The trail of discrete X-ray sources in the early-type galaxy NGC 4261: Anisotropy in the globular cluster distribution?

Chandra images of the elliptical galaxy NGC 4261 have revealed an anisotropy in the spatial distribution of the off-nuclear X-ray sources, interpreted by Zezas et al. as evidence of an association with a young stellar population. Our independent analysis of archive X-ray (Chandra) and optical (INT and HST) observations confirms the anisotropy of the X-ray sources but conducts to a different interpretation for their origin. We find that nearly half of the X-ray sources are associated with a globular cluster (optical counterpart), suggesting that they are accreting low-mass X-ray binaries. Where color index information is available, the X-ray sources are found to reside in red (metal-rich) systems. The luminosity function of the X-ray sources is also consistent with the one drawn from a population of low-mass X-ray binaries. We further investigate the properties of the sample of pointlike sources obtained from archival optical images that we suggest are good globular cluster candidates and for which we find that the projected spatial distribution is nonhomogeneous. In addition, we show that the distributions of the optical and X-ray populations are very similar, which leads us to conclude that the spatial anisotropy of the X-ray sources in NGC 4261 is mostly a reflection of the anisotropy of the globular cluster population. © 2005. The American Astronomical Society. All rights reserved.