Our Supermassive Black Hole Is Slurping Down a Cool Hydrogen Smoothie

The world’s most powerful radio telescope is getting intimate with Sagittarius A*, revealing a never-before-seen component of its accretion flow

Artist impression of ring of cool, interstellar gas surrounding the supermassive black hole at the center of the Milky Way [NRAO/AUI/NSF; S. Dagnello]

As we patiently wait for the first direct image of the event horizon surrounding the supermassive black hole living in the core of our galaxy some 25,000 light-years away, the Atacama Millimeter/submillimeter Array (ALMA) has been busy checking out a previously unseen component of Sagittarius A*’s accretion flow.

Whereas the Event Horizon Telescope (EHT) will soon deliver the first image of our supermassive black hole’s event horizon, ALMA’s attention has recently been on a cool flow of gas that is orbiting just outside the event horizon, before being consumed. (The EHT delivered its first historic image on April 10, not of the supermassive black hole in our galaxy, but of the gargantuan six-billion solar mass monster in the heart of the giant elliptical galaxy, Messier 87, 50 million light-years away.)

While this may not grab the headlines like the EHT’s first image (of which ALMA played a key role), it remains a huge mystery as to how supermassive black holes pile on so much mass and how they consume the matter surrounding them. So, by zooming in on the reservoir of material that accumulates near Sagittarius A* (or Sgr A*), astronomers can glean new insights as to how supermassive black holes get so, well, massive, and how their growth relates to galactic evolution.

While Sgr A* isn’t the most active of black holes, it is feeding off limited rations of interstellar matter. It gets its sustenance from a disk of plasma, called an accretion disk, starting immediately outside its event horizon—the point at which nothing, not even light, can escape a black hole’s gravitational grasp—and ending a few tenths of a light-year beyond. The tenuous, yet extremely hot plasma (with searing temperatures of up to 10 million degrees Kelvin) close to the black hole has been well studied by astronomers as these gases generate powerful X-ray radiation that can be studied by space-based X-ray observatories, like NASA’s Chandra. However, the flow of this plasma is roughly spherical and doesn’t appear to be rotating around the black hole as an accretion disk should.

Cue a cloud of “cool” hydrogen gas: at a temperature of around 10,000K, this cloud surrounds the black hole at a distance of a few light-years. Until now, it’s been unknown how this hydrogen reservoir interacts with the black hole’s hypothetical accretion disk and accretion flow, if at all.

ALMA is sensitive to the radio wave emissions that are generated by this cooler hydrogen gas, and has now been able to see how Sgr. A* is slurping matter from this vast hydrogen reservoir and pulling the cooler gas into its accretion disk—a feature that has, until now, been elusive to our telescopes. ALMA has basically used these faint radio emissions to act as a tracer as the cool gas mingles with the accretion disk, revealing its rotation and the location of the disk itself.

“We were the first to image this elusive disk and study its rotation,” said Elena Murchikova, a member in astrophysics at the Institute for Advanced Study in Princeton, New Jersey, in a statement. “We are also probing accretion onto the black hole. This is important because this is our closest supermassive black hole. Even so, we still have no good understanding of how its accretion works. We hope these new ALMA observations will help the black hole give up some of its secrets.” Murchikova is the lead author of the study published in Nature on June 6.

ALMA image of the disk of cool hydrogen gas flowing around the supermassive black hole at the center of our galaxy. The colors represent the motion of the gas relative to Earth: the red portion is moving away, so the radio waves detected by ALMA are slightly stretched, or shifted, to the “redder” portion of the spectrum; the blue color represents gas moving toward Earth, so the radio waves are slightly scrunched, or shifted, to the “bluer” portion of the spectrum. Crosshairs indicate location of black hole [ALMA (ESO/NAOJ/NRAO), E.M. Murchikova; NRAO/AUI/NSF, S. Dagnello]

Located in the Chilean Atacama Desert, ALMA is comprised of 66 individual antennae that work as one interferometer to deliver observations of incredible precision. This is a bonus for these kinds of accretion studies, as ALMA has now probed right up to the edge of Sgr A*’s event horizon, only a hundredth of a light-year (or a few light-days) from the point of no return, providing incredible detail to the rotation of this cool disk of accreting matter. What’s more, the researchers estimate that ALMA is tracking only a minute quantity of cool gas, coming in at a total only a tenth of the mass of Jupiter.

A small quantity this may be (on galactic scales, at least), but it’s enough to allow the researchers to measure the Doppler shift of this dynamic flow, where some is blue-shifted (and therefore moving toward us) and some is red-shifted (as it moves away), allowing them to clock its orbital speed around the relentless maw of Sgr A*.

“We were able to shed new light on the accretion process around Sagittarius A*, which is a typical example of a class of black holes that have little to eat,” added Murchikova in a second statement. “The accretion behavior of these black holes is quite complex and, so far, not well understood.

“Our result is potentially important not only for our galaxy, but to any galaxy which has this type of underfed black hole in its heart. We hope that this cool disk will help us uncover more secrets of black holes and their behavior.”

We’re Really Confused Why Supermassive Black Holes Exist at the Dawn of the Cosmos


Supermassive black holes can be millions to billions of times the mass of our sun. To grow this big, you’d think these gravitational behemoths would need a lot of time to grow. But you’d be wrong.

When looking back into the dawn of our universe, astronomers can see these monsters pumping out huge quantities of radiation as they consume stellar material. Known as quasars, these objects are the centers of primordial galaxies with supermassive black holes at their hearts.

Now, using the twin W. M. Keck Observatory telescopes on Hawaii, researchers have found three quasars all with billion solar mass supermassive black holes in their cores. This is a puzzle; all three quasars have apparently been active for short periods and exist in an epoch when the universe was less than a billion years old.

Currently, astrophysical models of black hole accretion (basically models of how fast black holes consume matter — likes gas, dust, stars and anything else that might stray too close) woefully overestimate how long it takes for black holes to grow to supermassive proportions. What’s more, by studying the region surrounding these quasars, researchers at the Max Planck Institute for Astronomy (MPIA) in Germany have found that these quasars have been active for less than 100,000 years.

To put it mildly, this makes no sense.

“We don’t understand how these young quasars could have grown the supermassive black holes that power them in such a short time,” said lead author Christina Eilers, a post-doctorate student at MPIA.

Using Keck, the team could take some surprisingly precise measurements of the quasar light, thereby revealing the conditions of the environment surrounding these bright baby galaxies.


Models predict that after forming, quasars began funneling huge quantities of matter into the central black holes. In the early universe, there was a lot of matter in these baby galaxies, so the matter was rapidly consumed. This created superheated accretion disks that throbbed with powerful radiation. The radiation blew away a comparatively empty region surrounding the quasar called a “proximity zone.” The larger the proximity zone, the longer the quasar had been active and therefore the size of this zone can be used to gauge the age (and therefore mass) of the black hole.

But the proximity zones measured around these quasars revealed activity spanning less than 100,000 years. This is a heartbeat in cosmic time and nowhere near enough time for a black hole pack on the supermassive pounds.

“No current theoretical models can explain the existence of these objects,” said Joseph Hennawi, who led the MPIA team. “The discovery of these young objects challenges the existing theories of black hole formation and will require new models to better understand how black holes and galaxies formed.”

The researchers now hope to track down more of these ancient quasars and measure their proximity zones in case these three objects are a fluke. But this latest twist in the nature of supermassive black holes has only added to the mystery of how they grow to be so big and how they relate to their host galaxies.

Supermassive black hole with torn-apart star (artist’s impress
A supermassive black hole consumes a star in this artist’s impression (ESO)

These questions will undoubtedly reach fever-pitch later this year when the Event Horizon Telescope (EHT) releases the first radio images of the 4 million solar mass black hole lurking at the center of our galaxy. Although it’s a relative light-weight among supermassives, direct observations of Sagittarius A* may uncover some surprises as well as confirm astrophysical models.

But as for how supermassive black holes can possibly exist at the dawn of our universe, we’re obviously missing something — a fact that is as exciting as it is confounding.