Astronomers investigating behaviour of the universe shortly after the Big Bang have made a surprising discovery: the properties of the early universe are determined by the smallest galaxies. These galaxies are over 1000 times less massive than the Milky Way and contributed nearly one-third of the UV light during re-ionisation. The field of view of this image is 400,000 light years across, when the universe was only 700 million years old. Other studies often ignore these small 'dwarf' galaxies as they weren't thought to form stars, because the UV light from nearby larger galaxies was too strong and suppressed these tiny neighbors.
Astronomers agree that the universe became fully re-ionised roughly one billion years after the Big Bang. About 200 million years after the birth of the cosmos, ultraviolet (UV) radiation from stars began to split neutral hydrogen into electrons and protons. It took another 800 million years to complete the process everywhere. This epoch of re-ionisation marked the last major change to gas in the universe, and it remains ionised today, over 12 billion years later.
However, astronomers aren't in agreement on which type of galaxies played the most important role in this process. Most have focused on large galaxies. The new study by researchers at the Georgia Institute of Technology and the San Diego Supercomputer Center indicates scientists should also focus on the smallest ones.
A view of the entire simulation volume showing the large scale structure of the gas, which is distributed in filaments and clumps. The red regions are heated by UV light coming from the galaxies, highlighted in white.
The researchers used computer simulations to demonstrate the faintest and smallest galaxies in the early universe were essential. These tiny galaxies – despite being 1000 times smaller in mass and 30 times smaller in size than our own Milky Way galaxy – contributed nearly 30 percent of the UV light during this process.
A zoomed-in view of the most massive dwarf galaxy in the simulation, seen when the universe was only 700 million years old. This galaxy only has 3 million solar masses of stars, compared to 60 billion solar masses in our Milky Way. The yellow points represent the older and cooler stars in the galaxy, and the blue points show the young and massive stars forming just before this snapshot of the simulation. The haze around the stars show the gas distribution in the galaxy with blue and red representing hot and cold temperatures, respectively.
"It turns out these dwarf galaxies did form stars, usually in one burst, around 500 million years after the Big Bang," said Prof. John Wise, of the Georgia Institute of Technology, who led the study. "The galaxies were small, but so plentiful that they contributed a significant fraction of UV light in the re-ionisation process."
The team's simulations modelled the flow of UV stellar light through the gas within galaxies as they formed. They found that the fraction of ionizing photons escaping into intergalactic space was 50 percent in small galaxies (more than 10 million solar masses). It was only 5 percent in larger galaxies (300 million solar masses). This elevated fraction, combined with their high abundance, is exactly the reason why the faintest galaxies play an integral role during re-ionisation.
"It's very hard for UV light to escape galaxies because of the dense gas that fills them," said Wise. "In small galaxies, there's less gas between stars, making it easier for UV light to escape because it isn't absorbed as quickly. Plus, supernova explosions can open up channels more easily in these tiny galaxies in which UV light can escape."
The team's simulation results provide a gradual timeline that tracks the progress of re-ionisation over hundreds of millions of years. About 300 million years after the Big Bang, the universe was 20 per cent ionised. It was 50 per cent at 550 million years. The simulated universe was fully ionised at 860 million years after its creation.
"That such small galaxies could contribute so much to re-ionisation is a real surprise," said Prof. Michael Norman, of the University of California San Diego and one of the co-authors of the paper. "Once again, the supercomputer is teaching us something new and unexpected; something that will need to be factored into future studies of re-ionisation."
A rendering above of a simulation that follows the formation of the first galaxies in the universe. The field of view is adjusted to account for the expansion of the universe, where the scale bar represents 32,600 light-years (10,000 parsecs). The video shows hot and ionised gas in blue, and cold and neutral gas in red. The intensity of each pixel is set by the gas density, and the stars are not shown in this visualisation. The video runs from 200 million to 800 million years after the Big Bang.
In the image at the top of the page, 75,000 light years from Earth, a galaxy known as Segue 1 has some unusual properties: It is the faintest galaxy ever detected. It is very small, containing only about 1,000 stars. And it has a rare chemical composition, with vanishingly small amounts of metallic elements present.
A team of scientists recently analyzed the galaxy's chemical composition and come away with new insights into the evolution of galaxies in the early stages of our universe — or, in this case, into a striking lack of evolution in Segue 1. Commonly, stars form from gas clouds and then burn up as supernova explosions after about a billion years, spewing more of the elements that are the basis for a new generation of star formation.
Not Segue 1: In contrast to all other galaxies, as the new analysis shows, it appears that Segue 1's process of star formation halted at what would normally be an early stage of a galaxy's development.
"It's chemically quite primitive," says Anna Frebel, an assistant professor of physics at MIT, and the lead author of a new paper detailing the new findings about Segue 1. "This indicates the galaxy never made that many stars in the first place. It is really wimpy. This galaxy tried to become a big galaxy, but it failed."
But precisely because it has stayed in the same state, Segue 1 offers valuable information about the conditions of the universe in its early phases after the Big Bang.
"It tells us how galaxies get started," Frebel says. "It's really adding another dimension to stellar archaeology, where we look back in time to study the era of the first star and first galaxy formation."
The analysis uses new data taken by the Magellan telescopes in Chile, as well as data from the Keck Observatory in Hawaii, pertaining to six red giant stars in Segue 1, the brightest ones in that galaxy. The astronomers are able to determine which elements are present in the stars because each element has a unique signature that becomes detectable in the telescope data.
In particular, Segue 1 has stars that are distinctively poor in metal content. All of the elements in Segue 1 that are heavier than helium appear to have derived either from just one supernova explosion, or perhaps a few such explosions, which occurred relatively soon after the galaxy's formation. Then Segue 1 effectively shut down, in evolutionary terms, because it lost its gas due to the explosions, and stopped making new stars.
"It just didn't have enough gas, and couldn't collect enough gas to grow bigger and make stars, and as a consequence of that, make more of the heavy elements," Frebel says. Indeed, a run-of-the-mill galaxy will often contain 1 million stars; Segue 1 contains only about 1,000.
The astronomers also found telling evidence in the lack of so-called "neutron-capture elements" — those found in the bottom half of the periodic table, which are created in intermediate-mass stars. But in Segue 1, Frebel notes, "The neutron-capture elements in this galaxy are the lowest levels ever found." This, again, indicates a lack of repeated star formation.
"It is very different than these other regular dwarf-type galaxies that had full chemical evolution," Frebel says. "Those are just mini-galaxies, whereas [Segue 1 is] truncated. It doesn't show much evolution and just sits there. We would like to find more"
Dwarf galaxies, astronomical modeling has found, appear to form building blocks for larger galaxies such as the Milky Way. The chemical analysis of Segue 1 sheds new light on the nature of those building blocks, as Frebel notes.
The research team expects to learn more about these faint galaxies when the next generation of telescopes is operational. For example, NASA's James Webb Space Telescope, scheduled to launch in 2018, will be able to see them. The team report their findings in a paper published today in the journal Monthly Notices of the Royal Astronomical Society.
The Daily Galaxy via RAS and MIT
Image credits: Credit: John Wise, Matthew Turk, Michael Norman, Tom Abel, Britton Smith