Cosmic Collisions

How Do Black Holes Grow?

Every large galaxy has a supermassive black hole in its center. These black holes are a million to a billion times more massive than our Sun. An open question of galaxy evolution is how these black holes reached their enormous sizes. Since black holes can't radiate on their own, we can only observe them when they are actively consuming matter, because as they feed, the infalling matter gets extremely hot. Currently, the "standard model" of black hole growth is that they are fed by galaxy mergers; when two galaxies crash together, the gas in each galaxy can be funneled into black hole, causing it to grow. The black holes of each galaxy can merge into a single, more massive black hole, leading to growth as well. However, this is a theoretical model; the question of whether black hole growth is actually driven by merging galaxies is hotly debated; to date, studies have found correlations or a lack of correlations between galaxy interactions and black hole growth depending on the populations examined. Now, with cutting-edge observations and your help, we can determine once and for all if mergers are necessary to grow a supermassive black hole.

The James Webb Space Telescope

Past infrared (IR) surveys have been relatively low resolution and restricted to only the most massive or nearby galaxies. With the launch of the James Webb Space Telescope (JWST) and the release of the first wide field surveys, a new set of fainter, redder, more distant galaxies is now available for study. This more complete population of galaxies, like our own Milky Way but in the very early Universe, can provide new insight on how supermassive black holes grow.

This image (source) shows the same area of the sky observed by both JWST and the last generation near- and mid-infrared telescope, Spitzer. The left panel is the Spitzer observation in the mid-IR, the middle panel is the Spitzer observation in the near-IR, and the right panel is the JWST observation in the mid-IR. This is to illustrate just how much more capable of an extragalactic observatory JWST is; in the past, only the brightest galaxies were detected, and they just looked like circles or blobs. Now, we can see much smaller, dimmer things.

In addition, for the first time in history, galaxy structure is visible in the infrared thanks to JWST's excellent angular resolution. Mergers are only reliably detected in resolved populations, either through visual classifications or through analysis of light distributions in a resolved source. This is therefore the first time a study of this type is possible. Together with your help, we will use this exquisite data set to understand how black holes have grown over the last 10 billion years.

Finding Black Holes

When black holes feed on matter, the light they emit is unlike anything else in the universe. Active black holes emit light at all wavelengths, from the radio to the X-ray. In some wavelengths, like the optical, this light can be hard to distinguish from starlight. However, in the infrared, the light follows a pattern called a power law, which can easily be identified even among other sources of thermal IR light, like old stars and dust. We use the brightness of different wavelength IR observations from James Webb to estimate the relative contributions of different sources using a process called Spectral Energy Distribution (SED) fitting. SED fitting allows us to extract a lot of different physical properties, including the fraction of light from the galaxy which is emitted by a central black hole.

Once we find a black hole using this process, then we check whether or not it lives in a merging system using the the data that you will help us create. Doing this for thousands of galaxies, and comparing what we observe to what we expect to find, will allow us to tell if black holes tend to live in merging systems and if this tendency is enough to explain their current sizes.

Identifying Mergers

This is where we need your help! Astronomers have come up with many clever ways to sort galaxies by type, but unfortunately there are not many ways to uniquely distinguish mergers from non-mergers. This is because they can happen between any kind of galaxy, and so their appearances, colors, and physical properties are extremely varied and mixed. The best way to find them is by visual inspection, to determine if they look like two galaxies colliding. To do that, we need a lot of fresh eyes on our images! Some of these galaxies are located as far back in time as just a few billion years after the Big Bang - you might be the first person to ever look at them!

When you classify galaxies here, you'll be looking at a 3-panel image. The first, on the left, is a color image which is our best estimate for what the galaxy would look like if you could see it up close with the naked eye. The middle grayscale image shows the object at approximately rest-frame 464 nanometers (the g-band), which is a wavelength where starlight is dominant over other sources. These two images, generally, show the light from the old stars in the galaxy. The rightmost image shows the same galaxy in the mid-infrared, at 7.7 microns, which shows us the dust in the galaxy. You'll be asked to identify the type of galaxy you see (disk, elliptical/spheroid, or irregular), as well as how that galaxy is interacting with its neighboring galaxies. If you can tell what kind of galaxy it is, we will also ask about more detailed features; this will help us learn more about the kinds of galaxies that are out there, and provide more insight into the types of galaxies that have growing black holes.

We look forward to seeing what you find out there!