The Warm Circumgalactic Medium: 10^5-6 K Gas Associated with a Single Galaxy Halo or with an Entire Group of Galaxies?

Abstract

In preparation for a Hubble Space Telescope (HST) observing project using the Cosmic Origins Spectrograph (COS), the positions of all AGN targets having high-S/N far-UV G130M spectra were cross-correlated with a large catalog of low-redshift galaxy groups homogenously selected from the spectroscopic sample of the Sloan Digital Sky Survey (SDSS). Searching for targets behind only those groups at z = 0.1-0.2 (which places the O VI doublet in the wavelength region of peak COS sensitivity), we identified only one potential {{S}}/{{N}}=15{--}20 target, FBQS 1010+3003. An O VI-only absorber was found in its G130M spectrum at z = 0.11326, close to the redshift of a foreground small group of luminous galaxies at z = 0.11685. Because there is no associated Lyα absorption, any characterization of this absorber is necessarily minimal; however, the O VI detection likely traces “warm” gas in collisional ionization equilibrium at T ≈ 3 × 10⁵ K. While this discovery is consistent with being interface gas between cooler, photoionized clouds and a hotter intra-group medium, it could also be warm, interface gas associated with the circumgalactic medium (CGM) of the single closest galaxy. In this case, a detailed analysis of the galaxy distribution (complete to 0.2 {L}^* ) strongly favors the individual galaxy association. This analysis highlights the necessity of both high-{{S}}/{{N}}> 20 COS data and a deep galaxy redshift survey of the region in order to test more rigorously the association of O VI-absorbing gas with a galaxy group. A Cycle 23 HST/COS program is currently targeting 10 UV-bright AGN behind 12 low-redshift galaxy groups to test the warm, group gas hypothesis.