Dwarfs and the first galaxies: diesel or jerky?

Image of the cluster Abell 370, one of the regions in the sky observed by the SHARDS Frontier Fields project.

Astronomers from the Centro de Astrobiología (CAB, CSIC-INTA), in collaboration with researchers from the UK, Mexico and Chile, are searching for clues about the origins of the first stars and structures formed in the Universe. For that purpose, they are using data from the most ambitious project carried out with the Hubble Space Telescope (HST) and the Gran Telescopio Canarias (GTC), the so-called Frontier Fields. The first results has just been published in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

One of the most interesting questions that astronomers have been trying to answer for decades is how and when the first galaxies formed. Concerning the how, one possibility is that the formation of the first stars within galaxies started at a steady pace, slowly building up a more and more massive system. Another possibility is that the formation was more violent and discontinuous, with  intense, but short lived bursts of star formation triggered by events such as mergers and enhanced gas accretion.

“Galaxy formation can be compared to a car”, explains Pablo G. Pérez-González, one of the coauthors of the paper, affiliated to the Centro de Astrobiología (CAB/CSIC-INTA) in Spain, and principal investigator of the international collaboration behind this study. “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. 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.”

In the recently published work, the astronomers searched for nearby analogs of the very first galaxies formed in the Universe, so that they could be studied in much more detail. Alex Griffiths explains: “Until we have the new James Webb Space telescope, we cannot observe the first galaxies ever formed, they are just too faint. So we looked for similar beasts in the nearby Universe and we dissected them with the most powerful telescopes we currently have”.

The approach followed in this paper was to combine the power of the most advanced telescopes, such as HST and GTC, with the aid of “natural telescopes”. Chris Conselice, coauthor of the paper and supervisor of Griffiths PhD thesis, professor at the University of Manchester, comments on the strategy: “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. 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”. Observations of some of these massive clusters acting as gravitational telescopes is the base of the Frontier Field survey.

In the work published in MNRAS, the authors combined the gravitational lensing power of some of the Universe’s most massive galaxy clusters with the exceptional GTC data coming from a project entitled the Survey for high-z Red and Dead Sources (SHARDS), in order to locate and study some of the smallest, faintest galaxies in the nearby universe.

“We were able to find these objects due to the high quality SHARDS data coupled with imaging data from the Hubble Space Telescope to detect hot gas heated by newly formed stars in very small galaxies. This hot gas emits in certain wavelengths, what we call emission lines, just as a neon light. Analyzing these emission lines can provide an insight into the formation and evolution of a galaxy”, the authors explain.  “The SHARDS Frontier Fields observations carried out with GTC have provided the deepest data ever taken for discovering dwarf galaxies through their  emission lines, allowing us to identify systems with recently triggered star formation”, explains Pérez-González, one of the coauthors of the paper and principal investigator of the GTC SHARDS Frontier Fields project.

“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”, adds now-Doctor Griffiths. “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”, conclude the authors.

This third image shows a comparison of GTC and Hubble data. Hubble’s observations present better spatial resolution since it is not affected by atmospheric turbulence, unlike GTC. But GTC data is ultradeep and allowed to detect previously unknown galaxies missed by HST.

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