Linking black-hole growth with host galaxies: The accretion-stellar mass relation and its cosmic evolution (1710.09399v2)

Abstract: Previous studies suggest that the growth of supermassive black holes (SMBHs) may be fundamentally related to host-galaxy stellar mass ($M_\star$). To investigate this SMBH growth-$M_\star$ relation in detail, we calculate long-term SMBH accretion rate as a function of $M_\star$ and redshift [$\overline{\rm BHAR}(M_\star, z)$] over ranges of $\log(M_\star/M_\odot)=\text{9.5--12}$ and $z=\text{0.4--4}$. Our $\overline{\rm BHAR}(M_\star, z)$ is constrained by high-quality survey data (GOODS-South, GOODS-North, and COSMOS), and by the stellar mass function and the X-ray luminosity function. At a given $M_\star$, $\overline{\rm BHAR}$ is higher at high redshift. This redshift dependence is stronger in more massive systems (for $\log(M_\star/M_\odot)\approx 11.5$, $\overline{\rm BHAR}$ is three decades higher at $z=4$ than at $z=0.5$), possibly due to AGN feedback. Our results indicate that the ratio between $\overline{\rm BHAR}$ and average star formation rate ($\overline{\rm SFR}$) rises toward high $M_\star$ at a given redshift. This $\overline{\rm BHAR}/\overline{\rm SFR}$ dependence on $M_\star$ does not support the scenario that SMBH and galaxy growth are in lockstep. We calculate SMBH mass history [$M_{\rm BH}(z)$] based on our $\overline{\rm BHAR}(M_\star, z)$ and the $M_\star(z)$ from the literature, and find that the $M_{\rm BH}$-$M_\star$ relation has weak redshift evolution since $z\approx 2$. The $M_{\rm BH}/M_\star$ ratio is higher toward massive galaxies: it rises from $\approx 1/5000$ at $\log M_\star\lesssim 10.5$ to $\approx 1/500$ at $\log M_\star \gtrsim 11.2$. Our predicted $M_{\rm BH}/M_\star$ ratio at high $M_\star$ is similar to that observed in local giant ellipticals, suggesting that SMBH growth from mergers is unlikely to dominate over growth from accretion.

• The paper demonstrates that, for a given stellar mass, SMBH accretion rates are significantly higher at greater redshifts, highlighting strong cosmic evolution.
• The paper shows that the SMBH-to-star formation rate ratio increases with galaxy mass, challenging traditional models of synchronized growth.
• The paper suggests that accretion, rather than mergers, predominantly drives SMBH growth, with the SMBH-to-stellar mass ratio showing only weak evolution since z ≈ 2.

Linking Black-Hole Growth with Host Galaxies: The Accretion-Stellar Mass Relation and Its Cosmic Evolution

The paper by G. Yang et al. investigates the relationship between supermassive black hole (SMBH) growth and host-galaxy stellar mass, expressing it as a function of stellar mass ($M_*$) and redshift ($z$). This paper spans a broad range of cosmic history, from $z = 0.4$ to $z = 4$, and focuses on galaxies with stellar masses ($\log(M_*/M_\odot)$) between 9.5 and 12. The research leverages extensive observational data, including deep field surveys like GOODS-South, GOODS-North, and COSMOS, to better constrain SMBH accretion rates and their dependence on both $M_*$ and redshift.

Key Findings

1. Accretion Rate Dependence on Stellar Mass and Redshift: The research shows that, at a given stellar mass, SMBH accretion rates are significantly higher at higher redshifts, indicating a strong cosmic evolution of SMBH activity. The accretion rate scaling with stellar mass is particularly pronounced in massive systems, with differences spanning several orders of magnitude across the redshift range.
2. SMBH and Galaxy Growth Discrepancy: The results do not support the traditional notion that SMBH and their host galaxies grow in lockstep. The SMBH accretion to star formation rate ratio ($\dot{M}_{\rm BH}/$SFR) is not constant; it becomes larger in massive galaxies, contradicting simplistic co-evolution models.
3. Weak Redshift Evolution of the SMBH-Stellar Mass Relation: The paper calculates the SMBH mass history and predicts that the SMBH to stellar mass ratio shows only weak evolution since $z \approx 2$. The ratio tends to be larger for massive galaxies, suggesting that SMBH accretion heavily influences massive galaxy formation.
4. Implications for SMBH Mergers: The findings imply that accretion, rather than mergers, is the principal growth mechanism for SMBHs, particularly in local giant ellipticals, because the predicted accretion-dominated growth fits well with observed relations in these systems.

Theoretical and Observational Implications

The results have several important implications. The increasing SMBH accretion-to-stellar mass ratio toward massive galaxies suggests that SMBHs in large galaxies might be more effective in accreting material, potentially due to deeper gravitational wells or variations in SMBH occupation fractions. This evolution pattern imposes constraints on theoretical models of galaxy and SMBH evolution, especially those involving AGN feedback processes, which are crucial for regulating accretion and star formation in massive systems.

Observationally, these findings motivate further investigation into the high-redshift epoch, particularly around $z \approx 2$, where significant SMBH growth occurs. Current and forthcoming surveys, with enhanced sensitivity and larger coverage, could provide more comprehensive data to refine the scaling relations defined here.

Moving forward, it will be essential to integrate these observational results with cosmic models that incorporate both accretion and merging processes to better understand the observed universal SMBH-galaxy relations. These models will need to account for the complexity of galaxy evolution, including the circumstances under which SMBH feedback becomes dominant, and under what conditions the observed deviations from the standard coevolutionary scenario arise.

This paper effectively bridges observational data with theoretical models, offering insights that are crucial for the ongoing efforts to grasp the broader narrative of cosmic structure formation and evolution over time.