Overmassive Black Holes and Their Place in the Universe

Introduction

This article is a summary of a scientific research paper titled Overmassive black holes at cosmic noon: linking the local and high-redshift Universe that I wrote for my Astrophysics 314 class at Texas A&M.

Supermassive Black Holes (SMBHs) are found at the center of most galaxies across the Universe. These SMBHs grow through both the accretion process and galaxy mergers. Recent observations have revealed the existence of overmassive black holes, which are a category of SMBHs that seem to be too large for their host galaxy based on the normal observations made in the local regions and the predictions of theoretical models. These black holes are very important for our understanding of the evolution of the Universe and its galaxies; however, there is still contention about how these black holes formed. There are two seeding models—light and heavy—that aim to explain the existence of these outliers. These overmassive black holes have been found at a variety of different redshifts in the Universe, ranging from our local region (z < 1) to cosmic noon ( z ~ 1-3) and even the early universe (z > 4).

What Are Overmassive Black Holes?

This paper highlights an analysis of black holes from the cosmic noon era of the Universe, a time in which galaxy and star formation was at its peak. Connections are drawn between the similarities of the physical properties found between intermediate and high-redshift overmassive black holes and their host galaxies, which help to reveal fundamental aspects of galaxy formation and evolution.

Overmassive black holes are SMBHs that have a significantly higher mass than what is expected when compared to their host galaxies and the stars that make them up. These black holes present a major challenge to our current understanding of galaxy evolution and ultimately have changed the models we believe contribute to the formation of supermassive black holes. This paper claims that the most favorable scenario is that of heavy seeding, or the seeding from black holes of ~ 104 - 105 M⊙. These seeds were most likely formed from the direct collapse of pre-galactic gasses instead of the traditional collapse of a star. This allowed the black holes to begin with a much higher mass, potentially contributing to the overmassive we observe in these low mass galaxies today. It is also believed that the data presented in this paper points to evidence that black hole feedback processes play a major role in the development and growth of an overmassive black hole.

Observations and Data

This paper selected the 12 overmassive black holes it used for observations based on 2 criteria. The first criterion a sample had to meet was being in a redshift range of z ~ 1-3. This ensures that the black holes observed come from the cosmic noon era of the Universe, certifying that all of them are equally comparable to each other. The second criterion used was the mass of the host galaxy being less than 1010 M⊙. This selection criterion ensures that the galaxies and black holes observed are low mass galaxies of a high enough redshift to be considered nonlocal.

The paper utilizes spectral energy distribution (SED) techniques in order to determine the mass of the host galaxies. Then, for the samples that data was available, they used X-ray data to verify their results from SED. The galaxies used in this data set range from a log M* of 9.0-9.5 M⊙. The black hole masses were then derived through the width of MgII or CIV/Hɑ broad emission lines (for z < 2 and z > 2 respectively). The overmassive black holes observed range in log MBH from 7.3-8.9 M⊙. From this data, it is clear that the black holes at the center of these low mass galaxies represent a large fraction of the overall mass of the galaxies, a characteristic that is unique to these measurements and is significantly higher than that of local dwarf galaxies. This uniqueness is shown in Figure 1, which displays the relationship between M* vs MBH for different redshift levels of both overmassive and normal black holes. It is clear that the overmassive black holes follow a different trend line than their supermassive counterparts.

This paper also calculates the Eddington ratios for each of the 12 black holes observed. It was found that the ratios ranged from 0.02-0.8, with an average value of 0.2. This implies that the low mass galaxies of this data set are accreting at sub-Eddington rates. These inefficient rates of accretion suggest that there could be a feedback process present, affecting the surrounding environment. These low Eddington ratios are believed to slow the rate of growth of the host galaxy, quenching star formation, and they are likely part of the reason why the galaxies observed evolved to be low mass galaxies.

Insights into Cosmic Evolution

These findings, along with their implications, are major for our understanding of the Universe and the processes and evolutions that help to shape it. The presence of overmassive black holes at the cosmic noon epoch of the Universe presents a challenge to our current understanding of the evolution and growth of black holes. The data presented suggests that black holes in the early Universe underwent phases of rapid growth that still cannot be fully defined.

Also of interest to astronomers is the high level of similarities between the physical properties of these systems and overmassive blackholes at higher redshifts detected by the James Webb Space Telescope (JWST) of z > 4. These similarities entail that, even across multiple different eras in the Universe (i.e. early Universe and cosmic noon), the evolution of black hole growth has developed in the same way. Two of these similarities—bolometric luminosity and Eddington ratio—are compared for three different overmassive systems of redshift. This distribution shows that there are similarities and overlaps in the physical characteristics of the host galaxies and black holes at different points in the evolution of the Universe.

These findings help to emphasize the growth mechanisms of black holes along their galaxies through complicated processes like Active Galactic Nuclei (AGN) outflows and other feedback mechanisms. The evidence and similarities between the intermediate and high redshift overmassive black holes and their host galaxies suggest that the growth of supermassive black holes found in low mass galaxies is consistent across the Universe, regardless of the time it took place. This is a major finding for the field of astronomy and helps to give more detail on how the Universe developed and evolved.

Conclusion

Due to the challenges presented in our understanding by overmassive black holes at both redshifts of z = 1-3 and z > 4, astronomers have been looking for clues as to how black holes can evolve to be such a significant portion of their host, low mass galaxy. Due to the similarities between physical properties—galaxy mass, black hole mass, Eddington ratio, and bolometric luminosity—astronomers have deduced that there is a common evolution of these systems across different epochs of the Universe, and overmassive black holes are not unique to any certain time period. This paper enhances our understanding of black holes and low mass galaxy evolution while calling for further research into the field to deepen our understanding of the processes required to form such systems.