Sagittarius A*, taken by NASA's Chandra X-Ray Observatory. Ellipses indicate light echoes.
NASAチャンドラX線宇宙望遠鏡が撮影した、いて座A*(Sgr A*)。左上の楕円で囲まれた部分は、いて座A*が過去に放ったX線が周囲のガスに反射しているところ。
Credit: NASA/CXC/Caltech/M.Muno et al.

Taking the First Picture of a Black Hole [7] What is Sagittarius A*?

The Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) have caused quite a stir with their recent observations of the supermassive black hole at the center of the Milky Way, also known as Sagittarius A*. These observations are expected to lead to remarkable scientific results, enhancing our understanding of black holes and our theories of space and time. But just twenty years ago, astronomers didn’t know for sure that Sagittarius A* was the site of a black hole. Let’s step back in time and find out how we arrived at this point.

Back in 1974, British astronomer Sir Martin Rees proposed that supermassive black holes could live at the centers of some galaxies, such as those harboring active galactic nuclei (AGN). Such galaxies shine incredibly brightly in many different wavelengths — as bright as 30 billion Suns or more — and they also spew out powerful jets of charged particles. Rees realized that black holes could be the cause of this energetic turmoil, a fact that is now confirmed.

In the same year, American radio astronomers Bruce Balick and Robert Brown at the National Radio Astronomy Observatory discovered a compact radio source at the very center of the Milky Way. At radio wavelengths it’s the brightest feature in the Galactic Centre, but it is fairly small.

Balick and Brown thought it looked like a faint quasar, a type of AGN that was more common in the distant past. But this object was in our own cosmic backyard — just 26 000 light-years away. They dubbed it Sagittarius A*, or Sgr A* for short, because it is located in the direction of the constellation of Sagittarius. The asterisks arose because in atomic physics, excited states of atoms are denoted by asterisks — and Sgr A* is an incredibly exciting discovery.


Sagittarius A*, taken by NASA’s Chandra X-Ray Observatory. Ellipses indicate light echoes.
Credit: NASA/CXC/Caltech/M.Muno et al.

Over the next two decades, astronomers studied this bizarre object at different wavelengths and began to fit together the various pieces of the puzzle. As improving technology provided sharper and sharper views, they observed the commotion going on around the object. Gas and stars were whirling around it at incredible speeds — up to five million kilometers per hour — demonstrating that the object must be very small but very massive, with a staggering gravitational influence. Then, shortly after its launch in 1998, the Chandra X-ray Observatory spotted the first X-ray emission from Sgr A*. When matter is swallowed by a black hole, it emits a final “scream” of X-ray radiation before it crosses the event horizon. These X-rays can penetrate the thick clouds of gas and dust that veil the region, providing tell-tale evidence that a black hole lurks within.

Since the mid-1990s, research teams in Germany and the USA have been meticulously tracking the orbits of stars around Sgr A*. The US team, led by Andrea Ghez at the UCLA Galactic Center Group, has been using the W. M. Keck Observatory to measure the positions of thousands of stars in the vicinity of the Galactic Centre. The German team uses ESO’s Very Large Telescope to precisely measure the orbits of 28 stars frantically speeding around Sgr A*. These dedicated groups have produced the most detailed view ever of the region surrounding Sgr A*, and in 2008 confirmed once and for all that Sgr A* is the site of a supermassive black hole.

“Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist,” commented Reinhard Genzel, leader of the German team from the Max Planck Institute for Extraterrestrial Physics. “The stellar orbits in the Galactic Centre show that the central mass concentration of four million solar masses must be a black hole, beyond any reasonable doubt.”

Images collected over 16 years have been assembled into a time-lapse video. The real motion of the stars has been accelerated by a factor 32 million.
Credit: ESO/ R.Genzel and S. Gillessen

We have learned a great deal about Sgr A* over the years, as constant research has slowly revealed more and more of its secrets. Today, we know it is more than 4 million times as massive as the Sun but is extremely small, only about 40 million kilometers across — approximately equivalent to the distance from Mercury to the Sun. It is also fairly quiet for a black hole — it doesn’t emit an enormous amount of radiation, indicating that it typically doesn’t consume a lot of material. Astronomers have, however, captured blinding flares of X-rays hundreds of times brighter than the usual emissions. These are thought to be caused by the breaking apart of an asteroid falling into the black hole, or the entanglement of magnetic fields lines within the inflowing gas. Astronomers have also tracked a dust gas cloud called G2 that is in orbit around Sgr A* and imaged its closest approach in 2014. These observations, along with many more, add to our knowledge of the behavior of this black hole and its surrounding turbulent environment.

The dusty cloud G2 passes the supermassive black hole

This annotated composite image shows the motion of the dusty cloud G2 as it closes in, and then passes, the supermassive black hole at the centre of the Milky Way. These new observations with ESO’s VLT have shown that the cloud appears to have survived its close encounter with the black hole and remains a compact object that is not significantly extended. The blobs have been colourised to show the motion of the cloud, red indicated that the object is receding and blue approaching. The cross marks the position of the supermassive black hole.
Credit: ESO/A. Eckart

But mysteries still remain about this cosmic beast at the center of our galaxy — and about black holes in general. Sgr A* has helped cement the idea that every large galaxy hosts a supermassive black hole at its core, but astronomers are still scratching their heads about how such supermassive black holes form, and how they affect their host galaxies.

The GMVA and the EHT teams are using Very-Long-Baseline Interferometry to image Sagittarius A* in greater detail than ever before, linking up telescopes around the world. As well as probing the tumultuous regions around the black hole, the astronomers are also looking for one of the last pieces of the puzzle — observing the event horizon. This is the radius around a singularity beyond which matter and energy cannot escape a black hole’s gravitational pull; it is literally the point of no return.

The EHT and GMVA teams aim to observe the event horizon’s shadow. Not only will that demonstrate its existence, but measuring its shape and size will provide an unprecedented way to verify Einstein’s theory of general relativity. But of course, this is no easy task…

This is the seventh post of a blog series following the EHT and GMVA projects. Next time, we’ll talk about the challenges involved in imaging a supermassive black hole.