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Black Hole Shocker! December 11, 2008

Posted by Sarah in science.
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Near-IR image of the galactic centre.

Astronomers have been able to confirm that the centre of our Milky Way Galaxy contains a supermassive black hole, read the headlines (here, here, here) yesterday. Brilliant! The galactic centre observations of the last 15 or so years, at both the Max Planck Institute for Extraterrestrial Physics in Garching and the University of California in LA are really exciting stuff. Videos like this one (and more here)of the stars whizzing round the galactic centre at immense speeds are great. Furthermore, and most excitingly, the mapping of these stars’ motions has allowed astronomers on both sides of the Atlantic to deduce independently that the central mass in our Galaxy is so dense and confined to such a small space that it can only exist in the form of a supermassive black hole.  The observation of the ultra-powerful radio source, Sagittarius A*, in the central region of the Galaxy supports this conclusion, as radio jets are thought to be associated with infalling matter around black holes.

Technology played a crucial part in the discovery: astronomers have only been able to track the stars in the densely crowded galactic centre to the required precision with the aid of adaptive optics, which correct for distortions arising in the Earth’s atmosphere and give the necessary boost to the resolution of the instrument.

So far all the excitement. What I don’t get about this story is: why now?! The studies have been going on for 16 years, and as far as I know they’re still ongoing. And the result is certainly not new – astronomers have suspected the presence of a SMBH in the galactic centre for over 20 years, and the observations yielded some pretty conclusive results quite some time ago. The most recent sets of observations have increased the numbers of stars observed, and scientists have been able to narrow down the errors on their results. But the only thing really of note I found in these stories is that one of the stars that was studied, called S2, has now completed a full orbit.

The media have picked it up in some bizarre ways too. The BBC first reported the “discovery” of a black hole in the centre of the Galaxy – I was half asleep when I read that headline and was very confused indeed! The title was later changed to “confirmed the presence of” – ah.

In The Telegraph: “Astronomers believe they have come up with concrete proof for the existence of black holes”. No Mr. Alleyne, actually, I think we were pretty sure already that they existed. Moreover, they credited their image, the same one as I used here, to PA.

The Guardian used the story quite nicely, publishing a little background piece on black holes, read it here.

The Times, on the other hand, wrote that “[d]etails of the research are published in the Astrophysical Journal”. Aha, a reference! And yes, in the current edition of ApJ I found “Measuring Distance and Properties of the Milky Way’s Central Supermassive Black Hole with Stellar Orbits”. But it’s by Andrea Ghez and her group at UCLA, who used the Keck telescopes. Nice work, Mr Henderson.

Anyways, I don’t mean to do bad things on anyone’s parade, the galactic centre research is very exciting and ESO have put together some great images and videos to publicise it. But the actual “news” story and its timing are a bit weird.

Image: ESO/S. Gillesen et al.



1. Nicole - December 11, 2008

I remember getting into this discussion with someone when the VLBI measurements of Sgr A* came out recently, and he pointed out that given the mass and size estimates and errors from previous studies, it was still theoretically possible (but highly unlikely) that it did not reach the critical density to be a black hole. Although I considered it a weak argument, and there was still enough convincing evidence of black holes already, I think the new study is precise enough to take away that uncertainty. So with its newly measured density, it can’t possibly be supported by neutron degeneracy, it has to be a black hole. (But I actually need to read the whole thing…)

Yeah, the headlines are as misleading as heck, but some people really, really want to know if we have strong evidence of black holes. And this is stronger yet than anything we’ve had! Especially taken by itself.

Sarah - December 12, 2008

Yes, that’s the conclusion I’d come to after talking to some people who know more about it than I do – the improvement in the quality of the data has pretty much ruled any other options that were theoretically still possible before – like a neutron star or a dense cluster of dark stars (I like the sound of that actually, very Star Wars!).

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