X-ray Properties of NGC 253's Starburst-Driven Outflow

Abstract

We analyze image and spectral data from $\approx$365~ks of observations from the {\it Chandra} X-ray Observatory of the nearby, edge-on starburst galaxy NGC 253 to constrain properties of the hot phase of the outflow. We focus our analysis on the $-$1.1 to $+$0.63 kpc region of the outflow and define several regions for spectral extraction where we determine best-fit temperatures and metal abundances. We find that the temperatures and electron densities peak in the central $\sim$250 pc region of the outflow and decrease with distance. These temperature and density profiles are in disagreement with an adiabatic spherically expanding starburst wind model and suggest the presence of additional physics such as mass loading and non-spherical outflow geometry. Our derived temperatures and densities yield few-Myr cooling times in the nuclear region, which may imply that the hot gas can undergo bulk radiative cooling as it escapes along the minor axis. Our metal abundances of O, Ne, Mg, Si, S, and Fe all peak in the central region and decrease with distance along the outflow, with the exception of Ne which maintains a flat distribution. The metal abundances indicate significant dilution outside of the starburst region. We also find estimates on the mass outflow rates which are $2.8\:M_{\odot}/\rm{yr}$ in the northern outflow and $3.2\:M_{\odot}/\rm{yr}$ in the southern outflow. Additionally, we detect emission from charge exchange and find it has a significant contribution ($20-42$\%) to the total broad-band ($0.5-7$ keV) X-ray emission in the central and southern regions of the outflow.