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The EHT has published the first image of a black hole. It is the event horizon of the singularity in M87 against its accretion disk. I've been through the reports in the popular press but none mention the light wavelengths this was taken at. Is it an image in visible light, infra-red or longer?

enter image description here

Dave Gremlin
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    @uhoh Thanks for that. I saw the story and photo, couldn't find what the wavelengths were and though 'those nice people at space.stackexchange will know'. To be honest I'd looked at www.phys.org and a few other sites but didn't think to try Wikipedia and I didn't realise the EHT had its own website so you could call this out for not having done enough research. Not sure who added the radio-telescope tag, I was under the impression that all the EHT telescopes were visual or IR –  Apr 11 '19 at 10:34
  • @Hobbes I wonder then if it is time to see if we can better codify where the line is for the future? Maybe the existence these two tags unfairly invites people to ask questions that will then be closed for example. Maybe we can think of a "rule of thumb" that will help people know where better to ask. The problem with closing a good question as off-topic is that it either stays on hold, then closed (and is therefore rendered unanswerable) or eventually gets moved, which is extra work for the mods. – uhoh Apr 11 '19 at 10:45
  • @DaveGremlin if you click the word "edited" below your question, to the left of your icon, you can see the revision history of your question, and that I added it. I did primarily because the question turns out to be about radiotelescopes, and adding an appropriate tag makes it easier for future readers to find it. It also helps in the migration process I believe. There needs to be some overlap in tagging between the old site (here) and the new site (Astronomy) if it is to be migrated. This probably is a good candidate for that. – uhoh Apr 11 '19 at 10:49
  • I'm not sure "It is the event horizon of the singularity in M87 against its accretion disk." is a very accurate statement. The image arises from the paths that microwaves from the accretion disk follow in this highly warped space-time. The end result is that fewer come to us along a straight path from the event horizon and more come from the sides, especially one side, but it would be a mistake to think of this as too direct a representation of what is there. – Steve Linton Apr 11 '19 at 11:34
  • When the close votes are "this question belongs on another site", the 5th close vote will alert a moderator: https://meta.stackexchange.com/questions/85017/how-do-i-move-my-own-question-to-another-stack-exchange-site – Hobbes Apr 11 '19 at 12:08
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    @Hobbes This has never happened for me. – called2voyage Apr 11 '19 at 13:37
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    Interesting. A bug, perhaps? – Hobbes Apr 11 '19 at 13:42
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    @Hobbes Maybe, I don't know. My recommendation is flag a question for moderator attention and mention migration, if you think it needs to be migrated and has some close votes. By the way, where at that link are you getting that information from? I don't see anything there that says the 5th close vote will alert a mod. – called2voyage Apr 11 '19 at 14:10

2 Answers2

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According to the EHT website The observations were done using radio telescopes observing at a wavelength of 1.3mm. Visible light or infrared from the accretion disk would be nearly impossible to detect at such large distances. The radio measurements from all the synchronized telescopes were then combined and converted into an image.

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    and https://en.wikipedia.org/wiki/Event_Horizon_Telescope –  Apr 11 '19 at 08:35
  • The Messier 87 galaxy was discovered by the French astronomer Charles Messier in 1781. Of course Messier did discover the galaxy using a telescope and his eyes. There was no radio astronomy at all at this time. Visible light is detectable over a distance of 53 million light-years from Earth from a super giant galaxy. –  Apr 11 '19 at 10:32
  • @Uwe I meant visible light from the accretion disk. I'll edit my answer –  Apr 11 '19 at 11:17
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What part of the EM spectrum was used in the black hole image?... Is it an image in visible light, infra-red or longer?

Microwaves, (millimeter waves actually), and the hairy edge of far-infrared

at a nominal frequency of 230 GHz or 1.30 mm wavelength, the bandwidth is roughly 2 to 6 GHz wide depending on how many channels of data were used to produce the published image.

From First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole the first of four articles published together:

4. Observations, Correlation, and Calibration

We observed M87* on 2017 April 5, 6, 10, and 11 with the EHT. Weather was uniformly good to excellent with nightly median zenith atmospheric opacities at 230 GHz ranging from 0.03 to 0.28 over the different locations. The observations were scheduled as a series of scans of three to seven minutes in duration, with M87* scans interleaved with those on the quasar 3C 279. The number of scans obtained on M87* per night ranged from 7 (April 10) to 25 (April 6) as a result of different observing schedules. A description of the M87* observations, their correlation, calibration, and validated final data products is presented in Paper III and briefly summarized here.

At each station, the astronomical signal in both polarizations and two adjacent 2 GHz wide frequency bands centered at 227.1 and 229.1 GHz were converted to baseband using standard heterodyne techniques, then digitized and recorded at a total rate of 32 Gbps.[...]

So if we use 230 GHz, the wavelength is given by $c/f$ or 1.30 millimeters. It's hard for me to say right now if the image comes from only one 2GHz wide channel, or all three, which means that the bandwidth is either about 0.9% or 2.1%, but that's still pretty narrow compared to images taken at optical frequencies. That's (ultimately) because interferometry is done digitally these days and the computational size and time scales fairly fast with the size of the baseband.

I should note that these days it's more and more common for astronomers to refer to all kinds of different wavelengths as "light" in a loose way. Circa 1.3 millimeter wavelength certainly could be thought of as far-infrared, though Wikipedia puts the cutoff at 1 mmm (300 GHz).

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