Diffuse X-Ray-emitting Gas in Major Mergers

Using archived data from the Chandra X-ray telescope, we have extracted the diffuse X-ray emission from 49 equal-mass interacting/merging galaxy pairs in a merger sequence, from widely separated pairs to merger remnants. After the removal of contributions from unresolved point sources, we compared the diffuse thermal X-ray luminosity from hot gas (LX(gas)) with the global star formation rate (SFR). After correction for absorption within the target galaxy, we do not see a strong trend of LX(gas)/SFR with the SFR or merger stage for galaxies with SFR > 1 M☉ yr−1. For these galaxies, the median LX(gas)/SFR is 5.5 × 1039 ((erg s−1)/M☉ yr−1)), similar to that of normal spiral galaxies. These results suggest that stellar feedback in star-forming galaxies reaches an approximately steady-state condition, in which a relatively constant fraction of about 2% of the total energy output from supernovae and stellar winds is converted into X-ray flux. Three late-stage merger remnants with low SFRs and high K-band luminosities (LK) have enhanced LX(gas)/SFR; their UV/IR/optical colors suggest that they are post-starburst galaxies, perhaps in the process of becoming ellipticals. Systems with LK < 1010 L☉ have lower LX(gas)/SFR ratios than the other galaxies in our sample, perhaps due to lower gravitational fields or lower metallicities. We see no relation between LX(gas)/SFR and Seyfert activity in this sample, suggesting that feedback from active galactic nuclei is not a major contributor to the hot gas in our sample galaxies.