The Milky Way galaxy, which stretches some 100,000 light-years across, is orbited by dozens of much smaller galaxies, some spanning only a few thousand light-years. Where these satellite galaxies came from, how they formed, and what they can teach us about the universe are among the most pressing questions in astronomy. But we do know these faint little galaxies have interesting pasts.
In a published today in the Astrophysical Journal, researchers from the Harvard-Smithsonian Center for Astrophysics (CfA) and the Institute for Computational Cosmology at Durham University conclude that the faintest satellite galaxies around the Milky Way are among the oldest galaxies in the entire universe. These little galaxies accumulated the vast majority of their mass roughly 13.5 billion years ago, not long after the Big Bang.
Essentially, the researchers found that the fainter a galaxy is, the more likely it is to be ancient. They used what's called the luminosity function (LF), or the distribution of galaxies according to how bright and massive they are.
By using supercomputers in Warsaw, Poland, and Durham, England, the researchers calculated the luminosity function of galaxies according to the standard model of cosmology, known as the Lambda Cold Dark Matter model. They found that there is a break or "kink" in the LF—a specific brightness where there are very few galaxies. As it turns out, this break in the LF corresponds to a time when galaxies stopped forming temporarily, and anything that is fainter than that luminosity likely formed long, long ago.
"After some exploration, we found that where this kink happens (i.e., what galaxy luminosity it occurs at) is set by the transition between galaxies forming during the Dark Ages to those that form after," Sownak Bose, a research fellow at the CfA and lead author of the new study, told Seniorhelpline in an email.
According to the standard model, the Dark Ages began about 380,000 years after the Big Bang, when the universe cooled and the first atoms of neutral hydrogen formed. Still without stars or galaxies, the universe was filled with hydrogen gas and dark matter. The dark matter coalesced into "halo" structures, after which the gravity from the dark matter pulled the hydrogen gas together to form the first stars and galaxies.
"It is believed that particles of dark matter clump together to form extended structures of dark matter under the influence of gravity," Bose says. "These structures act as 'gravitational sinks’ and are able to pull in ordinary baryonic matter, primarily in the form of hydrogen. When hydrogen collects in dense enough clumps, stars and eventually galaxies are able to form."
These dark matter haloes are not just a product of the past, but are believed to form the structure of the universe to this day.
"We expect that all galaxies—including the Milky Way—are in fact surrounded by haloes of dark matter that enabled the galaxies to form in the first place," Bose says. "These dark matter structures are in some ways the invisible skeletal framework on which the visible cosmos is built."
However, after the first stars and galaxies formed, the intense ultraviolet radiation ionized the remaining hydrogen gas, stripping the atoms of electrons and leaving behind charged particles. The dark matter haloes were no longer sufficient to trap the gas and form new stars, and galaxy formation stopped for roughly a billion years.
"When [hydrogen] gets heated up due to the radiation from the first galaxies, the tiny haloes of dark matter are no longer able to trap the heated gas," Bose says. "As the funnelling of fuel gets shut off, star formation eventually ceases and they are no longer able to grow in mass. Only more massive haloes of dark matter (which take hundreds of millions of years to get built up) have gravitational wells deep enough to contain the gas and form new stars."
The Big Pause
This pause in galaxy formation due to a jolt of ionization is what results in the "kink" that the research team calculated in the luminosity function. Galaxies before the "kink" are small, faint, and ancient galaxies that accumulated most of their mass as the Dark Ages came to an end, while galaxies that formed after the "kink" tend to be bigger and brighter. Over time, these galaxies merge and coalesce, forming larger galaxies such as our own Milky Way, which could , but did not form its current structure or accumulate the majority of its mass until much later.
Some galaxies, however, such as the satellites around the Milky Way, remain largely unchanged for billions and billions of years. These include the dwarf galaxies Segue-1, Bootes I, Tucana II, and Ursa Major I, all of which are believed to have formed along with the very first galaxies. Overall, Bose says astronomers have discovered about 54 galaxies orbiting our own, but only about 47 percent of the sky has been adequately searched, so estimates for the total number of satellite galaxies are between 100 and 150.
"As a rough estimate, on average about 80 percent of the satellites fall in the ‘ancient’ group, with the rest being formed later," Bose says.
But in the end, the fact that a swirl of the oldest galaxies in the universe encircles our own Milky Way might not be that surprising after all. Galaxies are thought to be distributed across the universe more or less evenly, and researchers have of some of those close to us before. The real value in this work is refining theories of dark matter and the evolution of the cosmos.
"I think the search for and study of ultra-faint galaxies is very exciting because it can help us understand better not only the process of galaxy formation, but also the nature of the dark matter itself," Bose says. "Different models of dark matter predict differences in how abundant these galaxies are, what physical properties they have, how old they can be etc. So if we have large statistical information on these populations available at hand, we can learn something very fundamental about the makeup of the cosmos."