Why didn't the Event Horizon Telescope team mention Sagittarius A*?

  • At the press conference this morning, the Event Horizon Telescope team didn't say much about Sagittarius A*, which was the target many of us have been waiting for.

    Is there any explanation anywhere for this omission?

  • HDE 226868

    HDE 226868 Correct answer

    3 years ago

    There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make no promises, and they're not assuming they'll be successful).

    That said, I'm not wholly surprised that we ended up seeing M87, rather than Sgr A*, for a couple reasons which the team mentions in their first paper:

    • As Glorfindel said, Sgr A*'s event horizon is much smaller, meaning matter orbiting the black hole has a shorter orbital period. This contributes to variability on the timescale of minutes. The observations of M87 took place over the course of a week - roughly the timescale over which that target varies, meaning the source should not change significantly over that time.

    • Second - and this is the reason I've seen cited more often - Sgr A* lies in the center of our galaxy, and so thick clouds of gas and dust lie between it and us. That results in scattering, which is a problem. There are ways to mitigate this, of course, and the team has spent a long time on this, but it's simpler to just look at the black hole that doesn't have that problem in the first place. That's why M87's black hole is an attractive target.

    Neither of these are impossible hurdles to overcome, but they're certainly very real difficulties that can't be ignored.

    I saw multiple reports before hand that it was expected that they'd release images of both. Did that turn out to be nothing more than speculation?

    @curiousdannii I'd gotten definite information only the day before saying that it was going to be just M87, but I hadn't heard anything but rumors before that. Out of curiosity, where did you hear the reports?

    mostly youtube videos I think.

    The EHT observations are at mm wavelengths; the clouds of gas and dust between us and the galactic center are basically transparent to those wavelengths, so that's not really an issue. (It's an issue for optical and near-infrared observations, of course.)

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