Using light echoes to measure the size of active galactic nuclei

Michael Richmond
Jan 17, 2014

Some of my colleagues and I at RIT (Andy Robinson, Billy Vazquez, Triana Almeyda) are part of an international team of astronomers who are trying to measure the size of a dusty region around black holes in distant galaxies. We're using a technique called reverberation mapping, comparing the optical and infrared light emitted by a set of eleven galaxies over the course of two years.

But before I can tell you exactly what we are doing, I'd better explain some of these terms.

Active Galactic Nuclei (AGN)

When we look at most galaxies, we see starlight -- the combined emission of mostly optical light from billions and billions of stars. Below is a picture of M104 taken by the Hubble Space Telescope.

Almost all the radiation is in the visible or near-visible range, since ordinary stars emit light in this part of the spectrum.

But in some galaxies, we see large amounts of energy coming from the center of the galaxy at other wavelengths: in X-rays, or radio waves. For example, the galaxy Centaurus-A appears to be a big ball of stars cut by a dust lane in the optical (upper-right panel below), but it also shows a jet of X-rays coming from its center (upper-left panel), and jets emitting radio waves in both directions from its center (lower-left panel).

Image courtesy of the Chandra X-ray Observatory.

We call these objects active galaxies. Something unusual is happening at the center of these galaxies, in their nucleus. Putting it all together, there's some exotic physics inside Active Galactic Nuclei, or AGN for short.

For example, if we look at the spectrum of gas at the center of these galaxies, we see evidence that the gas is moving very, very fast --- and in a pattern that suggests it is orbiting around the center.

Image of M84 from the Hubblesite archive.

We explain these motions by postulating that there is a supermassive black hole at the center of these galaxies, with a mass millions of times larger than the mass of our Sun.

A model for the structure of an AGN

Over the years, astronomers have come up with a model which accounts for most of the observed properties of AGN. You can find pictures like the one below in lots of textbooks and presentations.

Image from Urry and Padovani, PASP 107, 803 (1995)

The important parts, for our project, are

Again, if we pay attention only to a portion of this model, we ought to see different regions, depending on how we observe:

Variable luminosity and the time delay

Okay so far? Now, the final thing one needs to know to understand our project is that many AGN do not always emit exactly the same amount of energy; instead, they may grow brighter or dimmer by factors of 2 to 10. These variations can take anywhere from a few hours to a few months.

For example, the galaxy NGC 5548 has sharp peaks and dips in its optical brightness over a period of eight years:

Image taken from The Starburst-AGN connection by Brad Peterson which you can also find at astro-ph 0109495 .

This light is emitted by the hot gas in the accretion disk, very close to the black hole. When we look at an AGN with optical telescopes, we are seeing light as it comes directly from the accretion disk to our eyes.

Let's consider an over-simplified situation: an accretion disk that emits a single, brief "burst" of light. How will that appear to an observer using an optical telescope?

In the figure below, you can see a single "burst" of light travel from the center of an AGN (the black dot) to an observer far away (the white dot at the origin). Click on the picture to start the animation.

The lower panel of the figure shows the apparent brightness of the source. Before the "burst" reaches the observer, the object is dim. When the "burst" arrives, the object suddenly appears bright .... but then, after the burst is over, the object fades again.

Look again. How long after the burst occurs does the observer detect the visible light? In other words, at what time (position on horizontal axis in lower graph) does the visible light appear?

Right. The visible light appears at time 300.

But ... what if the observer is using an infrared telescope?

In this case, the "burst" of radiation from the accretion disk has to fly out and strike the dust and gas in the torus. That radiation will heat up the dust grains in the torus, causing them to emit their own burst of infrared light. Watch carefully -- again, click on the figure to start the animation.

In this case, the observer sees an extended period of bright light in the infrared. How long after the burst occurs does the observer see this infrared emission?

Right again -- the observer sees infrared light over an extended period, from about 300 to 500. Note that the center of the infrared burst is at time 400, so the infrared burst is on average delayed after the optical light burst.

And that's the key: the delay between the optical and infrared light we observe tells us something about the size of the torus of gas and dust. The longer the delay, the larger the torus must be.

Our project: Spitzer and ground-based observations

Now, I can finally tell you about our project. We are using two types of telescopes to monitor a group of eleven AGN:

We used these instruments to measure the brighness of our sample of AGN over and over and over during the period from Jan, 2012 to Jan, 2014. Our hope was that we would see

  1. an optical light curve with peaks and dips
  2. an infrared light curve with similar peaks and dips
  3. AND
  4. a delay between the optical and infrared light curves

We are still analyzing the measurements, but we have preliminary results for the first year of data on one particular galaxy, NGC 6418. See for yourself -- did we detect a time delay?

You can see some additional details on this work by reading the poster that Billy Vazquez presented at the 223rd meeting the American Astronomical Society in Washington, DC. Click on the image below for a large version, or, for the full glory, look at a PDF version.