Chandra Detection of Intracluster X-Ray Sources in Fornax

Abstract

Based on archival Chandra observations with a total exposure of 1.3 Ms, we study Xray point sources in the Fornax cluster of galaxies, with the primary aim of searching for intra-cluster X-ray source populations. We detect 1177 point sources out to a projected radius of ∼30 arcmin (∼180 kpc) from the cluster center and down to a limiting 0.5-8 keV luminosity of ∼ 3 × 10 37 erg s −1 . We construct source surface density profile, after excluding sources associated with foreground stars, known globular clusters, ultracompact 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 2 σ level (with a maximum of 3.6 σ) outside three effective radii of the central giant elliptical, NGC 1399. The luminosity function of the excess sources is found to be Corresponding author: Zhiyuan Li xiangyu.jin@mail.mcgill.ca, lizy@nju.edu.cn 2 Jin et al.significantly steeper than that of the GC-hosting sources (presumably low-mass Xray 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 intra-cluster light, thus providing strong evidence for the presence of intra-cluster 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.

On the Origin and Evolution of the Intra-Cluster Light: A Brief Review of the Most Recent Developments

“…These authors found that the average distance between ICL and DM is around 25 kpc within the first 140 kpc from the center, making the ICL a more reliable tracer of the mass distribution than, e.g., X-ray emissions (X-ray emission, however, has been important as well for the detection of the ICL. Indeed, some studies on Virgo (e.g., [94]) and Fornax (e.g., [95]) have shown that the X-ray band can be a sensitive and independent way to trace the ICL, although, given the current amount of data available, these studies would be limited to near groups/clusters.). A similar conclusion has been reached by other authors (e.g., [85][86][87][88]) with different methods.…”

The faint light in groups and clusters of galaxies

“…An interesting prospect is the detection of intracluster X-ray sources in Virgo and Fornax [121,122], likely associated with X-ray binaries in the ICL population [123]. This will not only open a new, and independent, window on the study of the ICL in other wavelengths rather than the optical or near-infrared, but could also provide constraints on progenitors of this light (their initial mass [124]).…”

A Chandra X-ray Survey of Optically Selected AGN Pairs

“…We performed X-ray source detection in the 0.5-2, 2-8, and 0.5-8 keV bands for each observation using the CIAO tool wavdetect. We follow the method described in Hou et al (2017) and Jin et al (2019). We have supplied the exposure and 50% enclosed count fraction (ECF) PSF maps and have adopted a false detection probability of P ≤ 10 −6 .…”