Ultramassive black holes in the most massive galaxies: $M_{\rm BH}-σ$ versus $M_{\rm BH}-R_{\rm b}$ (2012.04471v1)

Abstract: [Abridged] We investigate the nature of the relations between black hole (BH) mass ($M_{\rm BH}$) and the central velocity dispersion ($\sigma$) and, for core-S\'ersic galaxies, the size of the depleted core ($R_{\rm b}$). Our sample of 144 galaxies with dynamically determined $M_{\rm BH}$ encompasses 24 core-S\'ersic 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_{\rm b} < 0.5 $ kpc) and large-core galaxies ($R_{\rm b} \gtrsim 0.5$ kpc), we find that $M_{\rm BH}$ correlates remarkably well with $R_{\rm b}$ such that $M_{\rm BH} \propto R_{\rm b}^{1.20 \pm 0.14}$ (rms scatter in log $M_{\rm BH}$ of $\Delta_{\rm rms} \sim 0.29$ 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 $M_{\rm BH}-\sigma$ relation $M_{\rm BH} \propto \sigma^{ 4.88 \pm 0.29}$ with $\Delta_{\rm rms} \sim 0.47$ dex, however, at the high-mass end large-core galaxies (four with measured $M_{\rm BH}$) are offset upward from this relation by ($2.5-4) \times \sigma_{\rm s}$, explaining the previously reported steepening of the $M_{\rm BH}-\sigma$ relation for massive galaxies. Large-core spheroids have magnitudes $M_{V} \le -23.50$ mag, half-light radii Re $>$ 10 kpc and are extremely massive $M_{*} \ge 10^{12}M_{\odot}$. Furthermore, these spheroids tend to host ultramassive BHs ($M_{\rm BH} \ge 10^{10}M_{\odot}$) tightly connected with their $R_{\rm b}$ rather than $\sigma$. The less popular $M_{\rm BH}-R_{\rm b}$ relation exhibits $\sim$ 62% less scatter in log $M_{\rm BH}$ than the $M_{\rm BH}- \sigma$ relations.

• The paper shows that core size (R₍b₎) is a more reliable predictor of SMBH mass than velocity dispersion, with the relation exhibiting less scatter.
• The study classifies galaxies into Sérsic, normal-core, and large-core types, highlighting that large-core galaxies host ultramassive black holes which deviate from standard scaling laws.
• The paper finds a steepening in the M₍BH₎–σ relation at the high-mass end, suggesting that traditional methods may underestimate black hole masses in these massive systems.

Analysis of Ultramassive Black Holes and Their Galactic Correlations

The paper presented in this paper investigates the relations between black hole mass ($M_{\rm BH}$) and two specific parameters of the host galaxies: the central velocity dispersion ($\sigma$) and the core size ($R_{\rm b}$) of core-Sérsic galaxies. This paper utilizes a substantial sample of galaxies with dynamically determined black hole masses to explore these correlations and their implications, especially in the context of ultramassive black holes (UMBHs) which have masses exceeding $10^{10}M_{\sun}$.

Key Findings

1. Correlation of $M_{\rm BH}$ with Core Size:
   • Among the key findings, the paper emphasizes that the mass of supermassive black holes (SMBHs) correlates strongly with the size of the depleted stellar cores in their host galaxies. The relation $M_{\rm BH} \propto R_{\rm b}^{1.20 \pm 0.14}$ displays less scatter compared to the $M_{\rm BH}-\sigma$ relation, suggesting that the core size is a more reliable predictor of SMBH mass for certain galaxy types.
2. Separation into Galaxy Types:
   • The galaxies are classified into Sersic, normal-core, and large-core categories, with the latter category showing distinct properties. Large-core galaxies, often the most massive and brightest, host UMBHs and deviate significantly from the standard $M_{\rm BH}-\sigma$ relation, implying that traditional velocity dispersion-based methods may underestimate SMBH mass in these massive galaxies.
3. Steepening at the High-Mass End:
   • There is a noted steepening of the $M_{\rm BH}-\sigma$ relation at the high-mass end. This steepening is primarily attributed to the large-core galaxies, which are observed to offset upward from the expected trend for less massive systems. The research highlights the need for alternative scaling relations, such as $M_{\rm BH}-R_{\rm b}$, particularly for ultramassive black holes within these large-core systems.

Implications for Galaxy and Black Hole Co-Evolution

The findings imply significant implications for the co-evolution of galaxies and their central black holes, particularly in the most massive elliptical systems often formed through dry mergers. The large-core galaxies, with their distinct black hole mass signatures, suggest a history of significant merger events that did not substantially increase the velocity dispersion of their stars. This aligns with hierarchical formation theories predicting that large spheroids grow through successive mergers, where black hole masses simply add up without major changes in the galaxy's velocity dispersion.

Future Directions

The paper opens avenues for future research to explore the dynamics and formation scenarios of these massive structures further. As observations improve in sensitivity, especially with the advent of facilities capable of resolving finer details in these distant and massive systems, the role of UMBHs in galaxy evolution can be more precisely determined. Furthermore, this research is pertinent to gravitational wave astronomy, contributing to the paper of merging SMBHs detectable via pulsar timing arrays and the upcoming Laser Interferometer Space Antenna (LISA).

Conclusion

This rigorous analysis underscores the importance of core-Sérsic galaxies in understanding SMBH scaling relations. The introduction of the $M_{\rm BH}-R_{\rm b}$ relation offers a more accurate framework in predicting SMBH masses in massive galaxies. Consequently, these findings necessitate a reassessment of existing galaxy formation models and emphasize the nuanced role of different galactic properties in influencing black hole growth trajectories.