AN X-RAY AND OPTICAL INVESTIGATION OF THE INFRARED-LUMINOUS GALAXY MERGER MARKARIAN-266

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 spe ctacular gaseous nebula. In soft X-rays, the nebula has a diameter of approximate to 100 kpc, a temperature of a few x 10(6) K, and a lumino sity of about 10(42) ergs s(-1). The optical emission-line nebula is s maller (35 kpc) but much more luminous (10(44) ergs s(-1) x 10% L(bol) for Mrk 266). The cinematics 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 systematica lly with increasing radius. We argue that a full understanding of the properties of this system probably requires the presence of both an ac tive galactic nucleus (AGN) and a powerful starburst occuring in a pai r of colliding/merging disk galaxies. The presence of an AGN in the so uthwestern nucleus is demonstrated by the high-ionization type 2 Seyfe rt emission-line spectrum there. The most direct evidence for a young stellar component in Mrk 266 comes from our analysis of the stellar ab sorption lines, which show that the signature of an old stellar popula tion is weak and the signature of a young or intermediate-age populati on 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 contrib utes significantly). The kinematics and physical properties of the gas suggest that a ''superwind'' driven by the collective effect of the s tarburst's supernovae and stellar winds stirs up this gas, drives it o utward, heats it to X-ray-emitting temperatures, and compresses it to high densities and pressures. We show empirically that only those gala xy mergers that are bright in Ha (i.e., those that contain luminous st arbursts) 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.