X-Ray Emissivity of Old Stellar Populations: A Local Group Census

We study the unresolved X-ray emission in three Local Group dwarf elliptical (dE) galaxies (NGC 147, NGC 185, and NGC 205) using XMM-Newton observations; this emission most likely originates from a collection of weak X-ray sources, mainly cataclysmic variables and coronally active binaries. Precise knowledge of this stellar X-ray emission is crucial not only for understanding the relevant stellar astrophysics but also for disentangling and quantifying the thermal emission from diffuse hot gas in nearby galaxies. We find that the integrated X-ray emissivities of the individual dEs agree well with those of the solar vicinity, supporting an often assumed but untested view that the X-ray emissivity of old stellar populations is quasi-universal in normal galactic environments, in which dynamical effects on the formation and destruction of binary systems are not important. The average X-ray emissivity of the dEs studied in the literature, including M32, is measured to be {L}_{0.5-2 {{keV}}}/{M}_*=(6.0+/- 0.5+/- 1.8)× {10}²⁷ {erg} {{{s}}}^-1 {M}_⊙ ^-1. We also compare this value to the integrated X-ray emissivities of Galactic globular clusters and old open clusters and discuss the role of dynamical effects in these dense stellar systems.