Astronomers have “imaged” gas as it orbits a supermassive black hole some 2 billion light-years away.

3C 273
This Hubble image shows the ancient and brilliant quasar 3C 273, which resides in a giant elliptical galaxy in the constellation of Virgo. It was the first quasar ever to be identified. The cloudy streak to the left is a jet shot out by the central black hole and extends some 200,000 light-years.
ESA / Hubble & NASA

In the last few months, astronomers working with the Gravity instrument on the Very Large Telescope Interferometer in Chile have released a series of impressive measurements. These results include preliminary data that show a long-sought gravitational effect on a star’s light as it passed the Milky Way’s central black hole. But the result I want to talk about here involves a much larger, more distant black hole, the 300-million-solar-mass leviathan that powers the brightest “nearby” quasar, 3C 273.

(I put nearby in quotation marks because this active galaxy in the constellation Virgo lies so far away that its light has taken some 2 billion years to reach us.)

Quasars essentially look like brilliant dots in the sky. They are galaxy cores that blaze brightly due to the hot gas their supermassive black holes chow down on and burp out as gigantic plasma jets. But thanks to an inventive approach by the Gravity Collaboration, 3C 273’s pinprick is now transformed into a map of gas moving right around the black hole, exploring a region we’ve heretofore only been able to probe indirectly with spectra and flickers of light.

Taking full advantage of the freedom that having a dedicated black holes blog gives me (wahahaha), I’d like to take you on a deep-dive into the Gravity Collaboration’s observations of the gas around 3C 273’s black hole to explain the unique approach this work involved.

Shifting Images

Like other quasars, 3C 273 has a region of hot gas zooming around in the black hole’s vicinity that’s called the broad-line region. The name comes from the shape of the gas’s spectral lines, which are smeared out. In normal circumstances, a spectral line is a narrow thing — a single wavelength. But when the gas moves, the spectral line shifts: to longer, redder wavelengths if the gas is moving away from us, and to bluer, shorter wavelengths if the motion is toward us. This Doppler effect is the same reason that an ambulance siren cascades through the sound spectrum as it races past you.

If you could combine all the notes you hear as the ambulance passes, you would hear a broad, multi-note sound, with the central note being the siren’s actual or “rest” frequency. The same thing happens with moving gas, creating a broadened spectral line.

The amount of smearing tells us how fast the gas is moving. Gas orbiting close to a black hole can move faster around the beast than gas farther out, just as the inner planets of the solar system orbit the Sun at a faster clip than the outer ones do. Based on the smear, astronomers infer that the BLR is one of the closest regions to the black hole that we can detect.

Observers use the BLR as a diagnostic, indicating things like the black hole’s mass. But even though the BLR is central to quasar studies, astronomers don’t actually know what it looks like. Is it the inner part of a disk around the black hole? Is it a halo of whizzing clouds?

Previous studies have tackled this question by inferring the gas’s motion and size from spectral patterns or the travel time of light echoes across the region. Reporting in the November 29th Nature, the Gravity Collaboration has now taken a completely different approach to studying the BLR, using blurry images of the gas itself.

A view of the four 8.2-meter VLT Unit Telescopes at the Paranal Observatory in Chile.

The team paired up the VLT’s four scopes in six different ways. Each pair of telescopes is separated by a unique distance, and their combined data create an image with the same resolution you’d obtain using a telescope as wide as the scopes’ separation. Telescopes closer together see the big, broad-brush picture, while telescopes farther apart home in on finer detail.

But turbulence in the atmosphere wiggles the image a wee bit. Each telescope pair sees a slightly different shift in where the image’s center is, explains Gravity team member Jason Dexter (Max Planck Institute for Extraterrestrial Physics, Germany). The center’s location also changes depending on which wavelength the astronomers observe.

Instead of taking these shifts as confusion that needs to be overcome, the researchers have used the complexity in their favor. The key is that in the case of the BLR’s gas, each wavelength corresponds to a redshift or blueshift caused by the gas’s velocity. By measuring how the image shifts at a series of different wavelengths, the astronomers could see where in the image there was stuff going at the speed and in the direction that corresponds to that Doppler shift. So even though Gravity only sees a blurry image, by tracking how the center of that blur changes position from wavelength to wavelength, the team can reconstruct how the blurred-out gas is moving around the black hole.

Evidence of Rotation

BLR geometry
This diagram shows the principle geometry of the broad-line region (BLR) of the quasar 3C 273. The individual clouds are distributed in a thick ring (green shaded area) and rotate around the central black hole. The astronomers on Earth view this system at a slight angle (i).
© GRAVITY Collaboration

This charting reveals that one side of the BLR glow is moving toward us, the other away, just as you’d expect if the gas is rotating. The pattern matches what you might see if the gas inhabits a puffed-up disk, with clouds orbiting at a range of inclinations to our line of sight. Furthermore, the gas is rotating around the axis drawn by the black hole’s powerful jet, which is exactly what should happen if the gas is rotating around the black hole.

No one’s been able to clearly show this toward-and-away motion in the BLR’s spectra before. It’s been one of the great mysteries of this gas, Dexter says: Before, astronomers could only see a “featureless lump,” a big, fat emission line with no clear pattern in the gas’s motion. Gravity’s data prove the gas is indeed rotating.

Now that they can see (if only vaguely) the structure, researchers can estimate how big the BLR is. The size, about 145 light-days, is within the range of previous estimates but on the small side.

Details aside, here’s the takeaway: We’re actually watching gas orbit a gargantuan black hole more than a billion light-years from Earth.

Why does this feat matter? The BLR enables astronomers to study what happens near supermassive black holes. The better we understand the gas’s motion and the size of the region it moves through, the better we’ll understand how these black holes feed and how they power quasars.


Reference: Gravity Collaboration. “Spatially resolved rotation of the broad-line region of a quasar at sub-parsec scale.” Nature. November 29, 2018.


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