Universal X-ray emissivity of the stellar population in early-type galaxies: unresolved X-ray sources in NGC 3379

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: 2 April 2008
Accepted: 25 June 2008

Abstract

We use deep Chandra observations to measure the emissivity of the unresolved X-ray emission in the elliptical galaxy NGC 3379. After elimination of bright, low-mass X-ray binaries with luminosities 10³⁶ erg s^-1, we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass   10²⁷ erg s^-1  in the 0.5-2 keV energy band. This value is in good agreement with those previosuly determined for the dwarf elliptical galaxy M 32, the bulge of the spiral galaxy M 31 and the Milky Way, as well as with the integrated X-ray emissivity of cataclysmic variables and coronally active binaries in the Solar neighborhood. This strongly suggests that i) the bulk of the unresolved X-ray emission in NGC 3379 is produced by its old stellar population; and ii) the old stellar populations in all galaxies can be characterized by a universal value of X-ray emissivity per unit stellar mass or per unit K band luminosity.