This image shows the grism spectroscopy data collected by Hubble’s WFC3. Image Credit: NASA/ESA/Hubble

Dwarf galaxies may be small on the galactic scale, but new data from the Hubble Space Telescope (HST) shows they played a larger role in the development of the early universe than previously anticipated. The bulk of the universe’s stellar population was formed between two and six billion years after the Big Bang; however, we still have new stars forming all across the cosmos. By studying the early universe, astronomers hope to have a clearer picture and better understanding of galaxy evolution.

Hubble’s Wide Field Camera 3 (WFC3) is enabling astronomers to do just that, by observing a small sample of starburst galaxies within a sample of dwarf galaxies from the early universe. Their observations showed that the starburst galaxies were pumping out stars at a furious pace, far exceeding the normal rate. Previous starburst galaxy studies primarily focused on mid-range or high-mass galaxies, leaving out the smaller dwarf galaxies completely. Partially, this was because it was nearly impossible to even study the dwarf galaxies, until the highly sensitive WFC3 was developed.

Thanks to a unique feature–grism spectroscopy mode–astronomers have been able to discern the brightness and color of the early universe. From this data, scientists are able to then determine each galaxy’s distance and chemical composition as well as each starburst galaxy’s contributions to the stellar population within dwarf galaxies. A grism, a combination of a grating and a prism, breaks up the light into its constituent parts, forming a spectrum. Astronomers then analyze each galaxy’s spectrum to determine its characteristics. If a lot of star formation is happening within a galaxy, the newborn stars will emit intense radiation, heating up Hydrogen gas. Astronomers see the Hydrogen gas as very narrow but bright emission lines and these lines are very easy to spot against the faint continuum of the early universe.

Section of the sky studied by Hubble. Red circles represent dwarf galaxies from the early universe. Image Credit: NASA/ESA/Hubble

Researchers had suspected that stellar formation within dwarf starbursting galaxies were major contributors to early star formation, and for the first time, have been able to directly measure exactly how much of a contribution they made. The team was surprised to see that the starburst galaxies had a much more significant role than previously expected. When the majority of stars were forming in the universe, the smaller starburst galaxies were stellar powerhouses, pumping out new stars and doubling their mass in only 150 million years. In a normal galaxy, this amount of star formation would take around 1-3 billion years.

By understanding how these galaxies behaved and formed in the early universe, scientists hope to determine what makes these galaxies burst. Under normal circumstances, it is rare to see a galaxy in a state of “starburst” and it typically caused by an event like a galactic merger, tidal interactions, or shock waves from a supernova explosion. Through this discovery, astronomers are able to understand the relationship between a galaxy’s mass and the rate of star formation. This new data will help astronomers uncover the mystery of galactic evolution.

You can read the full press release on the discovery here.


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