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

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 greater than or similar to 10(36) erg s(-1), we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass L-x/M-* similar to 8.2 x 10(27) erg s(-1) M-1 circle dot 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.