The Early Universe Had a Lot of Black Holes

The Hubble Deep Field and its successor, the Hubble Ultra-Deep Field, showed us how vast our Universe is and how it teems with galaxies of all shapes and sizes. They focused on tiny patches of the sky that appeared to be empty and revealed the presence of countless galaxies. Now, astronomers are using the Hubble Ultra-Deep Field and follow-up images to reveal the presence of a large number of supermassive black holes in the early Universe.

This is a shocking result because, according to theory, these massive objects shouldn’t have been so plentiful billions of years ago.

The Hubble Ultra-Deep Field (HUDF) was released in 2004 and required almost one million seconds of exposure over 400 of the telescope’s orbits. Over the years, the same region has been imaged with other wavelengths and been updated and refined in other ways.

The Hubble has re-imaged the region multiple times, and astronomers have compared the new images to older images and identified more SMBHs from the Universe’s early times.

The results are in a paper titled “Glimmers in the Cosmic Dawn: A Census of the Youngest Supermassive Black Holes by Photometric Variability, ” which was published in The Astrophysical Journal Letters. Matthew Hayes, an associate professor in the Department of Astronomy at Stockholm University, Sweden, is the lead author.

Supermassive Black Holes (SMBHs) sit in the center of large galaxies like ours. While the hole itself isn’t visible, material being drawn into the hole collects in an accretion disk. As that material heats, it gives off light as an active galactic nucleus (AGN). Since black holes feed sporadically, only a portion of them were visible in the original HUDF. By re-imaging the same field at different times, the Hubble captured additional SMBHs that weren’t originally visible.

Our understanding of the ancient Universe and how it and its galaxies evolved depends on several factors. One of them is the requirement for an accurate idea of the number of AGN. AGN can be difficult to spot, and this method overcomes some of the obstacles.

AGN can emit X-ray, radio, and other emissions, but they don’t always stand out. “The challenge to this field comes from the fact that identifying AGN at the luminosity regimes of typical galaxies is observationally difficult,” the authors write. “This leads to SMBHs probably being undercounted, with potentially large numbers going unnoticed among the ostensibly star-forming galaxy population at high-z.”

The authors’ photometric variability method circumvents that. Since AGN accrete material at variable rates, observing changes in output from AGN is a better method of determining how many there are. “Here, we argue that the photometric variability that results from changes in the mass accretion rate of SMBHs can provide a completely independent and complementary probe of AGN,” Hayes and his co-authors write. “Monitoring for variability selects AGN from imaging data directly by phenomena related to the SMBH, without any biases of photometric preselection (color, luminosity, compactness, etc).”

The new paper presents preliminary results and reports the detection of eight interesting targets that display variability. Three of the eight are probably supernovae, two are clear AGN at about z = 2–3, and three more are likely AGN at redshifts greater than 6.

These findings are significant because they impact our understanding of black holes, how they form, and their place in the history of the Universe.

Astronomers understand how stellar-mass black holes form. They also believe that supermassive black holes grow so massive through mergers with other black holes. They’re even making progress in finding the in-between black holes called intermediate-mass black holes (IMBHs).

Since astronomers think that SMBHs grow through mergers, there should be more of them in the modern Universe and comparatively few, if any, in the ancient Universe. There simply hadn’t been enough time for enough mergers to take place to create SMBHs. That’s why there are alternate theories to explain black holes in the early Universe.

Astronomers theorize that a different type of star existed in the early universe. These massive, pristine stars could only form in the conditions that dominated the early Universe. They could’ve collapsed and become massive black holes.

Another theory suggests that massive gas clouds in the early Universe could have collapsed directly into black holes. Yet another theory suggests that so-called ‘primordial black holes’ could have formed in the first seconds after the Big Bang through purely speculative mechanisms.

The new observations should help clarify some of these ideas.

“The formation mechanism of early black holes is an important part of the puzzle of galaxy evolution,” said study lead author Hayes. “Together with models for how black holes grow, galaxy evolution calculations can now be placed on a more physically motivated footing, with an accurate scheme for how black holes came into existence from collapsing massive stars.”

“These sources provide a first measure of n_SMBH in the reionization epoch by photometric variability,” the authors explain in their paper. They say the sources identified in their work indicate the largest black hole population ever reported for these redshifts. “This SMBH abundance is also strikingly similar to estimates of n_SMBH in the local Universe,” the authors write.

Some theoretical models suggest that there were large numbers of AGN in the reionization epoch. The JWST shows us that there seem to be more SMBHs and AGN than astronomers thought. By finding more SMBHs and AGN, this research is adding to our understanding of black holes and the evolution of the Universe.

But there’s still more work to be done. The researchers think that a larger sample of AGN at high redshifts is needed to reduce uncertainties and strengthen their results, and the JWST can help. “JWST is required to push to detection of fainter AGN via variability,” the authors explain, adding that it would take years of monitoring for the space telescope to do so.

This work also underlines the HST’s ongoing contribution to astronomy. It may not be as powerful as the JWST, but it has the benefit of many years of observations already under its belt and keeps proving its worth as a powerful observatory in its own right.

“In contrast, HST’s legacy of deep NIR imaging already stretches back about 15 yr, providing an excellent baseline for monitoring.”