Paper Title: Massive Black Hole Seed Formation in Strong X-ray Environments at High Redshift
Authors: Kazutaka Kimura, Kohei Inayoshi, Kazuyuki Omukai
First Author’s Institution: Astronomical Institute, Graduate School of Science, Tohoku University, Aoba, Sendai, Japan
Status: submitted to the Astrophysical Journal [open access]
Environments of Massive Black Holes
Astronomers have found that nearly every observable galaxy hosts a central supermassive black hole (SMBH) – a black hole over a million times more massive than the sun. Using the James Webb Space Telescope (JWST), they have peered back into the early universe to observe some of the earliest galaxies, and these early galaxies also contain SMBHs. In fact, many of these galaxies seem to contain SMBHs that are more massive than we expected them to be this early in the universe.
We believe that most black holes form at the ends of the lives of massive stars, which collapse under their own gravity when they run out of fuel. We expect that this happened to the first stars too, forming “light” (as in “lightweight”) seeds of SMBHs with masses up to about 100 solar masses. Astronomers also think that the right conditions in the early universe could lead to the formation of a “heavy” seed with a mass on the order of 104 solar masses or more; this kind of black hole is called a direct collapse black hole (DCBH).
In order to form a DCBH, a gas cloud needs to collapse without fragmenting to form star clusters and stars – if this happens, there won’t be enough mass funneled to the center to form the DCBH. One important criterion for this to happen is the absence of coolants, like metals and molecular hydrogen (H2), which cause fragmentation. One way to eliminate H2 is with a particular type of ultraviolet (UV) light called Lyman-Werner radiation, which can destroy H2 molecules. However, recent results from the Hydrogen Epoch of Reionization Array (HERA) suggests that early galaxies emitted more X-rays than expected, which would increase the amount of H2 and make it less likely for DCBHs to occur. Today’s authors investigate the feasibility of forming DCBHs in these X-ray-bright environments.
Modeling Black Hole Formation
Today’s authors model the collapse of gas in dark matter halos that have a nearby neighboring halo. This neighboring halo emits radiation like UV light and X-rays that impact the formation of molecules like H2, which affects the gas chemistry. This radiation is typically emitted by hot gas in star-forming regions. The authors also track quantities like the gas density and temperature as the gas collapses. Once the gas collapses enough, a star forms at the center and continues to accrete surrounding material. Once this star evolves and reaches the end of its life, it becomes a black hole.
The authors considered different intensities of X-ray radiation relative to a critical value of Lyman-Werner radiation believed to destroy enough H2 to form a DCBH, which is called J21. They explore values of the X-ray intensity Jx relative to J21, considering ratios of 0, 10-6, 10-5, and 10-4.
Another important consideration was the baryonic streaming velocity. This is the relative bulk velocity between baryonic (i.e., “normal”) matter and dark matter in the early universe. Higher streaming velocities can delay the collapse of gas in dark matter halos, which affect gas properties like density, temperature, and chemistry. The streaming velocity depends on the redshift z, which is given by σbsm = 30 km/s (1+z)/(1+z0), where z0 = 1100. This “standard” relationship has been calibrated with measurements from observations. The authors consider two cases for the streaming velocity: vbsm = 0 and vbsm = 1σbsm, which are denoted in the figures below.
X-Rays and Seed Suppression
The first main results are shown in Figure 1. The authors find that a large number of intermediate-mass seeds form from H2 cooling, shown in blue, regardless of the X-ray intensity. A smaller number of higher-mass seeds form from H-H2 and H-H cooling, but these cooling processes become suppressed as the X-ray intensity increases. This is because the increased X-ray intensity converts more H to H2. Interestingly, accounting for the streaming velocity all but wipes out black hole formation via H2 cooling except at very high X-ray intensities.
Another main set of results are the black hole mass functions (that is, the number density of black holes for a given black hole mass), shown in Figure 2. Note that this differs from Figure 1 because we are now accounting for the number density of black holes, not just the number, and we are no longer categorizing black holes based on their cooling mechanism. As before, we see that most massive black holes are eliminated with a high X-ray intensity and no streaming velocity, but a higher streaming velocity tends to produce more massive black holes regardless of the X-ray intensity. These black hole mass functions provide a useful comparison to observational results from telescopes like JWST.
In summary, today’s authors find that a high X-ray intensity leads to less massive black holes due to the increased amount of H2, but accounting for the baryonic streaming velocity can produce massive black holes even with a large amount of X-rays. The number densities of these black holes appear to agree with observations from JWST, meaning that this could be a promising formation mechanism for the observed SMBHs in the early universe.
Astrobite edited by Sparrow Roch
Featured image credit: T. Klein, University of Wisconsin-Milwaukee
