The Dearth of Chemically Enriched Warm-Hot Circumgalactic Gas

TL;DR

This study uses high-quality X-ray observations to search for hot, chemically enriched circumgalactic gas around galaxies but finds no detectable absorption lines, challenging previous hypotheses about the CGM's role in missing baryons.

Contribution

It provides the first tight constraints on ionic column densities of hot CGM gas, showing it may not contain enough baryons unless metallicity is very low.

Findings

No detectable X-ray absorption lines in stacked spectra.

Upper limits on ionic column densities are tightly constrained.

Results challenge the idea that hot CGM accounts for missing baryons.

Abstract

The circumgalactic medium (CGM) around galaxies is believed to record various forms of galaxy feedback and contain a significant portion of the "missing baryons" of individual dark matter halos. However, clear observational evidence for the existence of the hot CGM is still absent. We use intervening galaxies along 12 background AGNs as tracers to search for X-ray absorption lines produced in the corresponding CGM. Stacking Chandra grating observations with respect to galaxy groups and different luminosities of these intervening galaxies, we obtain spectra with signal-to-noise ratios of 46-72 per 20-mA spectral bin at the expected OVII Kalpha line. We find no detectable absorption lines of CVI, NVII, OVII, OVIII, or NeIX. The high spectral quality allows us to tightly constrain upper limits to the corresponding ionic column densities (in particular log[N(OVII)(cm^{-2})]<=14.2--14.8). These nondetections are inconsistent with the Local Group hypothesis of the X-ray absorption lines at z~0 commonly observed in the spectra of AGNs. These results indicate that the putative CGM in the temperature range of 10^{5.5}-10^{6.3} K may not be able to account for the missing baryons unless the metallicity is less than 10% solar.