A deepXMM–Newtonstudy of the hot gaseous halo around NGC 1961

Abstract

We examine 11 XMM-Newton observations of the giant spiral galaxy NGC 1961, allowing us to study the hot gaseous halo of a spiral galaxy in unprecedented detail. We perform a spatial and a spectral analysis; with the former, the hot halo is detected to at least 80 kpc and with the latter its properties can be measured in detail up to 42 kpc. We find evidence for a negative gradient in the temperature profile as is common for elliptical galaxies. We measure a rough metallicity profile, which is consistent with being flat at $Z \sim 0.2 Z_{\odot}$. Converting to this metallicity, the deprojected density profile is consistent with previous parametric fits, with no evidence for a break within 42 kpc ($\sim$0.1R$_{\text{vir}}$). Extrapolating to the virial radius, we infer a hot halo mass comparable to the stellar mass of the galaxy, and a baryon fraction from the stars and hot gas of around 30%. 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 we argue that an extended stream of neutral Hydrogen seen to the NW of this galaxy is instead 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 determines the hot halo properties better than the stellar mass.