Abstract
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 ($Z 0.2
Z_ødot$). 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.
Users
Please
log in to take part in the discussion (add own reviews or comments).