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

TL;DR

This study measures the unresolved X-ray emission in NGC 3379, finding a universal emissivity value per unit stellar mass that aligns with other galaxies and local stellar populations, indicating a common origin from old stars.

Contribution

It provides the first measurement of the universal X-ray emissivity of old stellar populations across different galaxy types using deep Chandra data.

Findings

Unresolved X-ray emissivity in NGC 3379 is ~8.2x10^{27} erg/s/M_sun.

The emissivity value is consistent with other galaxies and local stellar populations.

Supports the idea of a universal X-ray emissivity for old stellar populations.

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 >10^{36 erg/sec, we find that the remaining unresolved X-ray emission is characterized by an emissivity per unit stellar mass L_x/M_stars ~8.2x10^{27} erg/s/M_sun in the 0.5-2 keV energy band. This value is in good agreement with those previousely determined for the dwarf elliptical galaxy M32, the bulge of the spiral galaxy M31 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.