Dynamically defined bulges reveal that early-type galaxies with compact bulges host more massive black holes
Abstract
We revisit the MBH−Mbulge relation for early-type galaxies by introducing a uniform bulge defined by dynamical decomposition. From the set of nearby galaxies with accurately measured black hole masses, we select 30 early-type galaxies that have been observed with wide-field integral-field unit instruments (MUSE, ATLAS3D, or PPAK). For each galaxy, we construct an orbit-superposition Schwarzschild model and obtain its stellar orbit distribution. In this framework, we define the bulge uniformly as comprising stellar orbits with circularity λz < 0.7 (or, in an alternative definition, λz < 0.2) and lying within r < 3 kpc. From these orbits, we reconstruct the spatial density distribution of the bulge and measure its mass (Mbulge), size (Rbulge) and surface density slope (γbulge). We find that the dynamical bulge defined by λz < 0.7 yields masses that are systematically consistent with the photometric bulge masses. Comparing to the bulge from photometric decomposition, using this dynamical bulge definition slightly decreases the scatter around the best-fitting from 0.56 to 0.47. More interestingly, we identify a clear trend: at a given bulge mass, galaxies hosting more compact bulges (i.e. smaller bulge sizes or steeper bulge density slopes) systematically contain more massive black holes; eight outliers show overmassive BHs together with ultra-compact bulges. We interpret this as direct observational evidence that the MBH−Mbulge relation evolves over a galaxy's merger history, with that history imprinted in bulge compactness. Incorporating compactness via an MBH−Mbulge−Re, bulge (or γbulge) relation brings the outliers into agreement and reduces the overall scatter of the sample to ∼ 0.3.
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BibTeX
@misc{shi2026dynamically,
title = {{Dynamically defined bulges reveal that early-type galaxies with compact bulges host more massive black holes}},
author = {Hanjing Shi and Ling Zhu and Sabine Thater and Yong Shi and Ana L. Chies-Santos and Luis Gabriel Dahmer Hahn and Zhengya Zheng},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.01469},
eprint = {2611.01469},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.01469},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Dynamically defined bulges reveal that early-type galaxies with compact bulges host more massive black holes
AU - Hanjing Shi
AU - Ling Zhu
AU - Sabine Thater
AU - Yong Shi
AU - Ana L. Chies-Santos
AU - Luis Gabriel Dahmer Hahn
AU - Zhengya Zheng
PY - 2026
DA - 2026/11/01
PB - arXiv
DO - 10.48550/arxiv.2611.01469
UR - https://arxiv.org/abs/2611.01469
AB - We revisit the M_BH−M_bulge relation for early-type galaxies by introducing a uniform bulge defined by dynamical decomposition. From the set of nearby galaxies with accurately measured black hole masses, we select 30 early-type galaxies that have been observed with wide-field integral-field unit instruments (MUSE, ATLAS3D, or PPAK). For each galaxy, we construct an orbit-superposition Schwarzschild model and obtain its stellar orbit distribution. In this framework, we define the bulge uniformly as comprising stellar orbits with circularity λ_z < 0.7 (or, in an alternative definition, λ_z < 0.2) and lying within r < 3 kpc. From these orbits, we reconstruct the spatial density distribution of the bulge and measure its mass (M_bulge), size (R_bulge) and surface density slope (γ_bulge). We find that the dynamical bulge defined by λ_z < 0.7 yields masses that are systematically consistent with the photometric bulge masses. Comparing to the bulge from photometric decomposition, using this dynamical bulge definition slightly decreases the scatter around the best-fitting from 0.56 to 0.47. More interestingly, we identify a clear trend: at a given bulge mass, galaxies hosting more compact bulges (i.e. smaller bulge sizes or steeper bulge density slopes) systematically contain more massive black holes; eight outliers show overmassive BHs together with ultra-compact bulges. We interpret this as direct observational evidence that the M_BH−M_bulge relation evolves over a galaxy's merger history, with that history imprinted in bulge compactness. Incorporating compactness via an M_BH−M_bulge−R_e, bulge (or γ_bulge) relation brings the outliers into agreement and reduces the overall scatter of the sample to ∼ 0.3.
ER -