Abstract
Abstract Building upon three late-type galaxies in the Virgo cluster with both a predicted black hole mass of less than ∼105 M ⊙ and a centrally located X-ray point source, we reveal 11 more such galaxies, more than tripling the number of active intermediate-mass black hole candidates among this population. Moreover, this amounts to a ∼36 ± 8% X-ray detection rate (despite the sometimes high, X-ray-absorbing, H i column densities), compared to just 10 ± 5% for (the largely H i-free) dwarf early-type galaxies in the Virgo cluster. The expected contribution of X-ray binaries from the galaxies’ inner field stars is negligible. Moreover, given that both the spiral and dwarf galaxies contain nuclear star clusters, the above inequality appears to disfavor X-ray binaries in nuclear star clusters. The higher occupation, or rather detection, fraction among the spiral galaxies may instead reflect an enhanced cool gas/fuel supply and Eddington ratio. Indeed, four of the 11 new X-ray detections are associated with known LINERs or LINER/H ii composites. For all (four) of the new detections for which the X-ray flux was strong enough to establish the spectral energy distribution in the Chandra band, it is consistent with power-law spectra. Furthermore, the X-ray emission from the source with the highest flux (NGC 4197: L X ≈ 1040 erg s−1) suggests a non-stellar-mass black hole if the X-ray spectrum corresponds to the “low/hard state”. Follow-up observations to further probe the black hole masses, and prospects for spatially resolving the gravitational spheres of influence around intermediate-mass black holes, are reviewed in some detail.
Highlights
If the low Eddington ratios associated with the supermassive black hole (SMBH) of these active galactic nucleus (AGN) are representative of the mean, they warn that the X-ray luminosities of potential intermediate-mass black holes (IMBHs) in lower-mass spiral galaxies will be challenging to observe
Accounting for each estimate’s associated uncertainty, d log(Mbh,i), the combined probability distribution function (PDF) yields the statistically most likely value for the black hole mass. When many such independent estimates are brought to bear on this derivation, as was the case for NGC 3319, one has a rather well-defined (Gaussian-like) PDF from which one can readily establish the probability of having detected an IMBH with Mbh < 102 Me) and SMBHs ( (105 Me)
There is a wealth of opportunity to further pursue the IMBH candidates identified here and those expected to reside in other low-mass galaxies
Summary
While galaxies suspected of harboring a supermassive black hole (SMBH) with a mass of around 105–106 Me have long been identified (e.g., Filippenko & Sargent 1985; Ho et al 1995; Greene & Ho 2007; Reines et al 2013; Yuan et al 2014; Graham & Scott 2015; Subramanian et al 2016; Liu et al 2018)— including POX 52 (Barth et al 2004; Thornton et al 2008), NGC 4395 (La Franca et al 2015; den Brok et al 2015; Brum et al 2019), and NGC 404 (Davis et al 2020)—there is an observational dearth of centrally located black holes with masses that are intermediate between stellar-mass black holes ( 102 Me) and SMBHs ( 105 Me). Chilingarian et al (2018) used the width and luminosity of the Hα emission line to identify IMBH candidates at the centers of 305 galaxies: ten of which have X-ray data that reveal a coincident point source and suspected AGN Four of these ten (which includes LEDA 87300) have a black hole mass estimate less than ∼105 Me. In addition, Moran et al (2014) has reported on 28 nearby (
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