The cool CGM of SDSS galaxies in absorption

See my paper for details (Anand et al. 2021).

Summary

In this work, I have developed a fully automated pipeline to model the quasar continuum and detect MgII absorbers in their spectra. I ran the developed pipeline on ~ 1 million quasar spectra from SDSS DR16 and compiled the largest MgII and FeII absorber catalogue available to date (see histogram of redshifts). The absorber catalogue is publicly available at this link for the community.

In the second half of the paper, I connected these MgII absorbers to the circumgalactic medium (CGM) of ~ 200,000 emission-line galaxies (ELGs) and ~ 1.1 million luminous red galaxies (LRGs) from the SDSS DR16 to characterize the properties of cool gas in their CGM. Thanks to these large sample sizes, our analysis characterized the scale dependence of MgII absorption in the halo of galaxies with much more accuracy than previously reported works. We find that both MgII absorption and its covering fraction are 2 - 5 times higher in the CGM of ELGs than LRGs within ~ 50 kpc from the galaxy. Also, there is a very sharp decline in the covering fraction for both kinds of galaxies, and at large distances, they are within the error bars (see Figure). The rapid decline in the covering fraction at ~ 50 kpc implies that MgII properties are regulated by galactic outflows in the inner part of CGM, while it is tightly linked with the dark matter halo in outer regions.

We also find that the stellar activity of ELGs play a very important role in enriching their CGM, where the MgII covering fraction correlates strongly with the star formation activity (SFR) of the galaxy (see Figure). In addition, we also see that MgII is rarely detected in the massive halos. The relative line-of-sight (LOS) velocity analysis also supports an outflow origin of MgII gas in ELGs, where the velocity dispersion of absorbing cloud relative to the halo is similar to the virial velocity of the dark matter halo. On the other hand, it is suppressed by 40-50 % in the LRGs, suggesting a different origin of MgII gas in their halo, possibly accretion or stripping from neighbouring halo. To summarize, our analysis combined with previous studies implied that cool circumgalactic gas has a different physical origin for star-forming versus quiescent galaxies.