Oct. 26 — Why do we care what happened 13 billion years ago?
A bold lead question for an interview with an astrophysicist looking at the early universe, but one that doesn’t seem to faze Brian O’Shea. The Michigan State University professor just smiles across the Skype connection and then chuckles.
“If you are interested in the Milky Way today, then understanding the early universe is really important,” he says, still smiling. “Through a process called hierarchical structure formation, little galaxies form bigger galaxies and bigger and bigger galaxies. It happens over and over again and you get a galaxy like the Milky Way. The main reason I’m interested in these galaxies in the very early universe is that fundamentally they are the progenitors of galaxies like the Milky Way. They are essentially the first building blocks. It’s one of those things where the initial state is really important for what happens at the end.”
O’Shea is no stranger to supercomputing or NCSA, dating back to his days as a student at the University of Illinois at Urbana-Champaign. As leader of a Petascale Computing Resource Allocations (PRAC) team that includes co-principal investigator Michael Norman and Hao Xu at the University of California, San Diego, John Wise of Georgia Tech, and Britton Smith of the University of Edinburgh, O’Shea’s been able to explore early galaxy formation and evolution. The team has published more than 16 papers, primarily in the Astrophysical Journal.
The team’s simulations on Blue Waters are, says O’Shea, the most sophisticated simulations that anyone has ever done of galaxies in the early universe. One of the things they’ve found through the simulations is that early on in the universe there are actually far fewer galaxies than they thought.
“The smaller galaxies are just gone and we don’t completely understand that. Our theory right now is that it is radiation from all of the bigger galaxies that suppresses star formation in the dwarfs,” says O’Shea.
When two galaxies merge with each other to form a larger galaxy it is primarily because the dark matter is drawn together by its gravitational potential, explains O’Shea. Gas is mostly along for the ride in galaxy formation, while stars in a galaxy contribute to, but don’t dominate, pulling together the galaxies. This leads to “dark” halos—matter that’s not seen, doesn’t have much gas in it, and has very few stars or even no stars at all.
“What we’re finding is that some of these dark matter halos have no stars at all. And something else that is really interesting is when you look at the satellite galaxies in the Milky Way the smaller ones are dark matter dominated. In the Milky Way there’s about 10 times as much dark matter as there are stars, and if you look at these really small dwarf galaxies it’s more like 1,000 times more dark matter than there is stars. That’s because it’s really hard to form the stars because of all these other physical effects. And in our simulations where we’re looking at galaxies that are more the size of those dwarf galaxies we see the same thing; that their dark matter clearly dominates. That there’s a hundred times more dark matter than there is stars, or a 1,000 times more dark matter than there is stars.
“So the pieces are actually starting to fall together in terms of understanding what’s going on in these galaxies around the Milky Way. What we’re seeing at high redshift early in the universe when we’re doing these simulations is that the bigger galaxies around are sort of stomping on their buddies. Radiation from the big galaxies keeps the small ones from doing anything exciting, keeps them from forming stars. About a billion years after the Big Bang there’s just enough radiation that the whole universe gets ionized and all of these little galaxies can never form stars after that, they’re just completely quenched for the rest of their lives,” says O’Shea.
What this means, he says, is that there should be a significant number of dark matter halos without any stars in the middle as well as small galaxies with only a few stars that won’t form additional stars. And that actually agrees with the galaxies seen by the Dark Energy Survey (DES) and the Sloan Digital Sky Survey where there’s a hundred more times dark matter than stars, or a thousand times more dark matter than stars in terms of the mass, and all of the star formation happened 13 billion years ago.
“Once you get to stars that are that old the margin of error is a couple billion years, simply because they evolve quite slowly so you can’t quite tell how old they are,” he says with a laugh. “But it’s consistent with this idea of everything happening really early on in the universe.”
Dwarf galaxies around the Milky Way are by and large relics of this first generation of galaxies. O’Shea is excited that analysis of data recorded by the DES has found more dwarf galaxies than previously observed. He says that discovery is the connection between the work his team has been doing on Blue Waters and the galaxy that we live in now.
“The work that we’re doing on Blue Waters is trying to understand how those first galaxies behave. A lot of those galaxies are still around and are still in orbit around the Milky Way, they’re just really hard to find. They’re relics, they’re fossils. The term galactic archaeology refers to looking at all these really old stellar populations, really old galaxies, really old stars, to figure out what was going on in the early universe. These ultra-faint dwarf galaxies that you find in DES are really fossils of what happened in that epoch right after the Big Bang that we are studying directly with our simulations.”
Yet for all we learn about the universe there are many, many things in the universe that remain unexplainable, despite decades of galactic research. Take black holes, for instance. There’s a black hole at the center of every massive galaxy, including the Milky Way. Yet no one is quite sure how this region of space forms in which nothing escapes, including light. Nor do researchers know how it grows.
O’Shea and other researchers believe, however, that black holes are “really intimately tied to how galaxies form.” He’s the principal investigator on a recently awarded PRAC allocation that will allow him to connect his work on the early universe to the present day Milky Way. He’ll lead a team that will use Blue Waters to conduct simulations on several aspects of galaxy evolution, including exploring precisely how the early galaxies and black holes turn into present-day galaxies.
Included in the team’s research is star formation. O’Shea is among the researchers who are convinced that early star formation “sets the stage for everything that happens afterwards.” And by sets the stage he means “they produce the metals that pollute the gas that all this star formation occurs in. They change the chemical composition and the actual behavior of the gas; it changes the future generation of star formation.”
The first stars in the universe, called primordial stars, formed during the Big Bang from just hydrogen and helium. Those gases are poor coolants, resulting in hot clouds of gas in the universe. Stars that form from hot gases are significantly larger than stars that form later from cooler gases. The later stars have carbon, nitrogen, oxygen and iron in them, among other things.
“The first generation stars are much bigger and brighter. One of the questions we’ve been trying to answer,” says O’Shea, “is how does that transition take place. We know that at some point there were stars that were primordial and then at some point later there were some stars that have metal. By the way, we call metal anything heavier than helium on the periodic table. That’s a term that astronomers use to mean it came from stars and it absolutely enrages chemists because metal has a very specific meaning in their field. I keep using that word in an indelicate way and I apologize. We’re really interested in the process by which the universe gets polluted with all of this other stuff on the periodic table. So I’m working on a project with Britton Smith and John Wise, and we’re trying to understand how that transition takes place.”
The difference between big stars and small stars is the difference between all of the stars blowing up in supernovae and only some of them blowing up in supernovae, he explains. The size of the stars also affects the radiation emitted.
And the first generation of stars “also probably produce the black holes that end up in the middle of all the galaxies today. We’re not sure about that but it is a plausible theory and some people think it’s true,” says O’Shea.
The mass of a star determines how long it lives. O’Shea says simulations, such as those done by his Michigan State colleague Ed Brown, have shown that smaller stars can live for 20 or 30 billion years, which is longer than the current age of the universe. So many of the smaller, dimmer stars that astronomers observe are most likely “relics of that initial time. But these little stars evolve really slowly. They’re actually offering really useful clues as to what’s going on early in the universe,” he says.
Dense of Void
Another issue O’Shea is exploring is that different parts of the universe are denser than other parts. There are galaxies in groups, like the Milky Way and Andromeda. But there are also voids—large regions that are millions and millions of light years across—that, as far as can be determined, are totally empty.
In his simulations run on Blue Waters, however, instead of being really, really dense or really, really empty these regions are somewhat dense or somewhat empty, but not definitively either dense or empty. Exploring a dense region that would be a galaxy cluster, an average region that the Milky Way would form from, and a low-density region that a void would form from and trying to understand the differences between the over-dense region, the average region and the under-dense region is another research focus O’Shea believes could yield clues about the early universe and its influence on the present day.
“I’m really interested in that, but we’re still in the middle of that analysis,” says O’Shea. “We got sidetracked with other science and there’s not that many of us [on the team]. That’s something that is really interesting because at present day, 13.7 billion years after the Big Bang, it’s really obvious that galaxies behave differently in different environments. So when galaxies are in clusters where they are all packed tightly together and orbiting around each other at hundreds of kilometers a second, galaxies look different in that environment compared to something like the Milky Way. And we want to know where along the line does that actually happen. Do you see differences between environments when you look at galaxies 400 or 500 million years after the Big Bang when the universe is 4 percent of the age that it is today, or does it take longer for those differences to develop? It’s an interesting question, it’s very relevant to modern day galaxy formation. We just haven’t answered it yet.”
Source: Barbara Jewett, NCSA