The Chandra view of galaxy mergers

From a Chandra survey of nine interacting galaxy systems the evolution of X-ray emission during the merger process has been investigated. It is found that the X-ray luminosity peaks ∼300 Myr before nuclear coalescence, and then dips, even though we know that rapid and increasing activity is still taking place at this time. It is likely that this drop in X-ray luminosity is a consequence of outflows breaking out of the galactic discs of these systems. In this work it is also shown that, for the systems close to the point of nuclear coalescence, LFIR becomes massively enhanced compared to the X-ray luminosity of these systems. We suggest that this enhancement may indicate a ‘top heavy’ initial mass function (IMF), with an enhanced fraction of massive stars. At a time ∼1 Gyr after coalescence, the merger remnants in our sample are X-ray faint when compared to typical mature elliptical galaxies. However, we do see evidence that these systems will start to resemble typical elliptical galaxies at a greater dynamical age, given the properties of the 3-Gyr system within our sample, supporting the idea that halo regeneration will take place within low LX merger remnants. As a part of this survey, detailed Chandra observations for the double nucleus merger system Markarian 266 (Mrk 266) and the merger remnant Arp 222 are presented for the first time. With the Mrk 266 observation, in contrast to previous studies, we now have good spectral information of the individual components part seen with the ROSAT High Resolution Imager (HRI). Additionally, the structure of the emission to the north of the system can clearly be distinguished and there is also a suggestion of some extension of X-ray emission to the south-east of the nuclear region, indicating that this galaxy could just be on the verge of large-scale galactic winds breaking out. Within Arp 222 an X-ray luminosity of 1.46 × 1040 erg s−1 has been detected, this is the lowest value of LX within our sample. The diffuse gas of Arp 222 has been modelled with a temperature of 0.6 keV and, from CO observations it has been found to host very little molecular gas, indicating that, from current observations, Arp 222 does not resemble a mature elliptical.