Galaxies grow and change with time, but the changes are so slow that we have to piece together the story of their lives from the snapshots we can take of the sky. With new observations and simulations of these processes, we are finally filling in some of the missing pieces. New observations are shedding fresh light on galaxies – from the earliest to form, to how galaxy form stars and stop forming stars, to the physics of galaxy clusters, the most massive objects in the Universe. Rachel Reddick caught up with Eliot Quataert after the KIPAC@10 galaxy evolution session.
One of the most fascinating frontiers is trying to find the youngest, most distant galaxies. The most distant galaxy reported so far is over 13 billion light-years away from us, which means we're seeing it a mere 350 million years after the Big Bang. This distant galaxy was spotted by the Hubble Space Telescope, but as Dan Stark mentioned, we’re always hoping to look deeper. Because these distant galaxies are greatly redshifted, they are easiest to see in the infrared. We’re looking forward to the launch of the James Webb Space Telescope, which will be able to see many more of these young galaxies.
We also expect JWST to tell us more about the gas surrounding galaxies in the early universe. Such gas was opaque, causing scientists to call this period the “dark ages.” That gas was ionized and made transparent by energetic light from the first stars and galaxies. But, there’s a problem: the galaxies we can currently see don’t produce enough energy to reionize the universe fast enough. Further, if reionization proceeds strongly enough, it may prevent some very small galaxies from ever forming. If so, that could explain why we see fewer small satellite galaxies around the Milky Way than most models predict. The hunt for more young, ionizing galaxies is on.
Star formation is one of the most important processes, and not just for the early universe. If it didn’t happen, the night sky would be empty! One of the major challenges for people such as Mariska Kriek and Yu Lu who work on galaxy formation models is to make sure galaxies have just enough star formation, but not too much. The result is a competition between two kinds of processes – those that heat gas and blow it out of the galaxy, and those that allow gas to cool and collapse into stars. Mechanisms that remove gas are generally referred to as feedback. This is because they all involve processes like star formation or black hole jets that first accrete some material, then heat up the rest enough to blow it away. Unfortunately, these generally happen on much smaller scales than galaxies themselves, which makes such processes very challenging to model accurately.
On the other side of the question, Norbert Werner and Aurora Simionescu focused on how star formation and gas accretion is fueled. Filaments of dust and gas are now visible in galaxy clusters, as is the sloshing of gas caused by mergers between these massive groups of galaxies. The interplay of this gas with the growth of galaxies and black holes is under investigation. Some of the most dramatic events in the universe, the jets of hot gas thrown out by the black holes at the center of galaxies, also contribute to the turbulence of gas in galaxy clusters.
While galaxy clusters are some of the most massive object in the universe, astronomers are also studying voids. Voids are what they sound like – large regions of space devoid of galaxies. Voids are actually quite important, as they help us understand how galaxies are distributed relative to each other. They can also tell us how much galaxies are affected by the acceleration caused by dark energy. Jeff Scargle described a new method for quantifying these empty spaces, an important step to better understanding them.
Overall, we have a much better picture of galaxy formation and evolution – and a long way to go. The general picture of galaxies growing from the early universe to the present day, accreting gas, forming stars, and eventually stopping star formation, is in place. The details of why and how are just waiting around the corner.
You can watch all the talks in this session on the KIPAC youtube channel.
You can also read more about KIPAC@10 on the conference blog home page.