An X-ray study of the galactic-scale starburst-driven outflow in NGC 253

Abstract: X-ray properties of hot interstellar gas in a starburst galaxy NGC 253 were investigated to gain a further understanding of starburst-driven outflow activity by XMM-Newton and Suzaku. Spectroscopic analysis for three regions of the galaxy characterized by multiwavelength observations was conducted. The hot gas was represented by two thin thermal plasmas with temperatures of kT ~0.2 and ~0.6 keV. Abundance ratios i.e., O/Fe, Ne/Fe, Mg/Fe and Si/Fe, are consistent between three regions, which suggests the common origin of the hot gas. The abundance patterns are consistent with those of type II supernova ejecta, indicating that the starburst activity in the central region provides metals toward the halo through a galactic-scale starburst-driven outflow. The energetics also can support this indication on condition that 0.01-50 {\eta}^0.5 % of the total emission in the nuclear region has flowed to the halo region. To constrain the dynamics of hot interstellar gas, surface brightness and hardness ratio profiles which trace the density and temperature were extracted. Assuming a simple polytropic equation of state of gas, T{\rho}^1-{\amma}) = const, we constrained the physical condition. {\gamma} is consistent with 5/3 at the hot disk and T is constant ({\gamma} = 1) in the halo. It is suggested that the hot gas expands adiabatically from the central region towards the halo region while it moves as free expansion from the inner part of the halo towards the outer part of the halo as the outflow. We constrained the outflow velocity to be >100 km s^-1 from the observed temperature gradient in the halo. In comparison with the escape velocity of ~220 km s^-1 for NGC 253, it is indicated that the hot interstellar gas can escape from the gravitational potential of NGC 253 by combining the outflow velocity and the thermal velocity.