Seeing Black Holes

This press release came out yesterday that was SO EXCITING to me that I was bouncing up and down. In today’s Nature, astronomers write about the successful attempt to resolve the supermassive black hole at the center of our galaxy using 1.3 mm VLBI. And now you are going, “huh? jargon say what?”

As most people now know, black holes are supremely dense objects. The gravitational field around them is so strong, they can severely warp space around them, and nothing, not even light, is fast enough to escape its gravitational pull once inside a certain radius. It is a weird concept, since it has essentially infinite density at a single point in space, yet we do find evidence that these things exist. For example, we can watch the orbits of stars around the very center of our Milky Way galaxy, and determine the mass of the dark object that lives there. It has so much mass in such a small area, that physics has no other explanation for what it could be, and any other explanation would be even weirder. Around some black holes, we can measure the orbits of gas, and measure the mass and limit the size.

Want to visualize where this is? If you look up towards the Southern sky (if you are in the Northern Hemisphere) towards the constellation Sagittarius the hunter, or teapot, and you are at a dark site, then you can look right at the center of the Milky Way galaxy, that band of light that stretches across the sky.

Remember, this is at the very center of the hustle and bustle of our Milky Way, whereas we are way out, 27,000 light years away, in some backwater spiral arm!

Astronomers want to actually measure the size of these black holes in order to test the theories. Although a black hole by definition gives off no light, the gas swirling around and into it does, giving off radiation at many wavelengths from the radio through the optical and into the x-ray. The Hubble telescope has shown us that supermassive (or millions of times the mass of our sun) black holes exist in the center of almost every galaxy that has a “bulge” in the center. The center, or bulge, of our Milky Way is not easily viewed by us in visible light since we have to look through the plane of the galaxy in order to see it, and there’s lots of attenuating dust in the way. But we can see signs of our black hole, dubbed Sgr A*, in the infrared, radio, and x-ray. In order to see something as tiny (at this distance) as the black hole, you need a big, big telescope.

Enter VLBI, or Very Long Baseline Interferometry. This is a technique by which you link up multiple telescopes across long distances, either in real-time or later in a computer, in order to make one giant telescope. The telescope formed from this can be as big as the Earth! It isn’t a very sensitive telescope, but it has very good angular resolution, meaning it can see very tiny, tiny things. In this study, they achieved a resolution of “37 micro-arcseconds – the equivalent of a baseball seen on the surface of the moon, 240,000 miles distant,” by linking up telescopes in Hawaii, Arizona, and California. The light that they were tuned to has a wavelength of 1.3 mm, so this can also be called “millimeter wave interferometry.”

So, go ahead and read the press release for more information on what they found and what theory they are testing. In summary, they were able to detect the structure and size of things around the black hole, thus squeezing in to see how small the black hole really is. But there is more work to be done! “Future investigations will help answer the question of what, precisely, they are seeing: a glowing corona around the black hole, an orbiting “hot spot,” or a jet of material.”

Other than the fact that this is really cool science, I was personally excited about this since I learned all about these experiments back in 2003 when I was at MIT Haystack Observatory for a summer internship. I’m really glad to see that the team is succeeding in their goal after years of hard work.

Sheperd S. Doeleman, Jonathan Weintroub, Alan E. E. Rogers, Richard Plambeck, Robert Freund, Remo P. J. Tilanus, Per Friberg, Lucy M. Ziurys, James M. Moran, Brian Corey, Ken H. Young, Daniel L. Smythe, Michael Titus, Daniel P. Marrone, Roger J. Cappallo, Douglas C.-J. Bock, Geoffrey C. Bower, Richard Chamberlin, Gary R. Davis, Thomas P. Krichbaum, James Lamb, Holly Maness, Arthur E. Niell, Alan Roy, Peter Strittmatter, Daniel Werthimer, Alan R. Whitney, David Woody (2008). Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre Nature, 455 (7209), 78-80 DOI: 10.1038/nature07245

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