X-ray emission from the stellar population in M 32

¹ Max-Planck-Institute für Astrophysik, Karl-Schwarzschild-Str. 1, 85740 Garching bei München, Germany e-mail: mikej@mpa-garching.mpg.de
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Received: 21 February 2007
Accepted: 22 July 2007

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 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 ~ 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  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.