EMBERS I: Low redshift post-starburst galaxies are frequently depleted in molecular gas relative to star forming progenitors

Abstract

The cold gas content of post-starburst galaxies (PSBs) provides important insight into the mechanisms that drive rapid quenching, but a multiphase assessment of both the atomic and molecular gas in PSBs does not yet exist. We introduce the Ensemble of Multiphase Baryons Evolving in Rapidly-quenching Systems, or EMBERS, a homogeneously selected, nearly mass- and redshift-complete survey of the global atomic (HI) and molecular gas (H2) in PSBs, observed with the Five Hundred-metre Aperture Spherical Telescope (FAST) and the Institut de radioastronomie millimetrique (IRAM) 30m telescope. We present new CO(1-0) observations for 52 PSBs with the IRAM 30m, which, combined with 9 archival observations, gives a total H2 sample of 61, of which 58/61 have ancillary HI measurements. We detect CO(1-0) in 34/61 galaxies, corresponding to molecular gas fractions (fH2 = MH2/M*) ranging from two to 250 per cent. By comparing with a stellar-mass matched star-forming (SF) control sample from xCOLD GASS, we find that PSBs on average are 0.3-0.6 dex depleted in H2. However, considering both HI and H2, individual PSBs host diverse gas reservoirs ranging from gas-rich in both phases, elevated in one phase, or gas-poor, the latter of which is common at lower stellar mass. The existence of gas-normal and gas-depleted PSBs in both phases suggests that some PSBs may rejuvenate their star formation, but the rapid shutdown of star formation in others is likely terminal. Despite this diversity, the majority of EMBERS PSBs are gas-poor compared to SF controls, with the typical PSB hosting gas reservoirs intermediate to those found in star-forming and quenched galaxies.

Figures (15)

• Figure 3: The total inferred molecular gas mass (left) and gas fraction (right) as a function of stellar mass for EMBERS PSBs coloured as blue stars for PCA selected galaxies and as red diamonds for E+A PSBs. In grey are the aperture corrected measurements from xCOLD GASS saintongeXCOLDGASSComplete2017a. Upper limits in both samples are denoted as downward arrows. All data points in the figure assume a CO-to-H$_2$ ratio of $\alpha_{\mathrm{CO}} = 4.3\ \mathrm{M}_\odot\,(\mathrm{K\,km\,s^{-1}\,pc^{2}})^{-1}$. The solid black lines denote the survey sensitivity threshold corresponding to 1.5 per cent gas fraction for log$_{10}$ M$_{\ast}$/M$_{\odot}$ > 10.5 and 2.5 per cent for log$_{10}$ M$_{\ast}$/M$_{\odot}$ < 10.5. The typical uncertainties on stellar and molecular gas masses for the PSBs are shown in the top left of the left panel. Although PSBs are in the process of quenching, we detect molecular gas in the majority of our sample, with bulk gas reservoirs spanning $\sim 2.5$ dex in gas fraction ranging from 0.8 per cent to $\sim 250$ per cent.
• Figure 4: Left: fraction of galaxies where CO is detected at the $\rm S/N>5$ level for our PSBs (blue), the entire xCOLD GASS sample (grey), and a BPT selected star-forming subset of xCOLD GASS (violet) in bins of stellar mass. The horizontal dashed line corresponds to a detection fraction of 50$\%$ where a robust median value can still be computed. Error bars are the binomial error for each bin. Low stellar mass PSBs are detected at a rate consistently lower than the xCOLD GASS comparison sample while higher mass PSBs are detected at a consistent or elevated rate. PSBs are detected at significantly lower levels than SF galaxies. Right: median gas fraction in bins of stellar mass for the same three samples of galaxies as the left panel. The downwards arrow denotes an upper limit due to a detection fraction $<50\%$ for that bin. Errors on each bin are the standard error on the median. Similar to detection fractions, the median gas mass is lower than the comparison sample for low stellar mass PSBs. The PSBs do not experience the downturn shown by the xCOLD GASS comparison, caused by the increase in quenched population in normal galaxies. When the quenched population is removed from the comparison, as is done for the SF subset of xCOLD GASS, our PSBs are consistently between $\sim$0.3 -- 0.6 dex lower in gas fraction than the assumed star-forming progenitors.
• Figure 5: Top: The ratio of molecular to atomic gas, $\rm R_{mol}$, as a function of stellar mass for both PSBs (blue and red) and all galaxies in both xGASS and xCOLD GASS (grey) detected in either CO or H$\text{\sc{i}}$. When CO is detected but not HI, an upward facing arrow denotes a lower limit on the ratio, while an upper limit is shown with a downwards arrow when HI is detected but not CO. Non-detections in both gas phases are omitted from this figure. Equal gas content in both gas phases is denoted as a horizontal dashed line at $\rm R_{mol}=0$. To the right of the plot are density histograms of $\rm R_{mol}$ for both EMBERS PSBs and xGASS/xCOLD GASS. Bottom: The same as the top panel but comparing PSBs to just star forming galaxies in xGASS/xCOLD GASS with log$_{10}~$M$_{\ast}$/M$_{\odot}>9.5$. $\rm R_{mol}$ spans $\sim 3$ dex in both our PSBs and the comparison sample, with some galaxies in both samples hosting larger molecular gas reservoirs than atomic. When compared to their progenitor population of star-forming galaxies in the same mass range, PSBs have similar $\rm R_{mol}$ distributions to a comparison sample, indicating that inefficient H$\text{\sc{i}}$-to-H$_2$ conversion is not the cause of quenching.
• Figure 6: Left: Offset from mass-matched controls drawn from all of xCOLD GASS as a function of stellar mass. The dashed black line corresponds to a galaxy as 'gas-normal' relative to the entire xCOLD GASS population. Non-detections in CO(1--0) are denoted with downwards arrows. The shaded area corresponds to the $\pm 1 \sigma$ spread of $\Delta$MH$_2$ for galaxies in the xCOLD GASS comparison sample. Right: the same but for a BPT selected SF control subset of xCOLD GASS as a control sample. This figure highlights the diversity of gas properties seen in our sample but also that the majority of PSBs in the sample when compared the star forming progenitors, are significantly gas poor by $\sim 0.5-1.3$ dex. The detection fraction for matches to EMB55 is $<50$ per cent what controls are drawn from the full xCOLD GASS sample, and so we denote it as a lower limit in the left panel.
• Figure 7: Gas mass offset from the BPT selected star-forming control sample of xCOLD GASS and xGASS for molecular and atomic gas for our PSBs. The PSB points are coloured in stellar mass bins of 0.5 with low mass PSBs in blue, intermediate mass PSBs in purple, and high mass PSBs in violet. If a galaxy is detected only in HI, the limit is denoted as a downward arrow, if a galaxy is only detected in CO the limit is denoted as a left-pointing arrow, and non-detections in both gas phases are denoted as 45 degree angles down and left. The horizontal and vertical dashed lines correspond to 'gas-normal' in a given gas phase compared to the SF population of controls while the diagonal dotted line is the 1-to-1 line where a galaxy is equally depleted in both gas phases. In the background is the gas mass offsets for the combined comparison sample relative to the SF subset. Contours enclose a percentage of the entire population of controls in steps of 10$\%$ (i.e., the outer contour encloses 100 per cent of the xCOLD GASS/xGASS sample). Along the right and top edges of the plot are projections of the variable along each axis for both the PSBs (blue) and controls (grey). This figures highlights how the majority of the PSBs in our sample exist in a transitionary state in cold gas between star-forming and quenched, a regime which is largely not populated by the xGASS and xCOLD GASS samples.