2023ApJ...957...91S

We study ultraviolet H I and metal-line transitions in the circumgalactic medium (CGM) of 15 massive, quenched luminous red galaxies (LRGs) at redshift z ∼ 0.5 and with impact parameters up to 400 kpc. We selected eight LRG-CGM systems to study general properties of the CGM around LRGs, while the other seven are already known to contain cool CGM gas from Mg II optical studies (Mg II-LRGs). In the general LRG population, we detect H I in four of eight LRGs, in all cases with N_H I_ < 10^16.7cm^–2. In contrast, all Mg II-LRGs show H I; for four LRGs, the H I column density is N_H I_ ≳ 10¹⁸cm^–2. The CGM of LRGs also shows low and intermediate ionized lines (such as C III, C II, Si III, and Si II) and highly ionized lines of O VI (we detect O VI around five of seven Mg II-LRGs and one of eight in the random sample). Next, we combine our sample with literature LRGs and ≲L^* galaxies, and we find that while for ≲L^* galaxies CGM H I Lyα absorption is stronger as galaxies are more massive, the cool CGM traced by H I Lyα is suppressed above stellar masses of M* ∼ 10^11.5M_☉. While most LRG-CGM systems show weak or nondetectable O VI (equivalent width < 0.2 Å), a few LRG-CGM systems show strong O VI 1031, which in most cases likely originates from groups containing both an LRG and a blue star-forming neighboring galaxy.