Why not take a picture of a closer black hole?

  • 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?


    Because black holes are dangerous and the IRB wouldn't let them get too close.

    53 million ly is just a short trip down the road, galactically speaking.

    I'd like to ask, so as better understand your question: Why take a picture of a closer black hole?

    Elliptical galaxy with a nice jet may have made M87 a very tempting target: https://duckduckgo.com/?q=Messier+87&t=ffsb&ia=images&iax=images&iai=https%3A%2F%2Fwww.universetoday.com%2Fwp-content%2Fuploads%2F2009%2F12%2FM87AGN-239x250.jpg Keep scrolling. The BH image looks to be taking over.

  • Ingolifs

    Ingolifs Correct answer

    3 years ago

    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*.



    black hole size comparison, self made
    Comparison of the apparent size of the largest nearby black holes



    And of course obligatory XKCD to remind us how small these objects really appear.


    Don't forget to say space is kind of dusty in the direction of Sgr A*. It's quite a bit clearer in the direction of M87.

    @FlorinAndrei Actually that doesn't really matter much, since the observations are made in radio where there's barely any extinction.

    @FlorinAndrei It may be dusty in the direction of Sgr A* but it’s messier in the direction of M87.

    @KonradRudolph I see what you did there :D

    Given that we are on the edge of the milky way, half the dust and contents of the milky way are between us and Sgr A*. I'm not sure the exact orientation of milky way relative to M87, but I believe they mentioned Sgr A* being obscured in the press conference. They also said Sgr A*, being much smaller, was also much more highly variable over time. I REALLY hope they make a time lapse video or something. That'd rule.

    Don't forget. Sgr A* would be seen rather from the side then from zenith/nadir

    @pela on the other hand, we never saw what he did in the direction of Sgr A*.

    I'm not sure this answer really fully answers the question. In the "short answer" paragraph, it states that M87's apparent size is 70% that of Sgr A*. That, by itself, would appear to make it a *worse* candidate than Sgr A*. The question wants to know why a closer black hole wasn't chosen. Sgr A* is closer - why not choose it (and indeed the question even wants to know why not "even ours")? The answer would be improved by elaborating on the other factors that make M87 a better candidate than Sgr A*

    @JBentley You're right, I skipped over Sgr A* because I assumed people knew they were also trying to image it. I'll edit my answer to make that explicit.

    I am wondering though, how many of those candidates are actually visible to the whole array. From what I understand, they need the south pole telescope to achieve their incredible baseline, so that should restrict observation targets to the southern hemisphere.

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Content dated before 7/24/2021 11:53 AM