2011ApJ...737...22A

Hot gaseous halos are predicted around all large galaxies and are critically important for our understanding of galaxy formation, but they have never been detected at distances beyond a few kpc around a spiral galaxy. We used the ACIS-I instrument on board Chandra to search for diffuse X-ray emission around an ideal candidate galaxy: the isolated giant spiral NGC 1961. We observed four quadrants around the galaxy for 30 ks each, carefully subtracting background and point-source emission, and found diffuse emission that appears to extend to 40-50 kpc. We fit β-models to the emission and estimate a hot halo mass within 50 kpc of 5x10⁹ M_☉. When this profile is extrapolated to 500 kpc (the approximate virial radius), the implied hot halo mass is 1-3x10¹¹ M_☉. These mass estimates assume a gas metallicity of Z = 0.5 Z_☉. This galaxy's hot halo is a large reservoir of gas, but falls significantly below observational upper limits set by pervious searches, and suggests that NGC 1961 is missing 75% of its baryons relative to the cosmic mean, which would tentatively place it below an extrapolation of the baryon Tully-Fisher relationship of less massive galaxies. The cooling rate of the gas is no more than 0.4 M_☉/yr, more than an order of magnitude below the gas consumption rate through star formation. We discuss the implications of this halo for galaxy formation models.