* Astronomy

Blobrana · L

NGC 1961

Blobrana · L

Title: A Deep XMM-Newton Study of the Hot Gaseous Halo Around NGC 1961
Author: Michael E. Anderson, Eugene Churazov, Joel N. Bregman

We examine 11 XMM-Newton observations of the giant spiral galaxy NGC 1961, with a total integration time of 289 ks (~100 ks after flaring corrections). These deep X-ray data allow us to study the hot gaseous halo of a spiral galaxy in unprecedented detail. We perform both a spatial and a spectral analysis; with the former, the hot halo is detected to at least 80 kpc and with the latter the halo properties can be measured in detail up to 42 kpc. In the region of overlap, there is good agreement between the two methods. We measure the temperature profile of the hot halo, finding a negative gradient as is common for elliptical galaxies. We also measure a rough metallicity profile, which is consistent with being flat at a sub-Solar value (Z~0.2Z solar). Converting to this metallicity, our deprojected density profile is consistent with previous parametric fits, with no evidence for a break or flattening within the inner 42 kpc (about 10% of the virial radius). We infer pressure and entropy profiles for the hot halo, and use the former to estimate the mass profile of the galaxy assuming hydrostatic equilibrium. Extrapolating these profiles to the virial radius, we infer a hot gaseous halo mass comparable to the stellar mass of the galaxy, and a total baryon fraction from the stars and hot gas of around 30%. We show that the cooling time of the hot gas is orders of magnitude longer than the dynamical time, making the hot halo stable against cooling instabilities, and argue that an extended stream of neutral Hydrogen seen to the NW of this galaxy is likely due to accretion from the intergalactic medium. The low metallicity of the hot halo suggests it too was likely accreted. We compare the hot halo of NGC 1961 to hot halos around isolated elliptical galaxies, and show that the total mass better determines the hot halo properties than the stellar mass.

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Blobrana · L

Title: Detection of a Hot Gaseous Halo Around the Giant Spiral Galaxy NGC 1961
Authors: Michael E. Anderson, Joel N. Bregman

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 Chandra ACIS-I instrument 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 \beta-models to the emission, and estimate a hot halo mass within 50 kpc of 5 x 10^9 solar masses. When this profile is extrapolated to 500 kpc (the approximate virial radius), the implied hot halo mass is 1-3 x 10^{11} solar masses. These mass estimates assume a gas metallicity of Z = 0.5 Z_{\odot}. 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 solar masses /year, 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.

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