The X-ray emission in post-merger ellipticals

The evolution in X–ray properties of early–type galaxies is largely unconstrained. In particular, little is known about how, and if, remnants of mergers generate hot gas halos. Here we examine the relationship between X–ray luminosity and galaxy age for a sample of early–type galaxies. Comparing normalized X–ray luminosity to three different age indicators we find that L X /L B increases with age, suggesting an increase in X–ray halo mass with time after a galaxy’s last major star-formation episode. The long-term nature of this trend, which appears to continue across the full age range of our sample, poses a challenge for many models of hot halo formation. We conclude that models involving a declining rate of type Ia supernovae, and a transition from outflow to inflow of the gas originally lost by galactic stars, offers the most promising explanation for the observed evolution in X-ray luminosity.