The supermassive black hole at the core of our galaxy.
This essay is due to appear in the next issue of UoA Scientific.
Sagittarius A (Sgr A), is a prominent source of radio waves in the constellation Sagittarius. Astronomers have believed that Sgr A*, the source’s core object, to be the supermassive black hole (SMBH) at the centre of our galaxy, around which billions of stars, including our sun, revolve.
On 12 May 2022, in several press releases held simultaneously around the world, the Event Horizon Telescope (EHT) collaboration unveiled the first direct image of Sgr A*, using several years’ worth of observation data and improved Earth Telescope techniques compared to what was used to obtain the first ever black hole portrait for M87* in 2019.
Literal to their name, black holes do not emit light. Therefore, what the image captures is the shadow “cast” by a black hole onto its accretion disk — extremely heated plasma orbiting the black hole at high speeds.
The long-anticipated radio-frequency image is a strong piece of evidence supporting the black hole nature of Sgr A*, and marks a milestone in an exciting new era of astrophysics and related fields.
An Elusive Attraction
I feel like I’ve finally got to see an old friend face to face.
Dr. Fiona Panther, U. Western Australia
On a Swedish winter day in 2020, three physicists shared the Nobel Prize in Physics, Roger Penrose, Reinhard Genzel and Andrea Ghez. The latter two shared half the prize “for the discovery of a supermassive compact object at the centre of our galaxy,” the modern Nobel committee was noticeably gingerly with words — it was speculated that the committee chose not to call their discovery a “black hole” just yet due to the lack of conclusive evidence to rule out other models by the time of the award.
While weighing in at the equivalent heft of over 400 million suns and surrounded by hot gas, Sgr A*, the Milky Way’s central black hole, is surprisingly among the dimmest and least active known in its class. Not helping the matter, there exists considerable interstellar dust between the galactic core and the Earth, which are over 26 thousand light years apart (246 quadrillion kilometres). In brief, it has been extremely challenging to observe anything at the centre of our galaxy.
Nevertheless, the scientific endeavour into understanding our galactic core has been ongoing for more than a hundred years, even from the days before Einstein’s theory of general relativity and the proposal of the black hole model.
At the turn of the 20th century, just as astronomers were starting to realise that some “island universes” — diverse and complex structures previously assumed to be part of the Milky Way — were separate galaxies far, far away (Curtis, 1917; Hubble, 1924), they also began to study the motion of stars by analysing the Doppler shifts in their emitted light. Soon, most galaxies were established to host a dense and massive object at their cores, around which everything revolved.
Since the 1950s, the nature of such massive objects were extensively studied and debated. An idea eventually prevailed that most galaxies — including our own — host an SMBH at their centres. Key advances during this period include theoretical formulations by Lynden-Bell (1969) and radio observations conducted by Balick and Brown (1974), which were one of the first to definitively measure the radio waves emitted by Sgr A* itself.
Indirect observations of Sgr A* and astrophysical bounds on its characteristics were continually improved. For example, the aforementioned 2020 Nobel prize was awarded to one of such efforts. Since 1995, scientists at both UCLA and the Max Planck Institute have been tracking several stars in tight orbits around Sgr A*, just like planets going around a star, except at much higher speeds. These stars can reach speeds as high as a few percents of the speed of light, and they seem to go around an invisible attractor. Careful orbital mechanical analyses of their trajectories agreed with predictions from general relativity, and allowed a good estimation of the mass of Sgr A* to be performed.
The Earth Telescope
To put the distance of Sgr A* in better perspective. The entire accretion disk looks as big in our night sky as a bagel placed on the surface of the moon. To directly image it requires an extremely high resolution. This means that very-long-baseline interferometry (VLBI) is the observational technique most likely to accomplish our goals.
In simple terms, VLBI combines the signal received by multiple radio telescopes in different places, correlates their time and orientations, and constructs a “virtual telescope” with the effective size equal to the separation between the telescopes — if telescopes around the globe are carefully chosen and coordinated, one can construct a telescope the effective aperture size of the earth.
Of course, if you think of an actual radio telescope or TV receiver the size of the earth that is capable of utilising every inch of its surface simultaneously, and compare that with the surface (lines?) traced by our real telescopes as they rotate with the earth, the signal we actually detect can only make up a tiny fraction of the full aperture, and requires vastly elaborate post-processing.
Not only did the scientists need to sync up, filter, and combine terabytes upon terabytes of raw data for each frame, but they needed to find the most likely source image. This is because there are infinitely many possible full images that could have given rise to each set of EHT raw readings, and a model-indifferent reconstruction algorithm had to be developed. While the full technical details of these are beyond the purview of this report, if you are an applied mathematics student, you might recognise this as a classic inverse problem.
Using 230GHz radio wave, the observation procedures for both Sgr A* and M87* began at about the same time, in early 2017. One might ask — rightfully so — why the Milky Way black hole, which is thousands of times closer to us, take much longer to image?
One of the main astrophysical reasons is in the sizes again. Sgr A* is much lighter and smaller than M87*, and that means the innermost stable orbit is much smaller, allowing matter in its accretion disk to complete a lap in mere minutes, in contrast to the case for M87*, which might take tens of hours.
Because VLBI relies on the rotation of the earth, and the pattern of the accretion disks changes significantly faster than that, additional processing steps must be taken to recover useful information. Within the constraints of EHT hardware, a mix of three error reduction strategies were employed. Variability reconstruction: instead of fitting the observed data to an image, the output is explicitly fit to a movie; variability circumvention: the data series are truncated into short enough segments so the source can be taken as basically static; variability mitigation: the shape of the accretion disk is assumed to be constant, and its changing details were simply absorbed into the error bars of the final fit.
In addition to variability, directions matter too. Because M87 is quite far off from our galactic disk, while Sgr A* lies right at the heart of it, the signal from M87 is actually subject to less distortion in the form of refractive and diffractive scattering caused by the dust in the Milky Way. Adequately correcting for these also meant the reconstruction took more time.
Almost a decade ago, the Earth Telescope was described to my high-school self as a far fetched-idea. It is remarkable how it has not only successfully been established, thanks to an extensive interdisciplinary collaboration system between astronomers, physicists, statisticians, signal engineers, and computer scientists, but is on track to become bigger and better.
A Unique Window of Physics
Thanks to the first image of Sgr A*, results from orbital mechanical (star-tracking) observations and measurements at the scale of the event horizon could be cross-checked against each other, highlighting the consistency of general relativity at this scale for the first time in scientific history. This, alongside the 2019 results for M87, suggest that general relativity is consistent with reality even in extreme conditions.
Further, similar to how M87* got an updated image a few months after the initial 2019 data release, we can expect EHT to release a version of the Sgr A* portrait with polarisation information too, where the magnetic field information around the black hole will be revealed.
As more observation facilities join the EHT project, and as more data are gathered, the tools developed to circumvent signal variability may also be able to give us the first animated movie of Sgr A*, revealing more about the dynamics of the black hole accretion disk, one of the most extreme environments we know. It has been reported, for example, that Sgr A* occasionally gives out flares in near infrared and X-ray, where the proposed mechanisms in literature are as of yet unverifiable.
In all, given its unique position as the nearest SMBH to us, Sgr A* is poised to provide for humanity, from today to the distant future, an exotic laboratory with which we could test the nature of spacetime and study fine details black hole astrophysics.
Across the galactic dust and debris, through the shadow of the black hole at the heart of our galaxy, for the first time, we can glimpse into the stories of our cosmic past, and wonder the directions of our future scientific endeavours.
I wish that Einstein would be happy if he could hear about this.
References and Further Reading
EHT
https://www.eso.org/public/news/eso2208-eht-mw/
(The full list of technical papers associated with this image release can be found on the EHT website above.
Sgr A*
Goss, Brown & Lo, The Discovery of Sgr A*, https://arxiv.org/abs/astro-ph/0305074
Lynden-Bell, Galactic Nuclei as Collapsed Old Quasars, Nature 223, 690–694 (1969).
Ghez, Klein, et al. High Proper‐Motion Stars in the Vicinity of Sagittarius A*: Evidence for a supermassive black hole at the center of our galaxy. The Astrophysical Journal. 509 (2): 678–686
