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Hot gaseous halos are predicted around all large galaxies and are criticallyimportant for our understanding of galaxy formation, but they have never beendetected at distances beyond a few kpc around a spiral galaxy. We used theChandra ACIS-I instrument to search for diffuse X-ray emission around an idealcandidate galaxy: the isolated giant spiral NGC 1961. We observed fourquadrants around the galaxy for 30 ks each, carefully subtracting backgroundand point source emission, and found diffuse emission that appears to extend to40-50 kpc. We fit $\beta$-models to the emission, and estimate a hot halo masswithin 50 kpc of $5\times10^9 M_{\odot}$. When this profile is extrapolated to500 kpc (the approximate virial radius), the implied hot halo mass is$1-3\times10^{11} M_{\odot}$. These mass estimates assume a gas metallicity of$Z = 0.5 Z_{\odot}$. This galaxy's hot halo is a large reservoir of gas, butfalls significantly below observational upper limits set by pervious searches,and suggests that NGC 1961 is missing 75% of its baryons relative to the cosmicmean, which would tentatively place it below an extrapolation of the baryonTully-Fisher relationship of less massive galaxies. The cooling rate of the gasis no more than 0.4 $M_{\odot}$/year, more than an order of magnitude below thegas consumption rate through star formation. We discuss the implications ofthis halo for galaxy formation models.

Detection of a Hot Gaseous Halo Around the Giant Spiral Galaxy NGC 1961