A Comprehensive Characterization of Galaxy-cool CGM Connections at $z<0.4$ with DESI Year 1 Data

TL;DR

This study addresses how the cool CGM traced by Ca II absorption relates to galaxy properties at $z<0.4$ by exploiting DESI Year 1 data to build high-S/N composite spectra. It employs a large sample of galaxy–quasar pairs with $r_p\le200$ kpc to measure the Ca II rest equivalent width $W_{0}^{\rm CaII}$ as a function of stellar mass, SFR, redshift, and galaxy type, using nonnegative matrix factorization for continuum removal and two-Gaussian fits for the Ca II doublet. The key findings include a strong $W_{0}^{\rm CaII}$–$M_*$ relation for star-forming galaxies ($\langle W_{0}^{\rm CaII}\rangle\propto M_*^{0.5}$ over $M_*\sim10^{8}-10^{11}\,M_{\odot}$), additional $W_{0}^{\rm CaII}\propto {\rm SFR}^{0.3}$ scaling among SF systems, and no significant minor-axis enhancement or AGN–galaxy difference at low redshift. The study also shows that the Ca II mass in the CGM is comparable to the Ca mass in the ISM, highlighting substantial cool CGM reservoirs and informing models of gas flows and feedback in galaxy evolution.

Abstract

We investigate the relationships between the cool circumgalactic medium (CGM), traced by Ca II absorption lines, and galaxy properties at $z<0.4$ using $\sim900{,}000$ galaxy-quasar pairs within $200\,\rm kpc$ from the Year 1 data of the Dark Energy Spectroscopic Instrument (DESI). This large data set enables us to obtain composite spectra with sensitivity reaching to the $\text{mÅ}$ level and to explore the Ca II absorption as a function of stellar mass, star formation rate (SFR), redshift, and galaxy types, including active galactic nuclei (AGNs). Our results show a positive correlation between the absorption strength and stellar mass of star-forming galaxies with $\langle W_{0}^{\rm Ca\ II}\rangle \propto M_{*}^{0.5}$ over 3 orders of magnitude in stellar mass from $\sim 10^{8}$ to $10^{11} \, M_{\odot}$, while such a mass dependence is weaker for quiescent galaxies. At a fixed mass, Ca II absorption is stronger around star-forming galaxies than quiescent ones especially within impact parameters $<30\,\rm kpc$. Among star-forming galaxies, the Ca II absorption further correlates with SFR, following $\propto \mathrm{SFR^{0.3}}$. However, in contrast to the results at higher redshifts, stronger absorption is not preferentially observed along the minor axis of star-forming galaxies, indicating a possible redshift evolution of CGM dynamics resulting from galactic feedback. Moreover, no significant difference between the properties of the cool gas around AGNs and galaxies is detected. Finally, we measure the absorption profiles with respect to the virial radius of dark matter halos and show that the total Ca II mass in the CGM is comparable to the Ca mass in the ISM of galaxies.

Figures (4)

• Figure 1: SFR--mass distribution of the foreground galaxies. The solid line divides the samples into star-forming and quiescent galaxies. The contours mark the levels of numbers at 10, 100, 500, 1000, 5000, and 8000.
• Figure 2: Examples of composite spectra as a function of impact parameters. The shades underneath are the associated uncertainties via bootstrapping. The solid blue lines are the best-fit absorption profiles of Ca$\;$ K and H. The vertical dashed lines are the central wavelengths of K and H lines. The median impact parameter is displayed in the lower right corner. Note that the scale of the y-axis changes as a function of impact parameters.
• Figure 3: Dependence of the Ca$\;$ K and H equivalent width on stellar masses as a function of impact parameters. Left: star-forming galaxies. Right: quiescent galaxies. The median stellar masses are listed in the lower left. The blue shaded bands in the right panel correspond to the two highest mass bins for star-forming galaxies in the left panel. The cyan and yellow square data points are the K line measurements from rubin22 multiplied by a factor of 1.69 to compare with our doublet measurements.
• Figure 4: Best-fit amplitudes $A_{100}$ as a function of stellar masses. The data points are plotted with their median stellar masses and respective ranges. The black line is the best-fit power law for the star-forming galaxies.