There Are Many Ways Galaxies and Supermassive Black Holes Dance

Title: Diverse pathways for supermassive black hole-galaxy coevolution

Authors: Bryan A. Terrazas, James Aird, and Alison L. Coil

First Author’s Institution: Department of Physics & Astronomy, Oberlin College, USA

Status: Preprint on Arxiv

Supermassive black holes (SMBHs) play a key role in galaxy evolution, co-evolving with their host galaxies in a process likened to a cosmic dance. However, the intricate “steps” of this co-evolution remain poorly understood. When SMBHs accrete matter, they produce immense heat, which affects their surroundings in two key ways: they suppress gas cooling and expel cold gas from the galaxy via powerful jets. This phenomenon, known as black hole feedback, reduces the availability of cold gas needed for star formation. This can result in ‘quieter’ galaxies, with limited fuel for star formation, called quiescent galaxies.

The effect of the black hole’s growth driven by accretion can be seen in the local galaxy population. Observations show that the SMBH mass correlates with galaxy properties such as the galaxy stellar mass. However, this relationship exhibits significant scatter, particularly for overmassive SMBHs in galaxies with low star formation activity. Galaxy simulations often rely on SMBH feedback to suppress star formation, but the implementation of feedback mechanisms varies widely across state-of-the-art simulations. Developing a proper physical model is challenging due to the vast range of scales involved in SMBH and galaxy evolution. 

In this work, the authors present an empirical model of SMBH growth, leveraging observational data to investigate the diverse pathways of galaxy-BH co-evolution. Their study focuses on the evolution of MBH​-M∗ (SMBH mass–galaxy stellar mass) relation from a redshift of z=2 (early universe) to the present day (z=0), shedding light on how populations of SMBHs grow alongside their host galaxies.

Setting the Stage for the Dance 

The authors use a model called UniverseMachine, which uses data about SMBH growth (specifically, the black hole accretion rate) and star formation between redshifts z=0 and z=2 to constrain black hole growth. They start with the MBH-M* star relation in the local universe (z=0) and then trace the growth of the black holes backward in time to the early universe. Each time step in the model is called a “snapshot,” and for each snapshot, they assign an accretion rate to the black holes based on the galaxy’s star formation rate to calculate the black hole masses. This way, they build an evolution of the BH masses from today to the early universe. 

The MBH​-M∗​ relation used in the study is different for two types of galaxies: quiescent (inactive) and star-forming. In the local universe (z=0) the quiescent galaxies tend to host more massive SMBHs than the star-forming ones. In Figure 1, the authors show the evolution of the MBH​-M∗​ relation over time (redshift, in this case). Red contours represent quiescent galaxies, while blue contours represent star-forming galaxies. The figure reveals that quiescent galaxies have always hosted larger SMBHs, even at z=2, while the star-forming galaxies have undermassive SMBHs, many of which seem to disappear from the plot at z=2. This is likely because either their stellar mass (M*) falls outside the plotted range, or their SMBHs grew primarily at later times (between z=2 and z=0). Additionally, the slope of the MBH-M∗​ relation becomes shallower at higher redshifts, suggesting slower SMBH growth relative to stellar mass in the quiescent galaxies.

Insights from the Dance

Figure 1 also suggests the undermassive black holes in star-forming galaxies have grown rapidly, accumulating mass over time to produce the distribution at z=0. To investigate this further, the authors tracked the evolution of individual SMBHs on the MBH-M* figure. They tested two variations of their model, where the accretion rate was biased toward galaxies with overmassive SMBHs, to see if these SMBHs could grow more. However, they found minimal growth for overmassive SMBHs in any scenario. This implies that these SMBHs accumulated most of their mass before z=2, while under massive SMBHs grew steadily from z=2 to z=0. This result aligns with recent JWST findings that show over massive SMBHs in the early universe, suggesting early growth for these black holes.

The authors also explored two additional models for the MBH-M∗​ relation: 1) a model where the relation depends on the star formation rate (SFR), leading to more scatter, and 2) a model with a tighter correlation, where MBH-M∗​​ is less connected to SFR. Figure 2 compares these models, and Figure 3 shows their effects on SMBH growth histories. The models with more scatter produce a wide range of SMBH growth patterns, showing the rapid growth of the undermassive black holes. In contrast, the model with the tighter correlation, with no dependence on star formation, results in a narrow range of MBH growth trajectories. In the model with scatter added, horizontal tracks (see Figure 2) are observed for over massive SMBHs, indicating significant stellar mass growth but minimal SMBH growth. Similarly, for undermassive SMBHs, the authors observe more vertical tracks (see Figure 2), suggesting rapid SMBH growth at later times.

The authors of today’s paper have produced an empirical model of SMBH and galaxy evolution that is computationally inexpensive, in contrast to the resource-intensive galaxy simulations. Their findings imply that some galaxies grow their SMBHs earlier, while others do so later. This suggests galaxy formation models may need to allow for SMBHs to form and grow gradually over time to capture this variety. The study also highlights the importance of including the scatter in the MBH-M*​ relation to capture the full range of SMBH and galaxy co-evolution histories. Models ignoring this scatter could miss key differences in how the SMBHs co-evolve with their host galaxies. 

Astrobite edited by Junellie Perez

Featured image credit: Pranav Satheesh