Ultramassive Black Holes in the Most Massive Galaxies: M BH –σ versus M BH –R b

Abstract

We investigate the nature of the relations between black hole (BH) mass ( M BH ) and the central velocity dispersion ( σ ) and, for core-Sérsic galaxies, the size of the depleted core ( R b ). Our sample of 144 galaxies with dynamically determined M BH encompasses 24 core-Sérsic galaxies, thought to be products of gas-poor mergers, and reliably identified based on high-resolution Hubble Space Telescope imaging. For core-Sérsic galaxies, i.e., combining normal-core ( R b  < 0.5 kpc) and large-core galaxies ( R b ≳ 0.5 kpc), we find that M BH correlates remarkably well with R b such that M BH ∝ R b 1.20 ± 0.14 (rms scatter in log M BH of Δ rms  ∼ 0.29 dex), confirming previous works on the same galaxies apart from three new ones. Separating the sample into Sérsic, normal-core and large-core galaxies, we find that Sérsic and normal-core galaxies jointly define a single log-linear M BH – σ relation M BH  ∝  σ 4.88±0.29 with Δ rms  ∼ 0.47 dex; however, at the high-mass end, large-core galaxies (four with measured M BH ) are offset upward from this relation by (2.5–4) ×  σ s , explaining the previously reported steepening of the M BH – σ relation for massive galaxies. Large-core spheroids have magnitudes M V  ≲ −23.50 mag, half-light radii R e ≳ 10 kpc, and are extremely massive, M * ≳ 10 12 M ☉ . Furthermore, these spheroids tend to host ultramassive BHs ( M BH ≳ 10 10 M ☉ ) tightly connected with their R b rather than σ . The less popular M BH – R b relation exhibits ∼62% less scatter in log M BH than the M BH – σ relations. Our findings suggest that large-core spheroids form via multiple major “dry” merger events involving super/ultramassive BHs, consistent with the flattening of the σ – L V relation observed at M V  ≲ −23.5 mag.