AnXMM-Newtonsearch for X-ray sources in the Fornax dwarf galaxy

Abstract

We report the results of a deep archive XMM-Newton observation of the Fornax spheroidal galaxy that we analyzed with the aim of fully characterizing the X-ray source population (in most of the cases likely to be background active galactic nuclei) detected towards the target. A cross correlation with the available databases allowed us to find a source that may be associated with a variable star belonging to the galaxy. We also searched for X-ray sources in the vicinity of the Fornax globular clusters GC 3 and GC 4 and found two sources probably associated with the respective clusters. The deep X-ray observation was also suitable for the search of the intermediate-mass black hole (of mass 10 4 M ) expected to be hosted in the center of the galaxy. In the case of Fornax, this search is extremely difficult since the galaxy centroid of gravity is poorly constrained because of the large asymmetry observed in the optical surface brightness. Since we cannot firmly establish the existence of an X-ray counterpart of the putative black hole, we put constraints only on the accretion parameters. In particular, we found that the corresponding upper limit on the accretion efficiency, with respect to the Eddington luminosity, is as low as a few 10 −5 .

A Catalog Sample of Low-mass Galaxies Observed in X-Rays with Central Candidate Black Holes

“…Their number density and characteristics is, in turn, of crucial importance to get information about the black hole seed population in the early Universe (Volonteri 2010;Natarajan 2014) and about their contribution in re-ionizing the hydrogen at high red-shift via the X-ray emission (Volonteri & Gnedin 2009). Furthermore, searching for the high energy emission expected from these objects and applying the fundamental plane relation at radio and X-ray wavelengths (see Merloni et al 2003) as recently done by several authors (see e.g., Reines et al 2013, Nucita et al 2013a, Nucita et al 2013b) allows one to infer the mass of the compact object (if any).…”

“…1 In this context, dwarf spheroidal galaxies are a sub-class of low mass galaxies with stellar mass in the range 10 3 -10 7 M ⊙ (Martin et al 2008) particularly interesting since they show large mass-to-light ratios which make them to be dominated by dark matter (Mateo 1997). The interest in dwarf galaxies is rapidly growing (see, e.g., McConnachie 2012) both for stellar populations studies (Amorisco & Evans 2012;Maccarone et al 2005) and searches for central IMBHs (see e.g., Reines et al 2013, Nucita et al 2013a It is known that massive black holes 2 are hosted in the nuclei of almost every galaxy characterized by a central bulge (Kormendy & Ho 2013). In these cases, black holes reveal themselves via stellar and gas kinematic in close targets or, in distant active galactic nuclei, by mean of the emitted radiation.…”

XMM-Newtonstudy of the Draco dwarf spheroidal galaxy

“…So far, 19 dSphs satellite galaxies of the Milky Way and 12 of M31 have been detected (Metz & Kroupa 2007). The dSphs are approximately spherical, have no gas or recent star formation, and are usually 1−2 orders of magnitude fainter than the faintest spiral galaxies known (Nucita et al 2013). They are the least luminous galaxies and their nature is fundamentally different from spiral and elliptical galaxies (Kormendy 1985).…”

Effect of black holes in local dwarf spheroidal galaxies on gamma-ray constraints on dark matter annihilation

“…It is notable that the last two dwarfs are included in the Fermi analysis. Additional searches for IMBHs in dSphs are at present underway [29,30]. The black hole in ω-Centauri [31], if it is conclusively found to be the remnant of a tidally disrupted galaxy [32] instead of a globular cluster [33], can be added to the list.…”