X-ray emission from the stellar population in M 32

Abstract

Using Chandra observations, we study the X-ray emission of the stellar population in the compact dwarf elliptical galaxy M 32. The proximity of M 32 allows one to resolve all bright point sources with luminosities higher than 8×10³³~erg~s^-1 in the 0.5-7 keV band. The remaining (unresolved) emission closely follows the galaxy's optical light and is characterized by an emissivity per unit stellar mass of ~4.3×10²⁷~erg~s^-1~M_⊙^-1 in the 2-10 keV energy band. The spectrum of the unresolved emission above a few keV smoothly joins the X-ray spectrum of the Milky Way's ridge measured with RXTE and INTEGRAL. These results strongly suggest that weak discrete X-ray sources (accreting white dwarfs and active binary stars) provide the bulk of the “diffuse” emission of this gas-poor galaxy. Within the uncertainties, the average X-ray properties of the M 32 stars are consistent with those of the old stellar population in the Milky Way. The inferred cumulative soft X-ray (0.5-2 keV) emissivity is however smaller than is measured in the immediate Solar vicinity in our Galaxy. This difference is probably linked to the contribution of young (age ≪ 1 Gyr) stars, which are abundant in the Solar neighborhood but practically absent in M 32. Combining Chandra, RXTE and INTEGRAL data, we obtain a broad-band (0.5-60 keV) X-ray spectrum of the old stellar population in galaxies.