On central black holes in ultra-compact dwarf galaxies

Abstract

Context. The dynamical mass-to-light (M/L) ratios of massive ultra-compact dwarf galaxies (UCDs) are about 50% higher than predicted by stellar population models. Aims. Here we investigate the possibility that these apparently elevated M/L ratios of UCDs are caused by a central black hole (BH) that heats up the internal motion of stars. We focus on a sample of ∼50 extragalactic UCDs from the literature for which velocity dispersions and structural parameters have been measured. Methods. To be self-consistent in our BH mass estimates, we first redetermine the dynamical masses and M/L ratios of our sample UCDs, using up-to-date distance moduli and a consistent treatment of aperture and seeing effects. On average, the homogeneously redetermined dynamical mass and M/L ratios agree to within 5% with previous literature results. We calculate the ra-pop between the dynamical and the stellar population M/L for an assumed age of 13 Gyr. Ψ > 1 indicates an elevated dynamical M/L ratio, suggesting dark mass on top of a canonical stellar population of old age. For all UCDs with Ψ > 1 we estimate the mass of a hypothetical central black hole needed to reproduce the observed integrated velocity dispersion Results. Massive UCDs (M > 10 7 M ) have an average Ψ = 1.7 ± 0.2, implying notable amounts of dark mass in them. We find that, on average, central BH masses of 10-15% of the UCD mass can explain these elevated dynamical M/L ratios. The implied BH masses in UCDs range from several 10 5 M to several 10 7 M . In the M BH -luminosity plane, UCDs are offset by about two orders of magnitude in luminosity from the relation derived for galaxies. Our findings can be interpreted such that massive UCDs originate from progenitor galaxies with masses around ∼10 9 M , and that those progenitors have SMBH occupation fractions of ∼60-100%. The suggested UCD progenitor masses agree with predictions from the tidal stripping scenario. Also, the typical BH mass fractions of nuclear clusters in such ∼10 9 M galaxy bulges agree with the 10-15% BH fraction estimated for UCDs. Lower-mass UCDs (M < 10 7 M ) exhibit a bimodal distribution in Ψ, suggestive of a coexistence of massive globular clusters and tidally stripped galaxies in this mass regime. Conclusions. Central BHs as relict tracers of tidally stripped progenitor galaxies are a plausible explanation for the elevated dynamical M/L ratios of UCDs. Direct observational tests of this scenario are suggested.

Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies

“…For VUCD3, we found that with a BH mass of M 4.4 10 6  , Γ with three-sigma error bars was 0.8±0.6 resulting in a total dynamical mass of 32  M 21 10 6  . For comparison, without a BH component Γ with 3σ error bars was 1.7±0.2 resulting in a total dynamical mass of M 66 8 10 6 ´, which is consistent with previous results (Evstigneeva et al 2007;Mieske et al 2013). For M59cO, we found that, with a BH mass of M 5.8 10 6  , Γ with 3σerror bars was 0.3±0.2, resulting in a total dynamical mass of M 32 10 22 24 6 -+  .…”

“…Recently, analyses of the integrated dispersions of UCDs revealed an interesting property; the dynamical mass appears to be elevated by∼50% for almost all UCDs above M 10 7  when compared to the mass attributed to stars alone (e.g., Haşegan et al 2005;Mieske et al 2013). These dynamical mass estimates have been made combining structural information from Hubble Space Telescope (HST) imaging with groundbased, global velocity dispersion measurements.…”

“…The metallicity of VUCD3 has been estimated to be between −0.28 and 0.35 in several studies (Evstigneeva et al 2007;Firth et al 2009;Francis et al 2012), and it has an [α/Fe]∼0.5 (Francis et al 2012). M59cO is located 10kpc in projection from the center of M59 and has M 13.26 V = - (Mieske et al 2013). Its metallicity has been measured in several studies with [Z/H] between 0.0 and 0.2, with [α/Fe]∼0.2 (Chilingarian & Mamon 2008;Sandoval et al 2015;Janz et al 2016).…”

M32 Analogs? A Population of Massive Ultra-compact Dwarf and Compact Elliptical Galaxies in Intermediate-redshift Clusters

“…At least some UCDs contain a central supermassive black hole (see Seth et al 2014;Janz et al 2015 for detections, andFrank et al 2011;Norris et al 2015 for non-detections). UCDs appear not to have high dark matter fractions (Frank et al 2011;Mieske et al 2013;Forbes et al 2014;Janz et al 2015).…”

The formation of ultra compact dwarf galaxies and massive globular clusters

“…As shown above, the variable IMF (scenario (1)) can explain the observed elevated M/L V values. The mass of a SMBH in scenario (2) required to explain the observed M/L V values, needs typically to be 10-15 per cent of the present-day UCD mass (Mieske et al 2013). The presence of SMBHs with such masses is indeed suggested or observationally confirmed at least for a few UCDs (e.g.…”