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 (0.5–8 keV) ∼ 10 ^38 erg s ^−1 . Based on the cumulative source surface density distribution as a function of projected vertical distance from the disk midplane, we identify a statistically significant (∼3.3 σ ) excess of ∼20 X-ray sources within a projected vertical off-disk distance of $0\buildrel{\,\prime}\over{.} 92\mbox{--}2\buildrel{\,\prime}\over{.} 5$ (∼4.4–12 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 the possible origins for the observed excess, including low-mass X-ray binaries (LMXBs) associated with globular clusters, supernova-kicked LMXBs, and 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 intracluster 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.

Keywords

• Virgo Cluster
• X-ray astronomy
• Late-type galaxies
• Disk galaxies
• X-ray binary stars