Clues from Quasars in the Early Universe

Supermassive black holes power the bright nuclei of young galaxies in the early universe — quasars. But how do these black holes gain supermassive status in less than a billion years?

Supermassive Centers

Quasars are truly superlative objects — they are the brightest objects we know of, and some astronomers claim that they’re also the most interesting! Quasars are so luminous that we can observe them at outrageous distances — the most distant quasar discovered shines from roughly 13 billion light-years away. This vast distance means that we see quasars as they were when the universe was less than a billion years old, on the edge of the epoch of reionization, when the first stars and galaxies suffused the universe with photons and put an end to the cosmic dark ages.

These ultra-bright objects are thought to be the nuclei of young galaxies, powered by the accretion of material onto a central supermassive black hole. The presence of supermassive black holes so early in the universe’s history poses a challenge for theorists. How, exactly, does a black hole amass so much material in just a few hundred million years?

Sensitive Spectroscopy

The size of a supermassive black hole in the early universe is determined by the masses of the smaller black hole “seeds” from which it forms as well as the rate at which it accretes gas from its surroundings. In order to determine the masses and accretion rates of young supermassive black holes, a team led by Jinyi Yang (Steward Observatory, University of Arizona) analyzed infrared spectra of 37 quasars with redshifts between 6.3 and 7.64 — roughly 700 to 900 million years after the Big Bang.

Yang and collaborators calculated the masses of the black holes in their sample to be in the range of 300 million to 3.6 billion solar masses and found that they accrete at 0.26 to 2.3 times the Eddington limit — the theorized point at which the outward push of radiation generated by accretion is so strong that it balances the inward pull of gravity.

Gauging Growth

The authors estimate that the black holes in their sample must have arisen from black hole seeds no smaller than 1,000 to 10,000 solar masses — but it’s not clear what process could generate these seeds. The collapse of the first generation of stars is one possibility, but these massive stars likely only formed black holes of a few hundred solar masses. Another option is the collapse of gas clouds directly into black holes without first forming stars, but this process is thought to be extremely rare.

Even if the seeds are small, rapid accretion could still bulk these early-universe black holes up to the masses we observe. However, in order to reach billions of solar masses, early-universe black holes would need nearly a billion years of sustained accretion at a rate far exceeding the Eddington limit — anywhere from a few to a few thousand times this limit, depending on the seed mass — and most of the quasars in this study are accreting too slowly. The jury is still out on how supermassive black holes in the early universe gain their impressive size — perhaps in addition to being the most luminous and most interesting objects in the universe, quasars are also the most mysterious!

Citation

“Probing Early Supermassive Black Hole Growth and Quasar Evolution with Near-infrared Spectroscopy of 37 Reionization-era Quasars at 6.3 < z ≤ 7.64,” Jinyi Yang et al 2021 ApJ 923 262. doi:10.3847/1538-4357/ac2b32