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

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 M_B≳ -20.5 mag have B-band luminosities that scale with the square of the stellar velocity dispersion: L_B ∝ σ². We show that the non-linear `super-quadratic' relation M_bh∝ L_B^2 - L_B^{2.5} from Graham & Scott yields black hole masses, M_bh, that agree with the M_bh-σ relation down to at least M_bh=10^4 M_{⊙}. We predict that 30 of the 100 galaxies have M_bh≤ 10^5 M_{⊙}, with IC 3602 having M_bh=10^4 M_{⊙} and IC 3633 having M_bh = (6-8)× 10^3 M_{⊙}. We additionally revise the black hole Eddington ratios, and their scaling with black hole mass, and we report a point-like Chandra source at the nucleus of five additional galaxies (NGC 4382, NGC 4387, NGC 4417, NGC 4467, and NGC 4472). Moreover, three of the galaxies predicted here to host an IMBH have a point-like Chandra source near their nucleus: IC 3442 (M_bh=2× 10^5 M_{⊙}); IC 3492 (M_bh=5× 10^4 M_{⊙}); and IC 3292 (M_bh=6× 10^4 M_{⊙}). Furthermore, IC 3442 and IC 3292 host a nuclear star cluster that is expected to house an IMBH. Finally, we present the (B - K)-M_K colour-magnitude diagram and discuss the implications for the M_bh-L_K and M_bh-M_*,galaxy relations, revealing why stripped galaxies, especially rare compact elliptical galaxies, should be excluded from M_bh-L scaling relations.