxy or its progenitors during the past starburst event. Adopting the latter interpretation, these results place one of only a few constraints on the radial extent of cool gas driven or stripped from a galaxy in the distant Universe. Future studies with integral field unit spectroscopy of spatially extended background galaxies will provide multiple sightlines through foreground absorbers and permit analysis of the morphology and kinematics of the gas surrounding galaxies with a diverse set of properties and environments.
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We present the evolution in the number density and stellar mass functions of photometrically selected post-starburst galaxies in the UKIDSS Deep Survey (UDS), with redshifts of 0.5<z<2 and stellar masses logM>10. We find that this transitionary species of galaxy is rare at all redshifts, contributing ~5% of the total population at z~2, to <1% 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% of quiescent galaxy formation, if the post-starburst spectral features are visible for ~250Myr. 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 250Myr, 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 logM~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.
Post-starburst galaxies can be identified via the presence of prominent Hydrogen Balmer absorption lines in their spectra. We present a comprehensive study of the origin of strong Balmer lines in a volume-limited sample of 189 galaxies with $0.01<z<0.05$, $log(mbox{M}_{star}/mbox{M}_{odot})>9.5$ and projected axis ratio $b/a>0.32$. We explore their structural properties, environments, emission lines and star formation histories, and compare them to control samples of star-forming and quiescent galaxies, and simulated galaxy mergers. Excluding contaminants, in which the strong Balmer lines are most likely caused by dust-star geometry, we find evidence for three different pathways through the post-starburst phase, with most events occurring in intermediate-density environments: (1) a significant disruptive event, such as a gas-rich major merger, causing a starburst and growth of a spheroidal component, followed by quenching of the star formation (70% of post-starburst galaxies at $9.5<log(mbox{M}_{star}/mbox{M}_{odot})<10.5$ and 60% at $log(mbox{M}_{star}/mbox{M}_{odot})>10.5$); (2) at $9.5<log(mbox{M}_{star}/mbox{M}_{odot})<10.5$, stochastic star formation in blue-sequence galaxies, causing a weak burst and subsequent return to the blue sequence (30%); (3) at $log(mbox{M}_{star}/mbox{M}_{odot})>10.5$, cyclic evolution of quiescent galaxies which gradually move towards the high-mass end of the red sequence through weak starbursts, possibly as a result of a merger with a smaller gas-rich companion (40%). Our analysis suggests that AGN are `on for $50%$ of the duration of the post-starburst phase, meaning that traditional samples of post-starburst galaxies with strict emission line cuts will be at least $50%$ incomplete due to the exclusion of narrow-line AGN.
We present the star formation histories of 39 galaxies with high quality rest-frame optical spectra at 0.5<z<1.3 selected to have strong Balmer absorption lines and/or Balmer break, and compare to a sample of spectroscopically selected quiescent galaxies at the same redshift. Photometric selection identifies a majority of objects that have clear evidence for a recent short-lived burst of star formation within the last 1.5 Gyr, i.e. post-starburst galaxies, however we show that good quality continuum spectra are required to obtain physical parameters such as burst mass fraction and burst age. Dust attenuation appears to be the primary cause for misidentification of post-starburst galaxies, leading to contamination in spectroscopic samples where only the [OII] emission line is available, as well as a small fraction of objects lost from photometric samples. The 31 confirmed post-starburst galaxies have formed 40-90% of their stellar mass in the last 1-1.5 Gyr. We use the derived star formation histories to find that the post-starburst galaxies are visible photometrically for 0.5-1 Gyr. This allows us to update a previous analysis to suggest that 25-50% of the growth of the red sequence at z~1 could be caused by a starburst followed by rapid quenching. We use the inferred maximum historical star formation rates of several 100-1000 Msun/yr and updated visibility times to confirm that sub-mm galaxies are likely progenitors of post-starburst galaxies. The short quenching timescales of 100-200 Myr are consistent with cosmological hydrodynamic models in which rapid quenching is caused by the mechanical expulsion of gas due to an AGN.
Post-starburst (or E+A) galaxies are characterized by low H$alpha$ emission and strong Balmer absorption, suggesting a recent starburst, but little current star formation. Although many of these galaxies show evidence of recent mergers, the mechanism for ending the starburst is not yet understood. To study the fate of the molecular gas, we search for CO (1-0) and (2-1) emission with the IRAM 30m and SMT 10m telescopes in 32 nearby ($0.01<z<0.12$) post-starburst galaxies drawn from the Sloan Digital Sky Survey. We detect CO in 17 (53%). Using CO as a tracer for molecular hydrogen, and a Galactic conversion factor, we obtain molecular gas masses of $M(H_2)=10^{8.6}$-$10^{9.8} M_odot$ and molecular gas mass to stellar mass fractions of $sim10^{-2}$-$10^{-0.5}$, comparable to those of star-forming galaxies. The large amounts of molecular gas rule out complete gas consumption, expulsion, or starvation as the primary mechanism that ends the starburst in these galaxies. The upper limits on $M(H_2)$ for the 15 undetected galaxies range from $10^{7.7} M_odot$ to $10^{9.7} M_odot$, with the median more consistent with early-type galaxies than with star-forming galaxies. Upper limits on the post-starburst star formation rates (SFRs) are lower by $sim10times$ than for star-forming galaxies with the same $M(H_2)$. We also compare the molecular gas surface densities ($Sigma_{rm H_2}$) to upper limits on the SFR surface densities ($Sigma_{rm SFR}$), finding a significant offset, with lower $Sigma_{rm SFR}$ for a given $Sigma_{rm H_2}$ than is typical for star-forming galaxies. This offset from the Kennicutt-Schmidt relation suggests that post-starbursts have lower star formation efficiency, a low CO-to-H$_2$ conversion factor characteristic of ULIRGs, and/or a bottom-heavy initial mass function, although uncertainties in the rate and distribution of current star formation remain.
Brightest Cluster Galaxies (BCGs) are mostly elliptical galaxies and very rarely have prominent star formation. We found that five out of 8,812 BCGs are E+A (i.e. post-starburst) galaxies, having the H$delta$~absorption line with an equivalent width $>2.5AA$ and no distinct emission lines in [O II] and H$alpha$. The E+A features we identified from the BCGs for the first time are not as significant as those in general galaxies, indicating that historically the star formation were not very violent.
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