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 MB ≿ −20.5 mag have B-band luminosities that scale with the square of the stellar velocity dispersion: LB ∝ σ 2. We show that the non-linear 'super-quadratic' relation Mbh ∝ L2B − L2.5B from Graham & Scott yields black hole masses, Mbh, that agree with the Mbh -σ relation down to at least Mbh = 104 M☉. We predict that 30 of the 100 galaxies have Mbh ≤ 105 M☉, with IC 3602 having Mbh = 104 M☉ and IC 3633 having Mbh = (6-8)×103 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 (Mbh = 2 × 105 M☉); IC 3492 (Mbh = 5 × 104 M☉); and IC 3292 (Mbh = 6 × 104 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)-MK colour-magnitude diagram and discuss the implications for the Mbh −LK and Mbh −M∗,galaxy relations, revealing why stripped galaxies, especially rare compact elliptical galaxies, should be excluded from Mbh −L scaling relations.