article image

Ancient fingerprints give fresh clues to how galaxies evolve

Paul Sutherland, Feature writer
May 27, 2015, 3:02 UTC

Sen—Astronomers are studying some of the most most distant galaxies in the Universe to learn more about other galaxies much closer to home, including our own Milky Way.

They have found a roundabout way to discover the chemical make-up of remote galaxies, that are much too far away for individual stars within them to be discerned, even with the most powerful telescopes.

The latest results using this technique, presented today at the 2015 meeting of the Canadian Astronomical Society in Hamilton, Ontario, will help scientists better understand how galaxies evolve.

Instead of studying stars, the team led by Trystyn Berg looked for gas released by supernovae—stars that have ended their lives in a violent explosion.

They analysed the make-up of this gas by using another clever bit of forensics. They measured light from even more remote, and brilliant, quasars as they shone though galaxies in the foreground.

Berg explained that all of the gas within the galaxy will absorb some of the background quasar light, leaving a chemical fingerprint of which elements are within the galaxy itself, giving astronomers some very useful clues about the evolutionary history of the galaxy.

Berg, a PhD candidate in physics and astronomy at the University of Victoria, in British Columbia, Canada, told Sen: “This technique has been used for the past three decades. It was pioneered to try and find the analogues of Milky Way in the early Universe.

“As supernovae release the elements into the gas of galaxies, the field naturally moved to measuring the chemistry. In addition, stellar astronomers have been using the chemistry of stars to understand how our own Milky Way formed. With the large samples of both stars and galaxies available, it made our comparison unique compared to previous works.”


The twin Keck Telescopes near the summit of Mauna Kea, Hawaii. Image credit: T. Wynne / JPL

Berg said it felt awesome to be working with light that had come from distant galaxies created during the first three billion years of the Universe, which is currently reckoned to be 13.82 billion years old. He told Sen: “I still can’t fathom that we can use the light that is missing because an object absorbed it ten billion years ago! In addition, the ability to measure amounts of elements at the same accuracy, and maybe even better, than the chemistry of stars is just as impressive!”

The team say they are the first to carry out a detailed comparison of this data to what is observed in nearby galaxies, including our own.

Using observations made with one of the twin Keck Telescopes on Hawaii, which are among the world’s largest optical instruments, Berg and his collaborators observed 30 distant galaxies, to add to a sample of 310 previously studied. They were then able to compare the chemistry in distant galaxies to nearly 2,000 stars within six nearby galaxies, including the Milky Way.

Berg told Sen: “In terms of the chemical content, we found that these galaxies appear similar to smaller dwarf galaxies that orbit the Milky Way. However they are not identical, which is very intriguing! I also want to emphasize that these are analogues to our own galaxy—it is crazy to think that we can see galaxies like our own so long ago!”

Announcing the results, Berg described the observations as stepping stones that allow astronomers to decide from which early galaxies the stars of modern galaxies came. He said: “We still don’t have an understanding of how parts of the Milky Way system formed, and our results now tell us what chemistry to go look for to answer this question.”

Berg told us he plans to build on his research with the aid of Hubble. He said: “We are bringing this technique closer to home using the Hubble Space Telescope. We are going to be looking at the gas outside of galaxies. By “closer”, I mean the light has taken at most 1.5 billion light years rather than 10 billion. The plan is to understand the properties of gas reservoirs in galaxies that will later form stars.”