Half-megasecond Chandra Spectral Imaging of the Hot Circumgalactic Nebula around Quasar Mrk 231

Abstract

A deep 400 ks ACIS-S observation of the nearest quasar known, Mrk 231, is combined with archival 120 ks data to carry out the first ever spatially resolved spectral analysis of a hot X-ray-emitting circumgalactic nebula around a quasar. The 65 × 50 kpc X-ray nebula shares no resemblance with the tidal debris seen at optical wavelengths. One notable exception is the small tidal arc ~3.5 kpc south of the nucleus where excess soft X-ray continuum emission and Si XIII 1.8 keV line emission are detected, consistent with star formation and its associated alpha-element enhancement, respectively. An X-ray shadow is also detected at the location of the 15 kpc northern tidal tail. The hard X-ray continuum emission within ~6 kpc of the center is consistent with being due entirely to the bright central active galactic nucleus. The soft X-ray spectrum of the outer (gsim6 kpc) portion of the nebula is best described as the sum of two thermal components with temperatures ~3 and ~8 million K and spatially uniform super-solar alpha-element abundances, relative to iron. This result implies enhanced star formation activity over ~10⁸ yr, accompanied by redistribution of the metals on a large scale. The low-temperature thermal component is not present within ~6 kpc of the nucleus, suggesting extra heating in this region from the circumnuclear starburst, the central quasar, or the optically identified gsim3 kpc quasar-driven outflow. The soft X-ray emission is weaker in the western quadrant, coincident with a deficit of Hα and some of the largest columns of neutral gas outflowing from the nucleus. Shocks may heat the gas to high temperatures at this location, consistent with the tentative ~2σ detection of extended Fe XXV 6.7 keV line emission.