Astronomers are finding that different types of galaxies are more prevalent at different times during the history of the Universe. For example, when the Universe was only around 2 billion years old, there were a very large number of galaxies with irregular shapes, massive amounts of gas and very rapid star-formation. As the Universe aged and stars churned through their gas supplies, galaxies started to form new stars more slowly. Older galaxies also generally have supermassive black holes millions to billions of times the mass of the Sun at their centers. These black holes suck in massive amounts of gas and dust that would normally be forming stars, causing their nearby surroundings to heat up and shine very brightly and look like stars. For this reason, they are referred to as “quasi-stellar objects”, or “quasars”. Just what causes this transition from gas-rich, rapidly star-forming galaxies to slower star-forming quasars is a hot topic in extragalactic astronomy!
Image showing prevalent galaxy populations at different times in cosmic history. Credit: NASA, ESA, M. Kornmesser; the CANDELS team (H. Ferguson)
Using observations and simulations, astronomers hypothesize that the transition between galaxy types in the early-Universe is the result of massive galaxies crashing into each other, causing one of the most dramatic events in the cosmos. When this happens, the centers of the crashing galaxies collide, forming a supermassive black hole that sucks in gas and dust, making it shine very bright as a quasar. However, because of all of the gas and dust, this bright quasar is blocked from our view, so all that we can see is a very faint glow in the dust letting us know that it is there. At the same time that this supermassive black hole forms, all of the gas and dust from the crashing galaxies causes a bunch of new stars to form (around 1,000 new stars per year!) leading to the messy, star-forming galaxies that we see so prevalently when the Universe was around 2 billion years old.
This brings us to Hot DOG galaxies, or “hot dust-obscured” galaxies, which are a special type of quasar that is in this phase of galaxy evolution. The supermassive black hole in the center of all of this gas, dust and star formation is actively churning up so much gas there isn’t nearly as much left for stars to keep forming. The area around the supermassive black hole gets so hot that it starts to push all of the gas out of the galaxy, which also helps to further slow down the formation of new stars. Eventually, most of the gas is gone from around the shining supermassive black hole and it can be seen by us from Earth as a regular quasar.
The farther away we look, the further back into time we are seeing, due to the finite speed of light (which is around 300 million meters per second, or, 670 million miles per hour). For this reason, if astronomers want to study how the Universe was 2 billion years ago, they would look for galaxies that are 2 billion light years away.
In order to detect objects that existed when the Universe was just 2-3 billion years old, we need to use very big and powerful telescopes capable of resolving objects with diameters around 80 times smaller than Jupiter (0.5 arcseconds). Currently, no single telescope is large enough to observe such distant objects, so the signals from several telescopes arranged in an array are stitched together to simulate one large telescope. This practice is known as “interferometry” and is how many discoveries in astronomy have been made.