X-ray emissivity of old stellar populations: a local group census

TL;DR

This study measures the X-ray emission from old stellar populations in dwarf elliptical galaxies, confirming their emissivity is similar to the Solar neighborhood and discussing implications for stellar and galactic astrophysics.

Contribution

It provides the first precise measurement of X-ray emissivity in dwarf ellipticals, supporting the universality of stellar X-ray emission in normal galactic environments.

Findings

X-ray emissivity of dwarf ellipticals matches Solar neighborhood values.

Average X-ray emissivity measured as (6.0 ± 0.5 ± 1.8) × 10^{27} erg s^{-1} M_\odot^{-1}.

Supports the quasi-universality of old stellar populations' X-ray emission.

Abstract

We study the unresolved X-ray emission in three Local Group dwarf elliptical galaxies (NGC 147, NGC 185 and NGC 205) using XMM-Newton observations, which 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 dwarf ellipticals agree well with that 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 dwarf ellipticals, including M32 studied in the literature, is measured to be $L_{0.5-2\ \rm {keV}}/M_{\ast} = (6.0 \pm 0.5 \pm 1.8) \times 10^{27} \ \rm{erg \ s^{-1} \ M_\odot^{-1}}$. 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.