The Hot Gaseous Halo of the Spiral Galaxy NGC 3628 in the Leo Triplet

Abstract

In a 14 ks {ital ROSAT} Position Sensitive Proportional Counter pointing we detected an extended (28{times}40 kpc) halo of hot tenuous gas around NGC 3628, with a flat emission distribution and a temperature of {ital T}{approx_equal}2{times}10{sup 6} K. The soft X-ray luminosity of the halo gas in the 0.1{endash}2.0 keV band is {approximately}8{times}10{sup 39} ergss{sup {minus}1}, and the total soft X-ray luminosity of NGC 3628 is approximately 2.1{times}10{sup 40} ergss{sup {minus}1}. The most likely heating source for the halo gas are the shocks resulting from a supernova-driven outflow from the circumnuclear starburst propagating through the halo interstellar medium. Both the morphology and the line ratios of associated optical emission-line gas are consistent with this interpretation. In many respects the soft X-ray halo of NGC 3628 resembles those of other nearby, edge-on starburst galaxies like, e.g., M82 and NGC 253. Its size and radiative cooling time indicate that halos like that of NGC 3628 have to be considered as potentially important contributors to the population of quasi-stellar object absorption-line systems. With two exceptions, most of the compact X-ray sources seen in projection within the halo of NGC 3628 are probably background active galactic nuclei. The remaining two sources might be condensationsmore » in the shock-heated halo gas. The soft X-ray properties of a few sources in the disk of NGC 3628 are consistent with those of high-mass X-ray binaries or powerful young supernova remnants. The PSPC spectrum of NGC 3627, which was within the field of view during our observations, also exhibits the typical characteristics of a starburst galaxy. It can be approximated by a power-law + Raymond-Smith model with a total soft X-ray luminosity of {ital L}{sub {ital x}}=1.1{times}10{sup 40} ergss{sup {minus}1}. {copyright} {ital 1996 The American Astronomical Society.}« less