Unexpectedly massive black holes dominate small galaxies in the distant universe

The supermassive black holes in the nuclei of early galaxies are far more enormous than previously thought, according to astronomers. These unexpectedly enormous black holes provide fresh perspectives on the beginnings of the lives of their host galaxies and the genesis of all supermassive black holes.

The total mass of stars in neighboring, mature galaxies, like our Milky Way, much exceeds the mass of the massive black hole located at the galaxy's core by a ratio of around 1,000 to 1. But in the recently discovered distant galaxies, that mass disparity decreases to 100 or even 10 to 1, and in some cases, to 1 to 1, suggesting that the mass of the black hole can match the total mass of the stars in its host galaxy.

This image, taken with NASA's newest flagship telescope, the James Webb Space Telescope (JWST), shows unusually enormous black holes in young galaxies. Astronomers' study of distant black holes were mostly restricted to quasars, which are incredibly luminous black holes that eat matter and utterly outshine the stars in their home galaxies, until JWST launched in late 2021.

"We can now see the stars in these host galaxies as well as lower-mass, yet still supermassive black holes in small, distant galaxies thanks to JWST," explains Fabio Pacucci, a Clay Fellow at the Center for Astrophysics | Harvard & Smithsonian (CfA). "This allows us to study, for the first time, early black holes and their host galaxies as they evolve together."

Lead author Pacucci presented the findings of a recent research at the 243rd meeting of the American Astronomical Society in New Orleans, Louisiana. The work was published in The Astrophysical Journal Letters.

Roberto Maiolino, a professor at the University of Cambridge (UK) and co-author of the study, says, "We have learned that distant, young galaxies violate the relation between black hole mass and stellar mass that is very well established in nearby, mature galaxies: these primeval black holes are undoubtedly overmassive relative to the stellar population of their hosts." "With JWST, it will be possible to pinpoint how the first supermassive black holes formed by finding black holes that are farther and smaller than those found so far, and which our study predicts to be quite abundant."

Pacucci et al. conducted a statistical analysis of 21 galaxies that were seen by three published JWST surveys and ranged in distance from around 12 to 13 billion light-years.

These 21 galaxies contain central black holes that are estimated to have masses tens or hundreds of millions of times that of the sun. These black holes are still supermassive, but they pale in comparison to the black holes that power the majority of the far-off quasars that have been observed so far, which have masses billions of times that of the sun.

According to co-author of the paper Xiaohui Fan, a professor at the University of Arizona, "overall, we see that black holes in the young galaxies observed by JWST are about ten to a hundred times more massive than the scaling relation in the local universe predicts." More than a dozen billion years ago, the ratio of star mass to black hole mass in early galaxies was far smaller than it is now. This finding has significant ramifications for our understanding of the early black hole population."

Precisely calculating this ratio ought to provide insight into the evolutionary history of supermassive black hole progenitors, also known as black hole seeds. Astronomers have roughly identified two major routes: "light" or "heavy" seeds.

About 100–1,000 times the mass of the sun would have been the comparatively modest mass of light black hole seeds. These light seeds would have originated as pieces of the first massive stars in the cosmos. Conversely, hefty black hole seeds would have originated at a mass of between 10,000 and 100,000 solar masses. Theoretically, massive seeds like this originated from the direct gravitational collapse of massive gas clouds.

The heavy seed method should help the prompt development of the extremely early supermassive black holes that the study team has found at increasingly larger distances over the previous twenty years, by providing the conditions for growth from a much higher starting point. The heavy seed theory is supported by the latest discoveries of overmassive black holes, as theoretical calculations and simulations of this pathway indicate that black holes should be at least as massive as the star component of the young galaxies they inhabit.

Astrophysically speaking, how galaxies then took shape and co-evolved around the primordial black hole seeds is still up for debate. Did the black holes form by merging with other black holes or by absorbing gas? And was it mostly within the galaxy that the star mass accumulated, or were mergers with larger galaxies required? However, Pacucci and this colleagues anticipate that with further JWST research, answers will start to emerge.

We know that the star to black hole mass ratio gradually approaches the local 1,000 to 1 ratio of the current universe over cosmic time. According to Pacucci, this occurs when the black hole and its host galaxy evolve together, combining to form new galaxies and producing a vast army of stars. "What we're still working on is seeing deeply enough into the universe to piece together how this all got started."

The paper's co-authors, in addition to Xiaohui Fan and Roberto Maiolino, are Stefano Carniani from Scuola Normale Superiore in Pisa, Italy, and Bao Nguyen from the University of Arizona. The JWST surveys that were utilized were the Galaxy Assembly with NIRSpec IFS survey (GA-NIFS), the Cosmic Evolution Early Release Science Survey (CEERS), and the JWST Advanced Deep Extragalactic Survey (JADES).