Ultramassive black holes in the most massive galaxies: M_(BH – σ) versus M_(BH) – R_(B)

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We investigate the nature of the relations between black hole (BH) mass (M_(BH)) and the central velocity dispersion (σ) and, for core-S´ersic 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 HST imaging. For core-S´ersic 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^(1.20±0.14)_(b) (rms scatter in log M_(BH) of ∆_(rms) ∼ 0.29 dex), confirming previous works on the same galaxies except three new ones. Separating the sample into S´ersic, normal-core and large-core galaxies, we find that S´ersic and normal-core galaxies jointly define a single log-linear M_(BH) − σ relation M_(BH) ∝ σ^(4.88±0.29) with ∆_(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)Mꙩ. Furthermore, these spheroids tend to host ultramassive BHs (M_(BH) ˃̰ 10^(10)Mꙩ) 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.
© 2021. The American Astronomical Society. We thank the referee for their suggestions that improved the paper. B.T.D acknowledges supports from a Spanish postdoctoral fellowship ‘Ayudas 1265 para la atracci´on del talento investigador. Modalidad 2: jóvenes investigadores.’ funded by Comunidad de Madrid under grant number 2016-T2/TIC-2039 and from the award of ‘Estancias movilidad en el extranjero ’José Castillejo’ para jóvenes doctores 2019’, grant number CAS19/00344, offered by the Spanish ministry of science and innovation for a research stay at Swinburne University of Technology (01/01/2020-31/03/2020). B.T.D acknowledges financial support from grant ‘Ayudas para la realización de proyectos de I+D para jóvenes doctores 2019.’ funded by Comunidad de Madrid and Universidad Complutense de Madrid under grant number PR65/19-22417. B.T.D and A.G.d.P acknowledge financial support from the Spanish Ministry of Science, Innovation and Universities (MCIUN) under grant numbers AYA2016-75808-R and RTI2018-096188- B-I00. J.H.K. acknowledges financial support from the European Union’s Horizon 2020 research and innovation programme under Marie Sk lodowska-Curie grant agreement No 721463 to the SUNDIAL ITN network, from the State Research Agency (AEI-MCINN) of the Spanish Ministry of Science and Innovation under the grant “The structure and evolution of galaxies and their central regions” with reference PID2019- 105602GB-I00/10.13039/501100011033, and from IAC project P/300724, financed by the Ministry of Science and Innovation, through the State Budget and by the Canary Islands Department of Economy, Knowledge and Employment, through the Regional Budget of the Autonomous Community.
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