Galactic superhaloes, globular clusters and high latitude X-ray sources

Abstract

I SHOW here that the observational features of unidentified, high latitude, X-ray sources¹ are consistent with the suggestion that they are a population of remote, optically undetectable, globular clusters. The principal objection to the hypothesis that the high latitude sources are members of any galactic population is their isotropy in galactic longitude. But if it is assumed that the most distant unidentified high latitude source radiates at 10³⁸ erg s⁻¹ (a typical value for a bright globular cluster X-ray source^2,3) and that it is the faintest unidentified high latitude source, visible at 1–2 Uhuru counts s⁻¹, its distance would be ∼ 200 kpc. Then, extrapolating to the brightest unidentified source at 7 counts s⁻¹, we get a lower bound on the minimum distance to any objects of about 80 kpc, and the volume toward the galactic centre ,between −90° < l^II < 90° in the ‘shell’ from 80 to 200 kpc, differs negligibly from the volume toward the anticentre, at 90° < l^II < 270°. Although a specific radiation model is used in this case, the luminosities do not affect the basic result.