Soft-band X/K luminosity ratios in late-type galaxies and constraints on the population of supersoft X-ray sources

Abstract

We study X-ray to K-band luminosity ratios (LX/LK) of late-type galaxies in the 0.3-0.7 keV energy range. From the Chandra archive, we selected nine spiral and three irregular galaxies with point source detection sensitivity better than 5 × 1036ergs-1 in order to minimize the contribution of unresolved X-ray binaries. In late-type galaxies cold gas and dust may cause significant interstellar absorption, therefore we also demanded the existence of publicly available Hi maps. The obtained LX/LK ratios vary between (5.4 and 68) × 1027ergs-1L-1K,⊙ exceeding by factors of 2-20 the values obtained for gas-poor early-type galaxies. Based on these results we constrain the role of supersoft X-ray sources as progenitors of Type Ia supernovae (SNe Ia). For the majority of galaxies the upper limits range from ∼ 3 to ∼15 per cent of the SN Ia frequency inferred from K-band luminosity, but for a few of them no meaningful constraints can be placed. On a more detailed level, we study individual structural components of spiral galaxies: bulge and disc, and, for grand design spiral galaxies, arm and interarm regions. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.