XMM-Newton Spectroscopy of the Starburst-Dominated Ultraluminous Infrared Galaxy NGC 6240

Abstract

We present a new XMM-Newton observation of the ultraluminous infrared galaxy (ULIRG) NGC 6240. We analyze the reflecting grating spectrometer (RGS) data, as well as data from the other instruments, and find a starburst-dominated 0.5-3 keV spectrum with global properties resembling those observed in M82 but with a much higher luminosity. We show that the starburst region can be divided into an outer zone, beyond a radius of about 2.1 kpc, with a gas temperature of about 10⁷ K, and a central region with temperatures in the range (2-6)×10⁷ K. The gas in the outer region emits most of the observed O VIII Lyα line, and the gas in the inner region the emission lines of higher ionization ions, including a strong Fe XXV line. We also identify a small inner part, very close to the active nuclei, with typical Seyfert 2 properties, including a large amount of photoionized gas producing a strong Fe Kα 6.4 keV line. The combined abundance, temperature, and emission measure analysis indicates supersolar Ne/O, Mg/O, Si/O, S/O, and possibly also Fe/O. The analysis suggests densities in the range of (0.07-0.28)ɛ^-1/2 cm^-3 and a total thermal gas mass of ~4×10⁸ɛ^1/2 M_solar, where ɛ is the volume filling factor. We used a simple model to argue that a massive starburst with an age of ~=2×10⁷ yr can explain most of the observed properties of the source. NGC 6240 is perhaps the clearest case of an X-ray-bright luminous AGN that is in a merger and whose soft X-ray spectrum is dominated by a powerful starburst.