Cool Gas in the Circumgalactic Medium of Massive Post Starburst Galaxies

TL;DR

This work uses stacked Mg II absorption along thousands of galaxy–quasar sightlines to probe the cool CGM around massive galaxies (log M*/M⊙ ≳ 11.4) in the redshift range 0.4 ≲ z ≲ 0.8. Galaxies are categorized by recent star-formation history into star-forming, quiescent, and post-starburst (PSB) populations using MFICA templates, enabling a comparative study of CGM gas content. The authors find a general decline of Mg II absorption with impact parameter, with PSB hosts showing a pronounced excess within ≲1 Mpc, while beyond ≈1 Mpc the absorption becomes similar across types, signaling a transition to the IGM near the virial radius. These results imply a strong coupling between recent starburst activity, fast cool-gas outflows, and the extent of cool CGM gas, bearing on quenching pathways and metal transport in massive galaxies; they also motivate targeted simulations to unravel the physical origin of extended, low-ionisation CGM around PSBs.

Abstract

Observing the interplay between galaxies and their gaseous surroundings is crucial for understanding how galaxies form and evolve, including the roles of long-lived cool gas reservoirs, starburst and AGN driven outflows. We use stacked Mg II absorption lines in the spectra of background quasars to study the cool gas out to 9Mpc from massive quiescent, star-forming and post-starburst galaxies with stellar masses $\log_{10}(M_{\mathrm{gal}}/M_\odot) \gtrsim 11.4$ and $0.4 \lesssim z \lesssim 0.8$ selected from the Baryon Oscillation Spectroscopic Survey (BOSS) CMASS galaxies. Consistent with previous studies, we observe a decline in absorption strength indicating a decrease in cool gas content with increasing distance from the galaxies, as well as decreasing star formation rate of the galaxies. Beyond 1Mpc, this decline levels off to the same absorption strength in all galaxy types, suggesting a transition from the circumgalactic medium (CGM) to the intergalactic medium (IGM) at approximately the virial radius of the host dark matter haloes. We find that post-starburst galaxies, that have experienced a recent burst of star formation that has rapidly quenched, exhibit significantly stronger Mg II absorption within 1Mpc than star-forming or quiescent galaxies of the same stellar mass. Because post-starburst galaxies are a potentially significant pathway for the formation of quiescent elliptical galaxies, our results have wide reaching implications for understanding the mechanisms involved in quenching star formation in galaxies. We speculate that the excess cool gas absorption out to 1Mpc around post-starburst galaxies is related to their observed high velocity ($\sim$1000\,km/s) cool gas outflows. Thus, strong, short-lived bursts of star formation impact the CGM around galaxies on Mpc distances and Gyr timescales.

Figures (10)

• Figure 1: The redshift distribution of the unique SDSS DR7 quasars (left panel) and unique SDSS DR12 CMASS galaxies (right panel) that make up the galaxy-quasar pairs studied in this paper. The galaxy histograms show the whole sample (grey filled), quiescent (red line), star-forming (blue line) and post-starburst (orange line) sub-samples. Each galaxy and each QSO may contribute to multiple pairs in our spectral stacks.
• Figure 2: Left: The stellar mass distribution of the unique SDSS DR12 CMASS galaxies that are part of the galaxy-quasar pairs studied in this paper. The histograms show the whole sample (grey filled), quiescent (red line), star-forming (blue line) and post-starburst (orange line) sub-samples. As noted in the text, there is significant disagreement in stellar masses between different CMASS value added catalogues, and therefore these histograms should be taken as indicative only. Right: The MFICA derived K and AF component fractions of the galaxy spectra, with colour scale indicating the logarithmic number of galaxies. The K component fraction represents the fraction of galaxy light between 3300-5100Å contributed by long-lived, low-mass stars. The AF component fraction represents the fraction of light contributed by intermediate mass A and F stars. Based on these two light fractions, the galaxy is classified as star forming ("SF"), quiescent ("Red") or post starburst ("PSB") as indicated by the demarcation lines. Green valley galaxies, lying between the quiescent and star-forming populations, are excluded from our analysis as they were found to contain significant contamination from the quiescent galaxy population in the relatively low SNR CMASS spectra.
• Figure 3: Stacked spectra around the Mg ii Mg ii absorption doublet in the rest-frame of the galaxies for the inner four (top) and outer four (bottom) impact parameter bins as given in the title of each subplot. Note the change in y-axis scaling between the upper and lower panels. The green dashed line shows the expected positions of the Mg ii Mg ii absorption doublet, and the orange dashed lines show the wavelengths between which the equivalent width is measured. For visualisation purposes only, we also show a double Gaussian profile fit to the Mg ii Mg ii absorption lines (purple dotted lines). The top left of each panel gives the number of pairs included in the stack, and the measured Mg ii Mg ii equivalent width of each stack. An absorption signature is seen at all impact parameters.
• Figure 4: The equivalent width of the Mg ii Mg ii absorption doublet as a function of average impact parameter for our work (red points), compared to results from the literature. Error bars are from a bootstrap analysis as described in the text. The green points are from Zhu2014 and the black and grey points are from PerezRafols2015, the black points show their variable smoothing method for removing the quasar continuum and the grey points show their mean subtraction method. Dotted lines show least squares linear fits to each dataset. In agreement with past results, we find that the strength of Mg ii Mg ii absorption in the CGM of galaxies decreases with impact parameter.
• Figure 5: Stacked spectra around the Mg ii Mg ii absorption doublet in the rest-frame of the galaxies for the inner four impact parameter bins, for galaxies of different spectral types: post-starburst (top), star-forming (centre) and quiescent (bottom). Note the change in y-axis scaling between rows. The dashed and dotted lines are the same as in Fig. \ref{['fig:allstacks']}. The top left of each panel gives the number of pairs included in the stack, and the measured Mg ii Mg ii equivalent width of each stack