X-Ray Imaging Spectroscopy of Stephan's Quintet and Seyfert 2 Galaxy NGC 7319

We observed Stephan's Quintet, a compact group of galaxies, with the Japanese X-ray satellite ASCA. The ASCA observation has revealed that the X-ray emission consists of two components. One is a soft X-ray emission characterized by thin thermal model with kT = 0.76^+0.05−0.10 keV and a metal abundance of 0.08 ± 0.04 solar. Since the soft X-rays are extended and the centroid is located at the region between NGC 7318 and NGC 7319, thin thermal emission originates from diffuse hot gas filling the Quintet. The total mass of hot gas is about 2 × 10¹⁰M⊙, which is only 10% of the galaxy mass. The low metal abundance indicates that most of the hot gas consist of primordial gas. The binding mass is about 6 × 10¹²M⊙ . The baryonic fraction is about 10%. The other component is hard X-ray emission located at NGC 7319, a Seyfert 2 galaxy. The X-ray spectrum is well fitted by the absorbed-power law emission plus a line. The photon index is 0.8^+0.8−1.1, and the absorption column is about 10²³ cm⁻². The X-ray luminosity in the 2–10 keV band is about 1.7 × 10⁴² erg s⁻¹. These characteristics are consistent with those of Seyfert 2 galaxies. We have revealed an obscured nucleus in NGC 7319. Our results well explain the flux of Hα and anisotropic radiation seen in the optical band.