NASA's Hubble Space telescope was recently able to make an extraordinary discovery by looking nine billion years into the past. This discovery, made possible by Hubble's near-infrared capabilities, involves a group of dwarf galaxies that, while young and small, are producing stars at an incredibly rapid rate. The number of stars they contain is likely to double in 10 million years.
This provides quite a contrast to our Milky Way galaxy, which is about a hundred times more massive than each of these galaxies, as it would take the Milky Way a thousand times longer to double its stellar population.
These findings are described in a paper published online on Nov. 14 in The Astrophysical Journal, of which Ariën van der Wel of the Max Planck Institute for Astronomy in Heidelberg, Germany is the lead author.
It is not surprising that galaxies had stars being produced at higher rates when the universe was still younger than today. In fact, this is true for most galaxies. However, the rate of star formation exhibited by the 69 dwarf galaxies observed – dwarf galaxies being the most common galaxy type in the cosmos – is extreme, even by those standards. The rapidity with which the stars are born probably represents an important phase in the dwarf galaxies' formation.
What Hubble saw, using its Wide Field Camera 3 and Advanced Camera for Surveys, was radiation from young stars, as it caused the oxygen around the stars to light up brightly. While the researchers were not specifically looking for this sort of behavior from these specific stars, the galaxies were extremely conspicuous because of their unusual coloration from their radiation. These galaxies were found in the regions of the sky known as the the Great Observatories Origins Deep Survey South and the UKIDSS Ultra Deep Survey (part of the UKIRT Infrared Deep Sky Survey).
In addition to images, Hubble also captures spectras, which indicate the galaxies' chemical composition. These spectra, confirm the rapid birth of stars by looking at the oxygen in the galaxies.
These galaxies would have been difficult to detect in the past. Until recently, astronomers only surveyed tiny patches of sky. However, thanks to the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), a three-year survey of the most distant galaxies in the universe – in other words, a survey of dwarf galaxies at an early stage in the universe's history – this type of discovery has been made possible.
The results of these findings have a variety of implications. One concern is their contradictory nature with existing studies, which have been carried out with great attention to detail, of dwarf galaxies that orbit the Milky Way as satellites. Such studies seem to suggest that star formation was a long process that stretched out over billions of years. However, the CANDELS findings indicate that certain galaxies -- in particular, dwarf galaxies – were forming stars extremely quickly early on (i.e. 9 billion years ago) in the universe's roughly 13.7 billion year-long existence.
In fact, the findings suggest that such galaxies were incredibly common at the time. We do not yet know, however, why it was the case that they were forming stars so quickly. It may be the case that star formation in dwarf galaxies is episodic and cyclical, as computer simulations can show. However, these simulations are unable to produce bursts of stellar activity as intense as what Hubble has observed.
With the launch of the James Webb Space Telescope, scientists will be able to look at these dwarf galaxies in further detail and at an even earlier era, as well as perhaps find other galaxies that are only just beginning to form stars. In doing so, they will be able to find out even more about the chemical composition and formation of early stars and galaxies.
