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Description
Based on archival Chandra observations with a total exposure of 1.3Ms, we study X-ray point sources in the Fornax cluster of galaxies, with the primary aim of searching for intracluster X-ray source populations. We detect 1177 point sources out to a projected radius of ~30' (~180kpc) from the cluster center and down to a limiting 0.5-8keV luminosity of ~3x10^37^erg/s. We construct the source surface density profile, after excluding sources associated with foreground stars, known globular clusters, ultra-compact dwarfs, and galactic nuclei. From this profile we statistically identify ~183 excess sources that are not associated with the bulk stellar content of the individual member galaxies of Fornax, nor with the cosmic X-ray background. Taking into account Poisson error and cosmic variance, the cumulative significance of this excess is at the >~2{sigma} level (with a maximum of 3.6{sigma}) outside three effective radii of the central giant elliptical, NGC 1399. The luminosity function of the excess sources is found to be significantly steeper than that of the GC-hosting sources (presumably low-mass X-ray binaries (LMXBs)), disfavoring the possibility that unidentified GCs are primarily responsible for the excess. We show that a large fraction of the excess can be related to the extended stellar halo of NGC 1399 and/or the diffuse intracluster light, thus providing strong evidence for the presence of intracluster X-ray sources in Fornax, the second unambiguous case for a galaxy cluster after Virgo. Other possible origins of the excess, including supernova-kicked LMXBs and stripped nucleated dwarf galaxies are discussed.
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