Black holes of unexpected mass dominate small galaxies in the distant universe

Astronomers have discovered that the supermassive black holes at the centers of early galaxies are much more massive than previously thought. These surprisingly large black holes offer new insights into the origins of all supermassive black holes, as well as the early stages of the life of their host galaxy.

In nearby mature galaxies like our Milky Way, the total mass of stars far exceeds the mass of the large black hole at the galaxy’s center by about 1,000 to 1. In new, distant galaxies, however, this difference in mass falls to 100 or 10 to 1, and even 1 to 1, meaning the black hole can equal the combined mass of the stars in its host galaxy.

This image of unexpectedly massive black holes in young galaxies comes from the James Webb Space Telescope (JWST), NASA’s latest flagship observatory. Until JWST, launched in late 2021, astronomers were generally limited in their studies of distant black holes to incredibly bright quasars, composed of monstrous, matter-devouring black holes that completely eclipse the stars in their host galaxies.

“With JWST, we can finally observe lower-mass, but still supermassive, black holes in small, distant galaxies, and we can also see the stars in these host galaxies,” says Fabio Pacucci, Clay Fellow at the Center for Astrophysics. | Harvard and Smithsonian (CfA). “This allows us to study, for the first time, early black holes and their host galaxies as they evolve together.”

Pacucci is the lead author of a new study published in Letters from the astrophysical journal reporting the results and presented these results at the 243rd meeting of the American Astronomical Society in New Orleans, LA.

“We learned that young, distant galaxies violate the relationship between black hole mass and stellar mass that is very well established in nearby mature galaxies: these primitive black holes are undoubtedly overmassive compared to the stellar population of their hosts”, explains Roberto Maiolino. , professor at the University of Cambridge (United Kingdom) and co-author of the study. “With JWST, it will be possible to identify how the first supermassive black holes formed by finding black holes that are more distant and smaller than those found so far, and which our study predicts will be quite abundant.”

For the study, Pacucci and his colleagues performed a statistical analysis of a set of 21 galaxies, located approximately 12 to 13 billion light-years away, and observed using three published JWST surveys.

These 21 galaxies harbor central black holes whose typical mass is estimated to be tens or hundreds of millions of times that of our Sun – still supermassive, but relatively puny compared to the black holes that power most of the distant quasars observed at this time. day, which are billions of times the mass of the sun.

“Overall, we find that black holes in young galaxies observed by JWST are about ten to one hundred times more massive than predicted by the scaling relationship in the local universe,” says Xiaohui Fan, a professor at the University of Arizona and co-author of the study. “The ratio between the mass of stars and the mass of black holes in the first galaxies was much lower then, more than a dozen billion years ago, than it is today. This result important implications for the study of the first population of black holes.”

The precise estimate of this ratio should help indicate the origin of supermassive black hole precursors – nicknamed black hole seeds. Generally speaking, astronomers have outlined two main pathways: “light” or “heavy” seeds.

The seeds of light black holes would have a relatively low mass, about 100 to 1,000 times the mass of the sun. These luminous seeds would have formed as the remains of the very first colossal stars in the universe. At the other end, heavy black hole seeds would have started to appear around 10,000 to 100,000 solar masses. Such heavy seeds theoretically arise from the direct gravitational collapse of titanic gas clouds.

The heavy seed pathway, by paving the way for growth from a much higher starting point, should facilitate the timely formation of the first-ever supermassive black holes that the team of researchers discovered over the past twenty years. recent years at increasingly greater distances. The new discoveries of supermassive black holes lend credence to the idea of ​​heavy seeds, as simulations and theoretical calculations of this pathway predict that black holes should be about as massive, or even more massive, than the stellar component of young galaxies in which they live.

How galaxies then took shape and co-evolved around the primitive seeds of black holes remains a largely open astrophysical question. Did black holes grow primarily by sucking in gas or by merging with other black holes? And did the stellar mass accumulate mainly inside the galaxy, or were mergers with other larger galaxies necessary? Pacucci and this team expect the answers to begin to materialize, however, with additional JWST studies.

“Over cosmic time, we know that the ratio of star to black hole mass gradually catches up to the local 1,000 to 1 ratio of the modern universe. This occurs as the black hole and its system “host galaxies evolve together, merging with other galaxies and forming legions of stars,” says Pacucci. “What we’re still working on is seeing deep enough into the universe to piece together how this all started.”

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