May 2, 2014
Faint Galaxy Sheds New Light On Unusual Aspects Of The Universe’s Early Evolution
John P. Millis, Ph.D. for redOrbit.com - Your Universe Online
In the normal course of evolution, galaxies initially formed stars as clouds of hydrogen and helium collapsed. Eventually the density and temperature of the cores would ignite nuclear fusion, allowing them to shine during what we call the main sequence phase of their lives.
Eventually, stars will fuse heavier and heavier elements, with stars like our sun eventually culminating as balls of carbon and oxygen – in varying percentages – that we call white dwarfs. More hefty stars will continue fusing more massive elements, and eventually explode in brilliant supernovae. The result is that much of the rest of the periodic table is filled out through the intense energy and plasma density of the blast – a process called nucleosynthesis.
Over time the remnants of these stars, both small and large, are used as the building blocks of new stars. However, instead of simple hydrogen and helium, the interstellar clouds gain greater and greater percentages of heavier elements. Our solar system, for instance, is still dominated by hydrogen and helium, but we find that some two percent of the solar composition is heavier elements.
Older galaxies should be characterized by higher quantities of these more massive elements, while younger galaxies should be “metal-poor”. But this is what makes the nearby galaxy Segue 1 so peculiar.
Located some 75,000 light-years from earth – virtually next door in cosmic terms – this dwarf galaxy contains a mere 1,000 stars. There are star clusters within our own Milky Way with considerably greater numbers. The difference is that galaxies like Segue are immersed in a dark matter halo, a differentiator from simple star clusters.
What makes this galaxy interesting, other than its small size, is the fact that it lacks heavy elements. “It’s chemically quite primitive,” said Anna Frebel, assistant professor of physics at MIT, in a recent statement. “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 simply because the galaxy seems to have stagnated, refusing to continue the process of star formation, doesn’t mean that it is uninteresting. In fact, it is quite the opposite. Segue 1 gives astronomers a look into the early Universe, a model of early galaxy formation.
“It tells us how galaxies get started,” Frebel told MIT's Peter Dizikes. “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.”
Using data from the Magellan telescopes in Chile, in addition to images from the Keck Observatory in Hawaii, the team analyzed six red giant stars representing the brightest samples in the galaxy. Using a technique called spectroscopy, the team was able to identify the elements that are present in the stars.
The objects were found to be so metal poor, that the amounts would have been developed in only a handful of supernovae events, perhaps as few as one. The initial events would have occurred shortly after the formation of the galaxy, meaning that stellar formation would have quickly slowed. “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 said.
Further analysis led scientists to realize that even heavier elements – those found in the bottom half of the periodic table, known as neutron-capture elements – are also distinctly lacking. In fact, the quantities were the lowest ever found in any galaxy. This also indicates a lack of star formation, particularly of medium and high mass stars which are usually responsible for the creation of such elements.
“It is very different than these other regular dwarf-type galaxies that had full chemical evolution,” notes Frebel. “Those are just mini-galaxies, whereas [Segue 1 is] truncated. It doesn’t show much evolution and just sits there.”
Dwarf galaxies are important because they are thought to be the foundation for larger galaxies, like the Milky Way, as they merge with other galaxies in the area. The more that we learn about primitive dwarf galaxies, like Segue 1, the more we can peer into the history of the formation of larger galaxies like our own.
A problem remains however: Segue 1 may not be a representative sample. Scientists, in general, are cautious to draw too many conclusions from a single data point. In order to get a clear picture of early galaxy formation and evolution, astronomers need to locate other metal-poor dwarf galaxies.
“We would really need to find more of these systems,” said Frebel. However, she is quick to note, “If we never find another one [like Segue 1], it would tell us how rare it is that galaxies fail in their evolution. We just don’t know at this stage because this is the first of its kind.”
The paper, “Segue 1: An Unevolved Fossil Galaxy from the Early Universe,” has just been published by The Astrophysical Journal. Along with Frebel, the co-authors of the paper are Joshua D. Simon, an astronomer with the Observatories of the Carnegie Institution, in Pasadena, Calif., and Evan N. Kirby, an astronomer at the University of California at Irvine.