EDP Sciences
Free access
Volume 473, Number 3, October III 2007
Page(s) 783 - 789
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20077336

A&A 473, 783-789 (2007)
DOI: 10.1051/0004-6361:20077336

X-ray emission from the stellar population in M 32

M. Revnivtsev1, 2, E. Churazov1, 2, S. Sazonov1, 2, W. Forman3, and C. Jones3

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 )

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 $8\times10^{33}~{\rm erg~s^{-1}}$ 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 ~ $4.3\times10^{27}~{\rm erg~s^{-1}}~M_\odot^{-1}$ 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 $\ll 1$ 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.

Key words: ISM: general -- galaxies: dwarf -- galaxies: individual: M 32 -- galaxies: general -- galaxies: stellar content -- X-rays: diffuse background

© ESO 2007