Monday, April 12, 2010

Light from a distant black hole pierces the Milky Way

Video credit: University of Michigan / Boston University / Cosmovision

We went looking for a small black hole in our neighborhood, maybe a few hundred light-years away, and instead we found a supermassive black hole nearly 7 billion light-years away.  Sometimes astronomy can be that way...

Back in February, my colleagues and I were looking at white dwarfs with the Keck I telescope in Hawaii.  Before our second night started, the astronomers at the neighboring telescope (Keck II, my telescope's twin) came over and asked if we had time to look at an object for them, because their telescope had the wrong camera for the observations they needed.  We had the time, and agreed.

Their target was discovered by the Fermi Gamma-ray Space Telescope, a space-based telescope that looks at gamma rays, the most energetic type of light in the Universe.  On February 1, the Large Area Telescope on Fermi, a telescope that maps the entire sky in gamma rays every three hours, had detected a new source of gamma rays.  This new source was long-lived, meaning it was not one of the enigmatic gamma-ray bursts that come and go in a few seconds. Since gamma rays are made by very energetic processes, the Fermi scientists knew that they were seeing something very exciting.

The new gamma ray source that Fermi saw was right in the band of the Milky Way as seen from Earth, in the middle of the constellation Cassiopeia.  Because most of the stars in our own galaxy appear to be in the band of the Milky Way,  Fermi scientists thought we must be seeing something in our own galaxy.  And since gamma rays have to come from very energetic things, we suspected it may be some kind of black hole in the Milky Way.  But while the dust and gas in the galaxy can act like a screen and block many kinds of light, gamma rays can travel right through.  So, if there is a distant galaxy that just happens to appear lined up with our Milky Way, we might be seeing something millions of times further away.  We needed more information.

The natural place to start was to look at pictures of this point of sky in other wavelengths of light.  Some of the astronomers therefore went and looked in X-rays, some in radio waves, and some in optical (visible) light.  And there was something there in all three of those types of light!  In fact, in visible light there are two faint little dots.  This isn't surprising -- there are so many stars in the Milky Way, you are bound to see one or more stars no matter where you look.  One of these two points of light was the source of the gamma-rays, and one was probably just a normal star.

This is where my white dwarf collaborators and I came into the picture, as we just happened to be sitting in front of the telescope at the right time.  We were asked to get a visible-light spectrum of both sources. Spectra, the splitting of light into its component colors, are like fingerprints.  Each type of object has its own unique spectrum.  Was the Keck telescope, one of the most powerful astronomical tools on the planet, able to tell us what these objects were?  We pointed the Keck telescope and its spectrometer at the two stars, recorded the data, and sent it to the gamma ray astronomers to analyze.

One of the faint spots was an ordinary star.  Ordinary stars don't produce gamma rays, so this first source was the wrong one.  The second spectrum was something very interesting: a quasar.  Quasars are ginormous black holes located at the centers of galaxies.  As the material near the black hole falls in, it heats up and releases the visible and X-ray light that we were seeing.  We had our gamma-ray source, but it wasn't nearby.  It was 7 billion light years away, and by chance happened to be lined up with the band of our Milky Way.

Fermi has seen many gamma-ray sources like this one.  When gas is falling into a black hole, it travels faster and faster, often colliding with other gas that is also falling in. The energy from all of this heat is enough to create jets, pencil-thin beams of matter that stream away from the black hole at nearly the speed of light (see the video at the top of this post for an artist's conception of these jets).  If these jets happen to be pointed at the Earth, we are seeing the most energetic part of the whole system, which means lots and lots of gamma rays.  The amount of gamma rays produced varies as the amount of energy in the jets changes.  Seven billion years later, these gamma rays arrive at the Earth, where we just happen to be looking with a gamma-ray telescope.

Some of the astronomers were a little disappointed.  Giant black holes are really cool, but there are thousands of them known.   Fermi has discovered many new types of gamma-ray sources in our own galaxy, and we were hoping this might be yet another unexpected discovery.  Instead, we had the chance alignment of a very, very distant galaxy with the stars of our own galaxy. 

My part in this whole adventure was small.  I took the optical light spectrum, and I wrote up a paragraph describing how I did that.  Twenty-one other astronomers were involved in the rest of the project, gathering data and analyzing from all kinds of telescope, and each contributing our little niche of expertise to a bigger problem that none of us would have solved individually.

Our paper on this discovery is still going through the review process, where another scientist is checking over what we did to look for any mistakes or problems with our logic.  Here's hoping she agrees with our analysis!

J. Vandenbroucke, R. Buehler, M. Ajello, K. Bechtol, A. Bellini, M. Bolte, C. C. Cheung, F. Civano, D. Donato, L. Fuhrmann, S. Funk, S. E. Healey, A. B. Hill, C. Knigge, G. M. Madejski, R. W. Romani, M. Santander-García, M. S. Shaw, D. Steeghs, M. A. P. Torres, A. Van Etten, & K. A. Williams (2010). Discovery of a GeV blazar shining through the Galactic plane Astrophysical Journal Letters arXiv: 1004.1413v1


  1. hiphopapotamos9:19 AM

    Thanks for writing such a fascinating post in terms that a an amateur, like myself, can understand. Science truly Rocks!

  2. Anonymous2:14 PM

    But we're looking dead on at one of these jets? That still seems pretty cool!