An X-Ray Study of the Galactic-Scale Starburst-Driven Outflow in NGC 253

Abstract

The X-ray properties of hot interstellar gas in a bright, nearby edge-on starburst galaxy, NGC 253, were investigated so as to gain a further understanding of starburst-driven outflow activity by utilizing XMM-Newton and Suzaku. Spectroscopic analyses for three regions of the galaxy characterized by multiwavelength observations, i.e., the superwind region, the disk region, and the halo region, were conducted. Various emission lines from O, Ne, Mg, Si, and Fe were observed in the spectra of each region. The hot gas was represented by two thin thermal plasmas with temperatures of $ kT$$ \sim$ 0.2 and $ \sim$ 0.6 keV. The abundance ratios, i.e., O$ /$ Fe, Ne$ /$ Fe, Mg$ /$ Fe, and Si$ /$ Fe, are consistent between the three regions, which suggests a common origin of the hot gas. The abundance patterns are consistent with those of type II supernova ejecta, indicating that the starburst activity in the central region provides metals toward the halo through a galactic-scale starburst-driven outflow. The energetics can also support this indication on the condition that 0.01–50 $ \eta^{1/2}$ % of the total emission in the nuclear region has flowed to the halo region. To constrain the dynamics of hot interstellar gas, surface brightness, and hardness ratio profiles, which trace the density and temperature, were extracted. Assuming a simple polytropic equation of state for gas, $ T\rho^{1-\gamma}$$ =$ const, we constrained the physical condition; $ \gamma$ is consistent with 5$ /$ 3 at a hot disk of $ <$ 3 kpc from the center along with the minor axis, and $ T$ is constant ($ \gamma$$ =$ 1) in the halo, the distance of which is between 3 and 10 kpc from the center. 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}$ based on the observed temperature gradient in the halo. In comparison with the escape velocity of $ \sim$ 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.