Distant blazar discovery supports rapid black hole formation in the early universe

Astronomers have discovered an important piece of the puzzle of how supermassive black holes were able to grow so quickly in the early universe: a special kind of active galactic nucleus so distant that its light has taken more than 12.9 billion years to reach us. This so-called blazar serves as a statistical marker: its existence implies the presence of a large but hidden population of similar objects, all of which should emit powerful particle jets.

This is where the discovery becomes important for cosmic evolution: black holes with jets are thought to be able to grow considerably more quickly than without jets. The research appears in a paper published in Nature Astronomy and another in The Astrophysical Journal Letters.

Active galactic nuclei (AGN) are extremely bright centers of galaxies. The engines driving their enormous energy output are supermassive black holes. Matter falling onto such black holes (accretion) is the most efficient mechanism known to physics when it comes to setting free enormous amounts of energy. That unmatched efficiency is why AGN are able to produce more light than all the stars in hundreds, thousands—or even more—of galaxies put together and in a volume of space smaller than our own solar system.

At least 10% of all AGN are thought to emit focused high-energy beams of particles, known as jets. These jets shoot out from the direct vicinity of the black hole in two opposite directions, sustained and guided by magnetic fields in the “accretion disk” of material: the disk formed by gas swirling around, and falling into, the black hole. For us to see an AGN as a blazar, something very improbable needs to happen: Earth, our base of observations, must be in just the right location for the AGN jet to point directly toward us.

The result is the astronomical analog of someone shining the beam of a really bright flashlight directly into your eyes: a particularly bright object in the sky. Characteristically for a blazar, we also see quick changes in brightness on time scales of days, hours, or even less than that—a consequence of random changes in the swirling accretion disk at the base of the jet and of instabilities in the jet’s interplay between magnetic fields and charged particles.

Finding active galactic nuclei in the very early universe

The new discovery was the result of a systematic search for active galactic nuclei in the early universe conducted by Eduardo Bañados, a group leader at the Max Planck Institute for Astronomy who specializes in the first billion years of cosmic history, and an international team of astronomers.

Since light takes time to reach us, we see distant objects as they were millions or even billions of years ago. For the more distant objects, the so-called cosmological redshift, due to cosmic expansion, shifts their light to far longer wavelengths than the wavelengths at which the light was emitted. Bañados and his team exploited this fact, searching systematically for objects that were redshifted so far that they did not even show up in the usual visible light (of the Dark Energy Legacy Survey, in this case) but that were bright sources in a radio survey (the 3 GHz VLASS survey).

Among 20 candidates that met both criteria, only one designated J0410–0139 met the additional criterion of showing significant brightness fluctuations in the radio regime—raising the possibility that this was a blazar.

The researchers then dug deeper, employing an unusually large battery of telescopes, including near-infrared observations with ESO’s New Technology Telescope (NTT), a spectrum with ESO’s Very Large Telescope (VLT), additional near-infrared spectra with the LBT, one of the Keck telescopes and the Magellan telescope, X-ray images from both ESA’s XMM-Newton and NASA’s Chandra space telescopes, millimeter wave observations with the ALMA and NOEMA arrays, and more detailed radio observations with the US National Radio Astronomy Observatory’s VLA telescopes to confirm the object’s status as an AGN, and specifically a blazar.

The observations also yielded the distance of the AGN (via the redshift) and even found traces of the host galaxy in which the AGN is embedded. Light from that active galactic nucleus has taken 12.9 billion years to reach us (z=6.9964), carrying information about the universe as it was 12.9 billion years ago.

‘Where there is one, there’s one hundred more’

According to Bañados, “The fact that J0410–0139 is a blazar, a jet that by chance happens to point directly towards Earth, has immediate statistical implications. As a real-life analogy, imagine that you read about someone who has won $100 million in a lottery. Given how rare such a win is, you can immediately deduce that there must have been many more people who participated in that lottery but have not won such an exorbitant amount.

“Similarly, finding one AGN with a jet pointing directly towards us implies that at that time, there must have been many AGN in that period of cosmic history with jets that did not point at us.”

Long story short, in the words of Silvia Belladitta, a post-doc at MPIA and co-author of the present publication, “Where there is one, there’s one hundred more.”

Light from the previous record-holder for the most distant blazar has taken about 100 million years less to reach us (z=6.1). The extra 100 million years might seem short in light of the fact we are looking back more than 12 billion years, but they make a crucial difference. This is a time when the universe is changing rapidly. In those 100 million years, a supermassive black hole can increase its mass by an order of magnitude.

Based on current models, the number of AGN should have increased by a factor of five to ten during those 100 million years. Finding that there was such a blazar 12.8 billion years ago would not be unexpected. Finding that there was such a blazar 12.9 billion years ago, as in this case, is a different matter altogether.

Helping black holes grow since 12.9 billion years ago

The presence of a whole population of AGN with jets at that particular early time has significant implications for cosmic history and the growth of supermassive black holes in the centers of galaxies in general. Black holes whose AGN have jets can potentially gain mass faster than black holes without jets.

Contrary to popular belief, it is difficult for gas to fall into a black hole. The natural thing for gas to do is to orbit the black hole, similar to the way a planet orbits the sun, with increased speed as the gas gets closer to the black hole (“angular momentum conservation”). In order to fall in, the gas needs to slow down and lose energy. The magnetic fields associated with the particle jet, which interact with the swirling disk of gas, can provide such a “braking mechanism” and help the gas to fall in.

This means the consequences of the new discovery are likely to become a building block of any future model of black-hole growth in the early universe: they imply the existence of an abundance of active galactic nuclei 12.9 billion years ago that had jets, and thus had the associated magnetic fields that can help black holes grow at considerable speed.