Screaming to the Stars: Quasar Echo’s

Posted By Pamela on Sep 21, 2007 | 9 comments

quasar_simonnet.jpgIt is possible to map a room using sound, the sea using sonar, and to generally just get at the shape of things based on how the absorb and emit waves. This is true both in our Earthly locations (caves, canyons) and also in the centers of galaxies. In the past several days, I’ve seen a couple different journal articles on how quasar flickering is being used to map galaxy cores. This isn’t a new idea, but it is an idea whose technological time has finally come.

Here’s how it works: A supermassive black hole in the center of a galaxy eats something and emits a burst of light. Some of that light flies immediately to us on a straight line. That’s what our telescope catches first. Some of the light travels outward and interacts with surrounding clouds of material. Sometimes, the light will get absorbed by the clouds and will then be re-emitted in all directions – and some of this new light will now start traveling our way. Depending on the density, composition, temperature, and orbital velocities of the clouds, the light will interact in different ways and look different when it reaches us. Areas where the gas is cool and calm will emit narrow lines. Clouds that have high velocities emit broader lines.

Now, if we see that first blast of light followed by the appearance of broad lines followed by narrow line emission, that could mean the area emitting broad lines is closer in (the lights gets to the clouds first and then gets to us next), and the area emitting narrow lines is even farther out. The duration of time that we see light from the different regions also helps us understand how big they are. It will take a longer period of time for light from larger regions to finish getting to us. Think of it this way, imagine that you are looking at a hullahoop of material almost edge on. The initial burst of light its the entire ring at the same time, but the material nearest us has its light get to us first. Light from the backside of the hoop has to travel all the way across the hoop and then across space to us. The difference in time between when light from the near side and far side reach us (combined with the speed of light and the geometry of the galaxy), allows us to calculate the size of the hoop.

To make the measurements necessary to do these mapping projects, astronomers use spectroscopes on extremely large telescopes. Quasars aren’t found anywhere near our galaxy, and while they are some of the most luminous objects in the universe, they appear faint because they are far away. It takes large scopes to capture enough light from these distant objects to make out the lines used for echo mapping.

And large scopes have now been around long enough for scientists to have collected enough data over enough time to start producing initial maps of the distant galaxy cores.

This is one of those sets of of observations that is particularly exciting because it shows how far we, as astronomers, have come in the past decade or so. When I was an undergrad in the mid-nineties no one knew for certain what powered quasars. People talked about the angry monster in the center, and said it might, maybe, probably be a black hole. Today we have solid evidence of supermassive black holes in the center of many nearby galaxies and are using blackhole theories to map out distant galaxies.

The next person who says we’ve learned nothing new in the past (some number of) years is going to get directed toward a supermassive black hole 😉

Image credit: NASA Education and Public Outreach at Sonoma State University – Aurore Simonnet


  1. “Areas where the gas is cool and calm will emit narrow lines. Clouds that have high velocities emit broader lines.”

    Ok, this one has my brain hurting. I understand the calm gas cloud’s spectral behavior but I don’t see the connection between cloud speed and spectral line width. If the cloud was moving at speed toward or away from us, we would see a dopler shift but wouldn’t the line width still be the same?

  2. mcenhillk:
    We would see a narrow but shifted line if the motion was uniform, but it’s chaotic, some moving toward us, some sideways and lastly some moving away. Thus there’s a smearing effect on the lines and they broaden.
    Correct me (as always) if I’m wrong, Pammie…
    This basically is a lot like the light echo in Monoceros (V838 Mon.) but on a vastly larger scale.

  3. That really is fascinating. Thanks for posting this.

    Not to be a grammar Nazi, but I think that might be “Echos” with no apostrophe.

  4. AH! Got it!

    So the more gas swirling around in different directions, the wider the spectral lines. That makes sense.


  5. I was only parroting what I’ve read so many times, see the link:
    A cool animation titled “Light Echo: The Movie”
    Richard want a cracker, brawwwk!
    Misued apostrophe’s (LOL) bug me too!

  6. what is the temperature of Quauar?

  7. Actually, if you are going to be a “grammar” Nazi, then you may as well become a spelling Nazi, too!! The plural of ECHO is ECHOES !!!

    Regards- D

  8. estudiar a fondo las astros y todo lo que esta màs alla de la capa de atmosfèrica, es dar solucion a los paradigmas de nuestro origen y creaciòn.

  9. I didnt know the word NAZI was so popular; expescally amongst educated people.


  1. Astrolink [International Edition] » Blog Archive » Astrosphere for September 21st, 2007 - [...] Pamela Gay covers research about echoes of radiation from quasars being used to study the centres of distant [...]
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