Study reveals new scaling relations between black hole mass and galaxy properties
A new study using 151 galaxies with dynamically measured black hole masses finds that the size of a galaxy's partially depleted stellar core is a better predictor of black hole mass than stellar velocity dispersion, particularly at the highest mass end. The research identifies a significant upturn in the standard black hole mass–velocity dispersion (M_BH–σ) relation driven by large-core galaxies hosting ultramassive black holes. This matters because it suggests repeated major dry mergers—not just the processes traced by velocity dispersion—are the dominant growth channel for the universe's most massive black holes.
Researchers analyzed a sample of 151 galaxies with dynamically measured supermassive black hole masses to probe two key scaling relations: the well-known M_BH–σ relation (black hole mass versus stellar velocity dispersion) and, for a subsample of 30 core-Sérsic galaxies, the M_BH–R_b relation (black hole mass versus the radius of the galaxy's central light-depleted core). The updated M_BH–R_b relation follows a power law of M_BH ∝ R_b^(1.16 ± 0.10) and exhibits roughly 30–47% less scatter in log(M_BH) than the M_BH–σ relation for the same sample. Sérsic and normal-core galaxies (R_b < 0.5 kpc) together define a log-linear M_BH–σ relation with a slope of 4.95 ± 0.29, but large-core galaxies (R_b > 0.5 kpc)—including six with direct black hole mass measurements—deviate significantly upward, lying 1–4 times the intrinsic scatter (0.39 dex) above this relation. The authors interpret this high-mass upturn as a consequence of successive major, gas-poor (dry) mergers, which grow both the core size and black hole mass while adding relatively little to the galaxy's velocity dispersion. This scenario also naturally explains an observed flattening of the σ–luminosity relation at absolute magnitudes brighter than M_V = −23.5 mag, providing a coherent picture of how the most extreme black holes and their host galaxies co-evolve.
What's missing
The study relies on a sample of 30 core-Sérsic galaxies for the M_BH–R_b relation, with only six large-core galaxies having direct black hole mass measurements; the remainder use indirect estimates, which may introduce systematic uncertainties not fully quantified here. The paper does not discuss whether selection biases in the galaxy sample (e.g., observational limits on resolving cores or measuring dispersions) could affect the inferred slope or scatter of either relation. Additionally, the dry merger interpretation is presented as the most natural explanation but is not directly tested against simulations or alternative formation scenarios within this work.
What different sources said
- arXiv astro-phCenter
The $M_{\rm BH}$$-$$R_{\rm b}$ relation and the high-mass end of the $M_{\rm BH}$$-$$\sigma$ relation
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