It 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