THE HOT GASEOUS HALO OF THE SPIRAL GALAXY NGC-3628 IN THE LEO TRIPLET

In a 14 ks ROSAT Position Sensitive Proportional Counter pointing we d etected an extended (28 x 40 kpc) halo of hot tenuous gas around NGC 3 628, with a flat emission distribution and a temperature of T similar or equal to 2 x 10(6) K. The soft X-ray luminosity of the halo gas in the 0.1-2.0 keV band is similar to 8 x 10(39) ergs s(-1) and the total soft X-ray luminosity of NGC 3628 is approximately 2.1 x 10(40) ergs s(-1). The most likely heating source for the halo gas are the shocks resulting from a supernova-driven outflow from the circumnuclear starb urst propagating through the halo interstellar medium. Both the morpho logy and the line ratios of associated optical emission-line gas are c onsistent with this interpretation. In many respects the soft X-ray ha lo of NGC 3628 resembles those of other nearby, edge-on starburst gala xies 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 pot entially important contributors to the population of quasi-stellar obj ect absorption-line systems. With two exceptions, most of the compact X-ray sources seen in projection within the halo of NGC 3628 are proba bly background active galactic nuclei. The remaining two sources might be condensations in the shock-heated halo gas. The soft X-ray propert ies 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 starb urst galaxy. It can be approximated by a power-law + Raymond-Smith mod el with a total soft X-ray luminosity of L(X) = 1.1 x 10(40) ergs s(-1 ).