Abstract
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' 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. Also as a part of this survey, detailed Chandra observations for the double nucleus merger system Mkn 266 and the merger-remnant Arp 222 are presented for the first time.
Highlights
It is widely believed that very few galaxies exist today that have not been formed or shaped in some way by an interaction with another galaxy. Toomre (1977) identified 11 galaxies that exhibit characteristics of on-going mergers, and arranged them in a chronological order, illustrating how spiral galaxies can merge to produce ellipticals
The central part of the system, a single relaxed body, displays an r1/4 optical surface brightness profile typical of elliptical galaxies (Schweizer 1982). In addition to this relaxed nucleus, the galaxy exhibits complex loops and ripples, notably two long tidal tails, indicative of the merger history of this system. From both UBVI images taken with the WCFPT instrument on the Hubble Space Telescope (HST) (Miller et al 1997) and N-body simulations (Hibbard & Mihos 1995), it has been estimated that nuclear coalescence took place ∼1 Gyr ago
The initial data processing to correct for the motion of the spacecraft and apply instrument calibration was carried out with the Standard Data Processing (SDP) at the Chandra X-ray Center (CXC)
Summary
It is widely believed that very few galaxies exist today that have not been formed or shaped in some way by an interaction with another galaxy. Toomre (1977) identified 11 galaxies that exhibit characteristics of on-going mergers, and arranged them in a chronological order, illustrating how spiral galaxies can merge to produce ellipticals. X-ray observations were initially carried out with the Einstein Observatory, providing limited spatial resolution This instrument was followed by ROSAT, which greatly improved the sensitivity of the observations, allowing the Xray properties of these merging galaxies to be probed. A study of a sample of interacting systems was carried out by Read & Ponman (1998) (from here on RP98), where the Xray luminosity and properties of the diffuse gas were investigated with ROSAT, alongside the point source population. With the generation of X-ray observatories this issue was addressed with an increase in spatial resolution This improvement, provided by Chandra, has allowed the investigation of both the diffuse gas and the point source population of galaxies to be carried out in greater detail.
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