Tracing the Cosmic Evolution of the Cool Circumgalactic Medium of Luminous Red Galaxies with DESI Year 1 Data

Abstract

We investigate the properties of the cool circumgalactic medium (CGM) of massive galaxies and their cosmic evolution. By using the year 1 dataset of luminous red galaxies (LRGs) and QSOs from the Dark Energy Spectroscopic Instrument survey, we construct a sample of approximately 600,000 galaxy-quasar pairs and measure the radial distribution and kinematics of the cool gas traced by Mg II absorption lines as a function of galaxy properties from redshift 0.4 to redshift 1.2. Our results show that the covering fraction of the cool gas around LRGs increases with redshift, following a trend similar to the global evolution of galaxy star formation rate. At small radii (< 0.3rvir), the covering fraction anti-correlates with stellar mass, suggesting that mass-dependent processes suppress the cool gas content in the inner region. In addition, we measure the gas dispersion by modeling the velocity distribution of absorbers with a narrow and a broad components -- sigma_n ~ 160 and sigma_b ~ 380 km/s -- and quantify their relative contributions. The results show that the broad component becomes more prominent in the outer region, and its relative importance in the central region grows with increasing stellar mass. Finally, we discuss possible origins of the cool gas around massive galaxies, including the contribution of satellite galaxies and the precipitation scenario.

Figures (18)

• Figure 1: Example of an LRG–QSO pair with detected Mg II absorption. Left: Image of the LRG–QSO pair from the Legacy Survey DR9 Dey2019. Right: The corresponding DESI spectrum of the background QSO in the LRG rest frame, showing the Mg II absorption line. The orange dashed lines mark the wavelengths of the Mg II doublet, while the green dashed lines mark the centers of the fitted absorption lines. This difference indicates the gas velocity relative to the galaxy.
• Figure 2: Distributions of stellar masses (left panel), halo masses (middle panel) and virial radius (right panel). The stellar masses are derived from CIGALE, while the halo masses are derived from SHMR and HMF. See the text for more details about the estimation of halo masses and virial radius.
• Figure 3: U-V and V-J color-color distribution of DESI LRGs. The color bar represents the source density computed using a kernel density estimate (KDE). The thick black lines show the Whitaker12 color cut separating star-forming and quiescent galaxies.
• Figure 4: Covering fraction as a function of impact parameter in physical space. Top: $f_c$ for absorbers with $0.4 \leq W_{0,\lambda2796}<1 \rm \, \AA$ (weak absorbers) Bottom: $f_c$ for absorbers with $W_{0,\lambda2796}\geq 1 \rm \, \AA$ (strong absorbers). The gas covering fractions for LRGs with stellar masses ranging from least to most massive are shown in the left to right panels. Colors transition from light blue to dark blue, indicating galaxy redshifts from lowest to highest. The errors are estimated based on binomial statistics. Solid lines and dashed lines illustrate the posterior median of the regression lines for $r_p \leq 400$ kpc and $r_p \leq 1000$ kpc, respectively.
• Figure 5: Posterior median and 68% credible interval of the regression parameters for the covering fraction $f_c$ in physical space. Left: parameter describing redshift dependence. Middle: parameter describing stellar mass dependence. Right: the slope of the gas distribution. Circles represent the MCMC-derived parameters obtained in this work, while triangles represent the best-fit parameters obtained in Lan2020 within $600$ kpc. Weak and strong absorbers are indicated by light brown and dark brown colors, respectively.