Hot gas accretion by massive spirals - A view from Chandra

The formation and growth of spiral galaxies through cosmological accretion of gas remains a poorly understood phenomenon from an observational viewpoint. As an example, it is unclear whether most of the gas accreting onto spirals at low redshift is in a cold or hot phase, and thus whether the hot gas halos predicted by many semi-analytical models of galaxy formation exist around spirals at all. A Chandra search for X-ray gas around two massive, quiescent edge-on spirals has revealed tentative evidence for an extended hot halo around the more massive galaxy, NGC 5746. No such gas has been detected around the other galaxy, the M31-sized NGC 5170. In contrast to the hot gas found around other spirals, the X-ray emission surrounding NGC 5746 appears to arise from the cooling of externally accreted material and not from disk outflows. In particular, the derived constraints on halo gas properties compare well to predictions for inflowing gas in cosmological simulations of disk galaxy formation and evolution.