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SYFY WIRE Bad Astronomy

Staring down the throat of an ancient, *extremely* distant black hole

By Phil Plait
Artist drawing of a blazar, a galaxy with a supermassive black hole spewing out energy. Credit: DESY, Science Communication Lab

Using one of the largest optical light telescopes in the world, astronomers have identified one of the rarest beasts in the Universe: An extremely distant type of galaxy called a BL Lac object, a galaxy's supermassive black hole spewing out a beam of energy so powerful it dwarfs the light of the entire galaxy it comes from.

While over a hundred BL Lac objects are known, this is the first one every seen at this incredible distance — the light we see from it has traveled 12 billion years to get to us. We see this galaxy as it was when the Universe was less than 2 billion years old.

And that's an interesting situation: We didn't think BL Lac objects could exist so early in the Universe's history.

Artwork depicting a supermassive black hole surrounded by an accretion disk and magnetic corona, with powerful jets launching away in opposite directions. Credit: NASA/CXC/M. Weiss

Every big galaxy in the Universe has a central supermassive black hole, generally a few million up to a few billion times the mass of the Sun. Material falling into this black hole forms a huge disk that gets incredibly hot. The magnetic field of the disk gets twisted up by the disk's rotation, and in the center forms a vortex like a magnetic tornado. The field is so strong it can pull matter away from the black hole, accelerating it to very nearly the speed of light.

We call galaxies like this active galaxies. We see lots of radiation from them, but only a small subset seem to emit gamma rays, the very highest energy form of light. We think that we can only see gamma rays from them when we happen to see the beam of material aimed almost exactly at us. If we see it from an angle then the gamma rays miss us.

Galaxies like that are called blazars, and this means we only see a galaxy as being a blazar if we're staring down the barrel of that beam. That's disconcerting. I'm glad they tend to be extremely far from us.

Artist drawing of a blazar, a galaxy with a supermassive black hole spewing out energy. Credit: DESY, Science Communication Lab

Blazars come in two flavors. One is called flat radio spectrum quasars (or FRSQs), and they tend to change their brightness on short timescales. We also see light from them characteristic of carbon and oxygen heated to high temperatures (emitted at very sharply defined colors called emission lines).

Galaxies in the other category are called BL Lac objects, named after the first one found, called BL Lacertae (a variably bright object in the constellation of Lacerta, the lizard). These are similar to FRSQs but don't show the bright emission lines. We think that FRSQs are youngish to mature galaxies, which eventually settle down a bit and become BL Lac objects.

That would imply that if we look far enough back in the history if the Universe, we'll stop seeing BL Lac objects and only see FRSQs. This new observation makes that conclusion a bit dicier.

An image of the BL Lac object 4FGL J1219.0+3653 (center) together with some foreground galaxies and a star (left) used to align a slit to let light from the galaxy through the telescope to the spectrograph. Credit: Paliya et al.

The object in question is called 4FGL J1219.0+3653 (let's call it J1219 for short), and was discovered to be emitting gamma rays by the orbiting Fermi gamma ray observatory. It's clearly a blazar, but what flavor?

Using the enormous 10.4-meter Gran Telescopio Canarias on the island of La Palma, a team of astronomers observed J1219, taking a spectrum of it. They found no emission lines at all, making it clear it's a BL Lac objects and not an FRSQ. They were also able to confirm its enormous distance of 12 billion light years, making it easily the most distant BL Lac galaxy ever seen, by about 800 million light years.

And that's a bit of a problem! If FRSQs eventually become BL Lac objects, then the youth of this object implies it can happen earlier than thought. Even so, the astronomers predict that there are only two — 2! — BL Lacs that should exist this far from us in the entire observable Universe. And here they've found one of them.

That's very cool. And it implies what must be done next: Find more of them. If another is found, OK then. Things are still good. But if, over time, more than that are found, it throws the evolutionary path idea of these objects into doubt. To be honest a lot about the behavior of blazars is unknown (it's not even completely clear why we see emission lines from FRSQs and not BL Lac objects), so at the moment this is an open area of research.

Thing is, it takes huge telescopes like the one on La Palma to get decent enough spectra of these terrifyingly distant beasts. So it may be a while before astronomers know if more BL Lac galaxies exist out there at the edge of the observable Universe. Maybe J1219 is alone. Maybe it has one or two siblings or more. We won't know for a while yet.

Peering this deeply into the Universe is extremely difficult, and something we've only been able to do relatively recently. It's still unknown in many ways.

And also, in many ways, hic sunt dracones — here be dragons. Fire-breathing monsters in universus incognita. Sounds about right.