New EHT Image of Sagittarius A*

By Joanne A Calitri   |   June 7, 2022
The black hole Sgr A* main photo and its relative images (photo by EHT Collaboration)

Location: Earth. Star-date 75825.4 (May 12, 2022): the long suspected supermassive black hole named Sagittarius A* (Sgr A*) at the center of the Milky Way Galaxy is confirmed by the Event Horizon Telescope (EHT) team via millimeter-wave radio telescopes. Prior evidence and other phenomenon of Sgr A* was noted by astronomer Harlow Shapley in 1918, engineer Karl Jansky in 1933, Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in 2002, and later with Genzel and Andrea Ghez earning a Nobel Prize in Physics in 2020.

In 2017, the EHT used a global network of eight telescopes as radio antennas, aka Very Long Baseline Interferometry, to observe at a frequency of 230 GHz (a wavelength of 1.3 mm), the highest radio wavelengths reliably observed from the Earth’s surface and get through the gas surrounding black holes. The radio signals arrive on the telescopes, are time tagged, recorded on hard discs and shipped to supercomputers that combine the data together, realigning all the signals between the telescopes to create one virtual telescope larger than the size of the Earth. Employing forward and inverse modeling on the data, strong representative images are averaged to make one main image. 

Images of black hole M87* (released in 2019) and Sgr A* look similar in appearance, although they are from different galaxies and are different distances and masses. They both produce the same warping of space-time, a phenomenon predicted by Einstein’s Theory of General Relativity (GR), now twice confirmed. Skeptics note: GR is used for location tracking on your smartphones. In 2019, I reported on M87*in the Montecito Journal.

Enter UCSB NSF Fellow Joseph Farah, a Ph.D. student in astrophysics with a focus on supernovae and one of the 300 EHT scientists researching black holes. He also works with Andy Howell, Ph.D. at the Las Cumbres Observatory in Goleta, and designs digital multiverse fantasy art with 3D open-source modeling. Read on for our interview with Farah.

Q. Top question you’re getting on Sgr A*? 

A. [laughs] A lot of people think, ‘Well, we already have an image of a black hole. Why get another one?’ But it’s important because black holes represent the most extreme environment in the universe to test Einstein’s Theory of General Relativity. 

Sgr A* and M87* span three orders of magnitude of mass. Sgr A* is a paltry four million times our Sun’s mass and 27,000 light years away. It is a thousand times smaller and closer than M87* which is 6.5 billion times our Sun’s mass and 55 million light-years away from the Earth. Yet, they look similar, it’s a cosmic coincidence, and excellent confirmation that GR is correct, not just in the vicinity of black holes, but the vicinity of black holes spanning three orders of magnitudes of mass. My colleague Sunny Vagnozzi at the University of Cambridge wrote a paper this week, basically going through an exhaustive list of alternative theories of gravity and checking whether the new image supported or refuted them. 

How was the May 2022 black hole photograph made? 

The EHT’s global network of radio telescopes required years of planning to coordinate and upgrade to the required 230 gigahertz, and during the actual observation, to get them to point to the exact same spot in the sky at the same time with the same cadence. It was a complicated endeavor.

It’s the largest physics experiment by data per night of observation, producing about 1.5 petabytes of data per night for 11 total approximate hours of observing, which would take over 30 years for each facility to send over the internet to the supercomputers at the MIT Haystack Observatory and the Max Planck Institute. So, it is recorded on data cartridges and shipped by aircraft to the correlating centers. Planes can only fly in and out of Antarctica twice a year, so the South Pole Telescope’s data takes six months to get to us.

The image that the public sees is representative of a fairly small subset of images, a sample of our parameter survey, which tests millions of different combinations of parameters across multiple algorithms to see which ones fit the data the best to make the representative set. We use forward and inverse modeling. Forward modeling takes millions of images and forward transforms them to see how well they fit the data. Inverse modeling takes the data and inverts it to get an image out. 

The first image of M87* was super-super hard to make from every aspect, especially because we used an instrument that had never been used or calibrated before to take a picture of something that had never been seen. We spent these past few years improving on that and analyzing the data. Doing it again and seeing similar horizon-level structure gives us more confidence we got it right the first time. 

Where does EHT go from here?

We want longer baselines, higher frequency, and better resolution. We are upgrading our telescopes to 345 gigahertz, which will increase our resolution by 50% immediately. We want to look at more black holes, put a telescope in space and have space baselines, improve our algorithms to make a better black hole image, and potentially expanding to making movies of the black holes we observe. 

The key result from my recent published paper is the method of selective dynamical imaging. It identified that the best time of the 2017 observations for a movie to be made would be a 100-minute period in the second half of the observation, and it showed a method for how to find such an ideal time in any future or past observation. A movie made using this method would not necessarily be in that region or with that duration. [ref:]

Your favorite sci-fi movies and songs? 

I love The Empire Strikes Back, Star Trek 4: The Voyage Home, and the original Matrix movie. I listen to disgustingly upbeat music, “Puebla” by Alvaro Soler, “Take Me Home remixed by BUNT” by Alexander Tidebrink, “Run Wild” by Laney Jones, and “Home” by Vexento.  

Please explain your motto: “If I fear, then I must.” 

One of my best friends and most inspirational people I’ve met, Sarah DuBois-Coyne PhD,never lets fear control her. I found in my decision-making that fear has never been a good reason not to do something, and I wouldn’t be where I am right now if I didn’t do certain things purely because I was afraid.  



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