Extreme circumgalactic HI and CIII absorption around the most massive, quenched galaxies
Luminous red galaxies (LRGs) are the most massive galaxies at $z\sim 0.5$ and, by selection, have negligible star formation. These objects have halo masses between those of $L_{*}$ galaxies, whose circumgalactic media (CGM) are observed to have large masses of cold gas, and clusters of galaxies, which primarily contain hot gas. Here we report detections of strong and extended metal (CIII 977) and HI lines in the CGM of two LRGs. The CIII lines have equivalent widths of $\sim 1.8$ \r{A} and $\sim 1.2$ \r{A} , and velocity spreads of $\sim 796$ km s$^{-1}$ and $\sim 1245$ km s$^{-1}$, exceeding all such measurements from local $\sim L_{*}$ galaxies (maximal CIII equivalent widths $\sim 1$ \r{A}). The data demonstrate that a subset of halos hosting very massive, quenched galaxies contain significant complexes of cold gas. Possible scenarios to explain our observations include that the LRGs' CGM originate from past activity (e.g., star formation or active galactic nuclei driven outflows) or from the CGM of galaxies in overlapping subhalos. We favor the latter scenario, in which the properties of the CGM are more tightly linked to the underlying dark matter halo than properties of the targeted galaxies (e.g., star formation).