Expected intermediate-mass black holes in the Virgo cluster. I. Early-type galaxies

Abstract: We expand upon the AMUSE-Virgo survey which imaged 100 early-type Virgo cluster galaxies with the Chandra X-ray Observatory, and we place an emphasis on potential intermediate mass black holes (IMBHs). Virgo early-type galaxies with absolute magnitudes $\mathfrak{M}B\gtrsim-20.5$ mag have $B$-band luminosities that scale with the square of the stellar velocity dispersion: $L_B\propto\sigma^2$. We show that the non-linear `super-quadratic' relation $M{\rm bh}\propto L_B^2$-$L_B^{2.5}$ from Graham & Scott yields black hole masses, $M_{\rm bh}$, that agree with the $M_{\rm bh}$-$\sigma$ relation down to at least $M_{\rm bh}=10^4\,M_{\odot}$. We predict that 30 of the 100 galaxies have $M_{\rm bh}\le10^5\,M_{\odot}$, with IC3602 having $M_{\rm bh}=10^4\,M_{\odot}$ and IC3633 having $M_{\rm bh}=$(6-8)$\times10^3\,M_{\odot}$. We additionally revise the black hole Eddington ratios and their scaling with black hole mass, and report a point-like Chandra source at the nucleus of five additional galaxies (NGC4382, NGC4387, NGC4417, NGC4467, and NGC4472). Moreover, three of the galaxies predicted here to host an IMBH have a point-like Chandra source near their nucleus: IC3442 ($M_{\rm bh}=2\times10^5\,M_{\odot}$); IC3492 ($M_{\rm bh}=5\times10^4\,M_{\odot}$); and IC3292 ($M_{\rm bh}=6\times10^4\,M_{\odot}$). Furthermore, IC3442 and IC3292 host a nuclear star cluster that is expected to house an IMBH. Finally, we present the ($B-K$)-$\mathfrak{M}K$ colour-magnitude diagram and discuss the implications for the $M{\rm bh}$-$L_K$ and $M_{\rm bh}$-$M_{\rm *,galaxy}$ relations, revealing why stripped galaxies, especially rare compact elliptical galaxies, should be excluded from $M_{\rm bh}$-$L$ and $M_{\rm bh}$-$M_{\rm *,galaxy}$ scaling relations.