2010ApJ...716.1514Y

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 active galactic nuclei (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 mÅ spectral bin at the expected O VII Kα line. We find no detectable absorption lines of C VI, N VII, O VII, O VIII, or Ne IX. The high spectral quality allows us to tightly constrain upper limits to the corresponding ionic column densities (in particular log[N_O vii(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.