<p>M87 was actually the easiest black hole for the Event Horizon Telescope (EHT) to attempt, and was thus sensibly its first target.  For the EHT to work you need 1) an active BH accreting matter such that it is a strong radio source, and 2) that it be close and massive enough to be angularly large.   While M87 is some 20x further away than, say M31 or M33, it is also extremely massive, thus offering a large resolvable horizon.   The only BH larger in angular size is the BH in our own Milky Way galaxy, which while weighing only 1/1500 as much at M87 is far closer by about 2000x, giving a larger target.  The small mass of the MW BH, however, means that it has a very short time scale for varying its rate of mass accretion, so that its appearance changes while the EHT is observing it.  This is a tricky technical issue that the EHT did not have to address for M87.</p>

<p>WRT other BHs, as a previous respondent did an excellent job of answering, M33 shows no evidence for harboring a central BH.  M31, in contrast, does have an ~800M solar mass BH, but it's 1) just a little below EHT's resolving power, and 2) provides very little radio emission.</p>

CC BY-SA 4.0
2020-02-08T21:05:19.580Z
2020-02-08T21:05:19.580Z
Tod R. Lauer
<p>Since we see the Triangulum galaxy M33 from a quite vertical position (contrary to our Milky Way and a bit the Andromeda galaxy) it should be easy to image the black hole in the center of it, shouldn't it? Why did they prefer to first image the black hole in a galaxy which is about 20 times farther and thus harder to photograph the black hole in its center?</p>

<black-hole><galaxy><photography><supermassive-black-hole><galactic-center>
Why don't they make an image of the black hole in the M33 galaxy?
user30007
# Why don't they make an image of the black hole in the M33 galaxy?

(Asked by: username_0 on 2020-02-05)

Since we see the Triangulum galaxy M33 from a quite vertical position (contrary to our Milky Way and a bit the Andromeda galaxy) it should be easy to image the black hole in the center of it, shouldn't it? Why did they prefer to first image the black hole in a galaxy which is about 20 times farther and thus harder to photograph the black hole in its center?

**Tags:** black-hole, galaxy, photography, supermassive-black-hole, galactic-center

---

## 1 answers

### Answer by username_1 on 2020-02-08. Score: 6

M87 was actually the easiest black hole for the Event Horizon Telescope (EHT) to attempt, and was thus sensibly its first target. For the EHT to work you need 1) an active BH accreting matter such that it is a strong radio source, and 2) that it be close and massive enough to be angularly large. While M87 is some 20x further away than, say M31 or M33, it is also extremely massive, thus offering a large resolvable horizon. The only BH larger in angular size is the BH in our own Milky Way galaxy, which while weighing only 1/1500 as much at M87 is far closer by about 2000x, giving a larger target. The small mass of the MW BH, however, means that it has a very short time scale for varying its rate of mass accretion, so that its appearance changes while the EHT is observing it. This is a tricky technical issue that the EHT did not have to address for M87.

WRT other BHs, as a previous respondent did an excellent job of answering, M33 shows no evidence for harboring a central BH. M31, in contrast, does have an ~800M solar mass BH, but it's 1) just a little below EHT's resolving power, and 2) provides very little radio emission.


---

