There are closer galaxies than Messier 87 for sure, even ours! It sparked my curiosity that they went with one 53 million light years away. Is there a reason for this?
I was surprised too when I first heard they were trying to image M87's black hole.
The short answer is because it's really, really big. It is 1500 times bigger (diameter) than our Sagittarius A*, and 2100 times farther away. This makes its apparent size about 70% of that of Sgr A*, which they are also attempting to image.
A cursory search of wikipedia's List of Largest black holes shows that there's no other black holes with a combination of size and closeness greater than these two.
A couple of other candidates are not too far off. Andromeda's black hole is 50x the size of ours, and at 100x the distance, it would appear half the size of Sgr A*. The Sombrero galaxy is 380 times farther way than Sgr A*, and has a black hole estimated to be 1 billion solar masses, which is 232 times Sr A*, resulting in an angular diameter about 60% of Sgr A*.
There appear to be many other considerations to which black holes were chosen, as explained in this similar question. At a guess these would include how obscured each black hole is with foreground dust/stars etc, how active (and therefore bright) the nuclei are, and their inclination w.r.t earth affecting which observatories could observe them at which times.
Edit: I've found another plausible candidate. NGC_1600 is 200 M light years away with a central black hole estimated to be 17 billion solar masses heavy. This would put it at about 40% the apparent diameter of Sgr A*.
And of course obligatory XKCD to remind us how small these objects really appear.
There are a few criteria necessary to see a black hole with the Event Horizon Telescope. They are, in importance:
- Active Feeding: you need a thick accretion disk with lots of matter accreting onto the black hole. M87 fits this criteria, and is a glut, consuming about 90 Earth masses a day.
- Apparent size. Even though it is 53 million light-years away, M87 is 6.5 billion solar masses. Since the radius of the event horizon scales linearly with mass, its distance is made up for by sheer scale.
As Ingolifs says, Sgr A* and M87* are the obvious candidates. At the press conference, Heino Falcke explained why they got a picture of M87* first:
But it would take some more time because Sagittarius A Star is 1000 times faster and smaller. Its like a toddler who is moving constantly. In comparison, M87 is much slower, like a big bear.
Another quick note - They are trying to get a photo of Sag. A*:
The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of Earth's sun, and our own Milky Way galaxy's central black hole, known as Sagittarius A*. This latter object, while still a supermassive black hole, is a runt compared to M87's beast, containing a mere 4.3 million solar masses.
Both of these objects are tough targets because of their immense distance from Earth. Sagittarius A* lies about 26,000 light-years from us, and M87's black hole is a whopping 53.5 million light-years away.
From our perspective, Sagittarius A*'s event horizon "is so small that it's the equivalent of seeing an orange on the moon or being able to read the newspaper in Los Angeles while you're sitting in New York City," Doeleman said during the SXSW event last month.
And in case you're wondering about Sagittarius A*: The EHT team hopes to get imagery of that supermassive black hole soon, Doeleman said today. The researchers looked at M87 first, and it's a bit easier to resolve than Sagittarius A* because it's less variable over short timescales, he explained.
The difficulty in imaging anything on the visible-light spectrum is dust. Sagitarrius A is clouded by dust clouds which can be penetrated with infrared. The M87 fulfilled the criteria of being big and relatively close while at the same time enabling light to reflect off the event horizon and not being blocked by dust clouds.