Images of Galactic Clusters Reveal Early Galaxies Underwent Periods of Violent Star Formation

Images taken using the Hubble Space Telescope have given astronomers clues about the formation and growth of the first galaxies, suggesting that early star formation happened in violent bursts.

A team of astronomers used data collected by Hubble and the ground-based Gran Telescopio CANARIAS (GTC), to observe and investigate some of the smallest and faintest nearby galaxies. They focused on those that resemble galaxies that existed in the early universe.

Until now, astronomers have had difficulty in determining how some of the earliest galaxies in the Universe formed. Theories suggest that star formation in early galaxies could have either happened smoothly or that it could have taken place in sudden violent bursts of activity.

"Galaxy formation can be compared to a car," researcher from Centro de Astrobiología in Spain, Pablo G. Pérez-González, said in a statement from the University of Nottingham. "The first galaxies might have had a 'diesel' star-forming engine, slowly but continuously adding up new stars, without much acceleration and gently turning gas into relatively small stars for long periods of time.

"Or the formation could have been jerky, with bursts of star formation producing incredibly large stars that disrupt the galaxy and make it cease its activity for a while or even forever."

Pérez-González, who led the investigation and is the lead author on a paper detailing the findings published in Monthly Notices of the Royal Astronomical Society, added: "Each scenario is linked to different processes, such as galaxy mergers or the influence of supermassive black holes, and they have an effect on when and how the carbon or oxygen, that are essential for our life, formed."

The problem of determining how early galaxies grew has continued because current telescopes aren't powerful enough to view the Universe's earliest galaxies. What the team led by Pérez-González did instead, was select nearby galaxies that are similar to these earlier galaxies and study the way in which they formed.

"Until we have the new James Webb Space telescope, we cannot observe the first galaxies ever formed, they are just too faint," University Nottingham researcher, Dr. Alex Griffiths, said. "So we looked for similar beasts in the nearby Universe and we dissected them with the most powerful telescopes we currently have."

The telescopes that the team used to conduct their investigation, including Hubble, were given a boost by a so-called "natural telescope" the phenomenon of gravitational lensing.

Gravitaional Lensing
A NASA graphic showing how gravitational lensing curves light and creates cosmic optical illusions. Researchers used the phenomenon to discover a weird object was actually the reflection of a distant galaxy. NASA

First predicted in Albert Einstein's theory of general relativity, gravitational lensing occurs when an object of tremendous mass, like a star, black hole, or an entire galaxy, curves space in its vicinity.

This has the effect of making the usually straight paths that light follows curve as well when they pass through this warped region of space. Not only can this lead to some spectacular illusions in the night sky, such as making one object appear at multiple points, it can also magnify the light from distant sources.

"Some galaxies live in large groups, what we call clusters, which contain huge amounts of mass in the form of stars, but also gas and dark matter," University of Manchester researcher and study co-author Chris Conselice said. "Their mass is so large that they bend space-time, and act as natural telescopes. We call them gravitational lenses and they allow us to see faint and distant galaxies with enhanced brightness and at a higher spatial resolution".

The team looked at the signatures of emissions in 1,098 galaxies, from the galaxy clusters Abell 370 and MACS J1149.5+2223, light from which takes approximately 5 billion years to reach the Earth, to search for gas heated by newly formed stars.

What they discovered was evidence that the galaxies they studied grew in a stop-start fashion, with bursts of star-forming activity followed by lulls.

"Our main result is that the start of galaxy formation is fitful, like a jerky car engine, with periods of enhanced star formation followed by sleepy intervals," said Griffiths.

The researcher went on to reveal that the reason these periods of star formation stop and then start again is still unknown.

Griffiths continued: "It is unlikely that galaxy mergers have played a substantial role in the triggering of these bursts of star formation and it is more likely due to alternative causes that enhance gas accretion, we need to search for those alternatives."

Galaxy cluster MACS J1149.5+223,
An image of the galaxy cluster MACS J1149.5+223. The galaxies within this cluster revealed to astronomers that the Universe may have evolved in fits and starts. S. Rodney John Hopkins University, USA) and the FrontierSN team; T. Treu University of California Los Angeles, USA), P. Kelly University of California Berkeley, USA) and the GLASS team; J. Lotz (STScI and the Frontier Fields team; M. Postman (STScI and the CLASH team; and Z. Levay (STScI/NASA/ESA