Astronomers have discovered a supermassive black hole scarfing down gas just 690 million years after the Big Bang.
Astronomers are like historians on steroids. They doggedly push back the curtain of cosmic time, peering back to ever-earlier eras in the universe. The latest discovery in this quest, announced today in the journal Nature, is the quasar J1342+0928. This black-hole-powered beacon blazes at us from a redshift of 7.54, or a mere 690 million years after the Big Bang.
This is not the earliest object astronomers have found: they’ve netted galaxies back to a mere 400 million years after It All Began. But J1342+0928 contains the earliest supermassive black hole detected, squeaking into first place some 50 million years ahead of the previous record holder.
Astronomers are actively hunting ancient quasars because they want to understand how the first supermassive black holes formed. To do that, they need to know how many there are in the early universe and how big they are at different times. This requires a census. As part of an ongoing, multi-survey search looking for the most distant quasars, Eduardo Bañados (Carnegie Institution for Science) and colleagues came upon J1342+0928. They were looking for sources shining at us from so far in the past that their light has been severely redshifted by cosmic expansion. J1342+0928 stuck out because it was invisible at shorter wavelengths but showed up at the longer, redder ones expected from objects in this distant era.
Based on J1342+0928’s brightness and how fast the gas whirls around the central black hole — determined thanks to how much the motion broadens the spectral line of singly ionized magnesium — the team estimates that the black hole has a mass of about 800 million Suns. That’s a little lower than the supplanted contender (J1120+0641, at 2 billion Suns) and within the ballpark for other supermassive black holes found a few hundred million years later.
Astronomers have been struggling for some time to understand how the universe grew such colossal black holes in less than a billion years. The options generally are that small black holes ate abnormally fast, or big clouds somehow collapsed directly into big black holes. The discovery of objects like J1342+0928 will one day help answer that question.
Not Just About the Black Hole
Follow-up work, published by several of the same team members and headed up by Bram Venemans (Max Planck Institute for Astronomy, Germany) in Astrophysical Journal Letters, tracked down the radio glow from gas and dust in J1342+0928’s host galaxy. Interpreting the data takes a fair bit of extrapolating, but broadly speaking, the galaxy appears to be small and very dusty.
Another galaxy with the same cosmic age, A1689-zD1, also has a lot of dust — but not as much as J1342+0928. In fact overall, the dustiness of the quasar’s host galaxy is higher than “normal” galaxies seen at similar cosmic times, but it parallels the levels found for other high-redshift quasars.
Dust comes from aging or dying stars. Early galaxies often pumped out stars rapidly, which might explain these levels. Still, it remains unclear what the results tell us about starbirth in these systems.
One of the most important aspects of the newly discovered quasar, however, is its larger environment. Using the quasar’s beam as a backlight, the team spotted the distinct spectral signs of neutral hydrogen gas in the vicinity. Astronomers have also seen neutral hydrogen around the second-earliest quasar, J1120+0641, but almost none around quasars a couple hundred million years later.
This result proves that J1342+0928 sits in the epoch of reionization, when radiation from early galaxies tore apart the hydrogen atoms filling the universe and left the hydrogen ionized — a state the universe is still predominantly in today. Reionization is a fundamental change, like dawn hitting the universe.
The team estimates that the hydrogen around J1342+0928 is about 50/50 split between neutral and ionized. That favors a late date for reionization, which would jibe with results based on the cosmic microwave background.
Eduardo Bañados et al. “An 800-million-solar-mass Black Hole in a Significantly Neutral Universe at a Redshift of 7.5.” Nature. Online December 6, 2017.
Bram Venemans et al. “Copious Amounts of Dust and Gas in a z=7.5 Quasar Host Galaxy.” Astrophysical Journal Letters, upcoming.
C. M. Carlisle. "The First Black Holes." Sky & Telescope. January 2017.