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 keV } / M _ { \ast } = ( 6.0 \pm 0.5 \pm 1.8 ) \times 10 ^ { 27 } { 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 .