Uncovering an Excess of X-ray Point Sources in the Halos of Virgo Late-type Galaxies

Abstract

We present a systematic search for extraplanar X-ray point sources around 19 late-type, highly inclined disk galaxies residing in the Virgo cluster, based on archival Chandra observations reaching a source detection sensitivity of $L\rm(0.5- 8~keV)\sim10^{38}\rm~erg~s^{-1}$. Based on the cumulative source surface density distribution as a function of projected vertical distance from the disk mid-plane, we identify a statistically significant ($\sim3.3\sigma$) excess of $\sim20$ X-ray sources within a projected vertical off-disk distance of $0.92'-2.5'$ ($\sim4.4-12\ \mathrm{kpc}$), the presence of which cannot be explained by the bulk stellar content of the individual galaxies, nor by the cosmic X-ray background. On the other hand, there is no significant evidence for an excess of extraplanar X-ray sources in a comparison sample of field late-type edge-on galaxies, for which Chandra observations reaching a similar source detection sensitivity are available. We discuss possible origins for the observed excess, which include low-mass X-ray binaries (LMXBs) associated with globular clusters, supernova-kicked LMXBs, high-mass X-ray binaries born in recent star formation induced by ram pressure stripping of the disk gas, as well as a class of intra-cluster X-ray sources previously identified around early-type member galaxies of Virgo. We find that none of these X-ray populations can naturally dominate the observed extraplanar excess, although supernova-kicked LMXBs and the effect of ram pressure are most likely relevant