Blog #19: An Explosive Signature of Explosive Galaxies

We learned in Astro 16 the conditions that allow a star to form. Ideally, stars form in large clouds of gas and dust.

We learned in Astro 17 that galaxies can collide. For example, the Milky Way Galaxy and the Andromeda Galaxy are in a direct collision course with each other.

So what does star formation and galaxy mergers have in common? The answer lies in what is known as Gamma-Ray Bursts (GRBs). Gamma-Ray Bursts are narrow beams of intense radiation that are the most powerful event in the Universe aside from the Big Bang itself. They occur when a truly massive star, with a mass of at least 15 $M_{\odot}$ dies and collapses in on itself. When a star of this size runs out of fuel and dies, it explodes in what is known as a "hypernova", which are considered to be substantially more energetic explosions than standard supernovae. In addition to the hypernova, the collapsing star also releases Gamma Ray Bursts.

Another important thing to note is that stars this massive have relatively short life-spans since they use up all of their fuel much faster than smaller stars. Therefore, galaxies that seem to have a lot of GRB's signal that the galaxy is forming new stars. In galaxies, there are two types of gasses: (1) hot, ionized gas, and (2) cold, neutral gas. Star formation, especially for the really massive stars, happens in regions of cold, neutral gas, which is hard to detect. This article shows how researchers observed this cold, neutral gas in galaxies within a 100 million light years away, which is about 50 times further than the Andromeda Galaxy. Because of their relative close distance to us, researchers were able to use the radio frequency on the 21 cm line to detect this cold neutral gas. This region of the galaxy had a lot of GRBs.

Looking at a density map of the cold, neutral gas, it seemed that the gas was disturbed, and was outside the main disk of the galaxy. This suggests that the galaxy had collided with another, smaller galaxy, which resulted in the scattered cold, neutral gas. The other important conclusion was that the collision could have "shock-compressed" the gas, which sparked the formation of massive stars. Since these massive stars give off GMRs when they die, the presence of GMRs could mean that a galaxy is the result of a galaxy collision and merger.

Unfortunately, this was observed only in galaxies that are within a 100 million miles from us. Since the cold, neutral gas is hard to detect because the 21 cm emission line is harder to see further out, it will be harder to conclude that the presence of GRBs are predictive of galaxy mergers. However, this research is on the path to show not only that GMRs are indicative of galaxies that are forming new stars, but the composition and history of the galaxy in which they reside.