Extended Thermal X-Ray Emission from the Spiral-Dominant Group of Galaxies HCG 57

We observed a group of galaxies, HCG 57, with ASCA. Regardless that theirmember galaxies are dominated by spiral galaxies, we detected extended thermalX-ray emission that is attributed to hot gas with a temperature of$1.04\pm0.10$ keV. This is the second clear detection of thermal X-ray emissionfrom a spiral-dominant group of galaxies after HCG 92. The luminosity of thethermal emission is about $5\times10^{41}$ erg s$^{-1}$ in the 0.5--10 keVband, which is higher than that of HCG 92, but relatively less luminous amonggroups of galaxies. The X-ray emission is extended over several membergalaxies, and is thus associated with the group rather than an individualgalaxy. The metal abundance cannot be well constrained with a lower limit of0.08 solar. The gas-to-stellar mass ratio is $\sim0.3$. Although this isrelatively low among groups, the hot gas is also a significant component evenin the spiral-dominant group. We suggest that the X-ray faintness ofspiral-dominant groups is due to the low surface brightness and somewhat lowgas mass, at least in the case of HCG 57.Comment: 12 pages, 4 figures, to appear in PASJ 54 No.