It is the most metal-poor galaxy ever discovered.
About 30 million light-years from Earth lies a unique, faint blue galaxy located in the constellation Leo Minor. Why is it to special? It could shed new light on the conditions at the birth of the universe.
The galaxy Leoncino, or “little lion,” contains the lowest level of heavy chemical elements, or “metals,” ever observed in a system of stars. In astronomy, any element other than hydrogen or helium is referred to as a metal, and metal-poor galaxies closely resemble the early universe.
"Finding the most metal-poor galaxy ever is exciting since it could help contribute to a quantitative test of the Big Bang," said co-author John J. Salzer, professor at IU Bloomington College of Arts and Sciences’ Department of Astronomy, in an IU news release. "There are relatively few ways to explore conditions at the birth of the universe, but low-metal galaxies are among the most promising."
The current accepted model of the start of the universe predicts the amount of helium and hydrogen present during the Big Bang, and the ratio of these in metal-poor galaxies provides a direct test of it.
But to find these types of galaxies, astronomers have to look far away from our own galaxy. There are just too many heavy elements in the Milky Way due to “stellar processing” — the slow process where stars generate heavier elements from hydrogen and helium and then explode as supernovae, redistributing these atoms back into the galaxy.
"Low metal abundance is essentially a sign that very little stellar activity has taken place compared to most galaxies," said Alec S. Hirschauer, a graduate student at IU and lead author, in the press release.
To determine the abundance of elements in a galaxy, astronomers use spectroscopic observations, which capture the emitted light waves. For this study, the observations were collected by spectrographs on two telescopes in Arizona: the Mayall 4-meter telescope at the Kitt Peak National Observatory and the Multiple Mirror Telescope at the summit of Mount Hopkins near Tucson.
"A picture is worth a thousand words, but a spectrum is worth a thousand pictures," Salzer said. "It's astonishing the amount of information we can gather about places millions of light years away."
Leoncino is not only a system with low levels of heavier elements, but it is also a “dwarf galaxy” that is just 1,000 light-years in diameter and composed of several million stars. The Milky Way, in comparison, contains roughly 200 billion to 400 billion stars. What’s more, Leoncino is also blue in color, due to the presence of recently formed hot stars. However, it is still quite dim, having the lowest luminosity level ever observed in this type of system.
"We're eager to continue to explore this mysterious galaxy," said Salzer, who hopes to use the Hubble Space Telescope to delve deeper into the galaxy. "Low-metal-abundance galaxies are extremely rare, so we want to learn everything we can."
The study was published May 12 in the Astrophysical Journal.