An X-Ray and Optical Investigation of the Infrared-luminous Galaxy Merger Markarian 266

January 1997 • 1997ApJ...474..659W

Authors • Wang, Jing • Heckman, Timothy M. • Weaver, Kimberly A. • Armus, Lee

Abstract • We discuss X-ray and optical spectra and images of the infrared-bright double-nucleus system Mrk 266 = NGC 5256, with an emphasis on its spectacular gaseous nebula. In soft X-rays, the nebula has a diameter of ~100 kpc, a temperature of a few × 10⁶ K, and a luminosity of about 10⁴² ergs s^-1. The optical emission-line nebula is smaller (35 kpc) but much more luminous (10⁴⁴ ergs s^-1 ~ 10% L_bol for Mrk 266). The kinematics of the emission-line nebula suggest that a radial outflow of gas is occurring at several hundred km s^-1. We also measure high gas pressures in the nebula that decline systematically with increasing radius.

We argue that a full understanding of the properties of this system probably requires the presence of both an active galactic nucleus (AGN) and a powerful starburst occuring in a pair of colliding/merging disk galaxies. The presence of an AGN in the southwestern nucleus is demonstrated by the high-ionization type 2 Seyfert emission-line spectrum there. The most direct evidence for a young stellar component in Mrk 266 comes from our analysis of the stellar absorption lines, which show that the signature of an old stellar population is weak and the signature of a young or intermediate-age population is strong throughout the inner part of the system.

We suggest that the ongoing galaxy collision has tidally redistributed a swarm of gas clouds. The Seyfert-like spectrum of the nebula implies that these gas clouds are photoionized by the AGN (though the starburst also contributes significantly). The kinematics and physical properties of the gas suggest that a ``superwind'' driven by the collective effect of the starburst's supernovae and stellar winds stirs up this gas, drives it outward, heats it to X-ray-emitting temperatures, and compresses it to high densities and pressures. We show empirically that only those galaxy mergers that are bright in Hα (i.e., those that contain luminous starbursts) have bright X-ray nebulae. This suggests that the luminous X-ray nebula in Mrk 266 is powered by the starburst outflow rather than by the collisions of gas clouds during the merger. This in turn would imply that powerful starbursts are able to heat (and possibly eject) a significant fraction of the interstellar medium in galaxies that are the products of mergers.

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Lee Armus

Senior Scientist