The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240
  We report on a recent ~150-ks long Chandra observation of the ultraluminous
infrared galaxy merger NGC 6240, which allows a detailed investigation of the
diffuse galactic halo. Extended soft X-ray emission is detected at the 3-sigma
confidence level over a diamond-shaped region with projected physical size of
~110x80 kpc, and a single-component thermal model provides a reasonably good
fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5
million K, an estimated density of 2.5x10^{-3} cm^{-3}, and a total mass of
~10^10 M_sun, resulting in an intrinsic 0.4-2.5 keV luminosity of 4x10^41 erg
s^{-1}. The average temperature of 0.65 keV is quite high to be obviously
related to either the binding energy in the dark-matter gravitational potential
of the system or the energy dissipation and shocks following the galactic
collision, yet the spatially-resolved spectral analysis reveals limited
variations across the halo. The relative abundance of the main alpha-elements
with respect to iron is several times the solar value, and nearly constant as
well, implying a uniform enrichment by type II supernovae out to the largest
scales. Taken as a whole, the observational evidence is not compatible with a
superwind originated by a recent, nuclear starburst, but rather hints at
widespread, enhanced star formation proceeding at steady rate over the entire
dynamical timescale (~200 Myr). The preferred scenario is that of a
starburst-processed gas component gently expanding into, and mixing with, a
pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature
(kT ~ 0.25 keV). This picture cannot be probed more extensively with the
present data, and the ultimate fate of the diffuse, hot gas remains uncertain.
Under some favorable conditions, at least a fraction of it might be retained
after the merger completion, and evolve into the hot halo of a young elliptical
galaxy.
