Just after the Big Bang, astronomers believe, the only elements in the universe were hydrogen, helium and a few other light elements. Metals formed only later, after hydrogen and helium combined to form stars. Because the current theory of the Big Bang includes clear predictions about how much helium and hydrogen were present at the birth of the universe, analyzing the ratio of these atoms in galaxies with low levels of metals provides one of the most valued ways of testing this model.
While metal-poor galaxies were plentiful around the birth of the universe, they’re much harder to find now. They’re particularly rare the closer you get to Earth, as our own Milky Way galaxy is rich in heavy chemical elements created over time by “stellar processing.” Stars produce metals through nucleosynthesis, then pass the metals back into the galaxy when they explode as supernovae.
The dim blue galaxy astronomers recently spotted in Leo Minor contains the lowest abundance of metals ever observed in a celestial body. It has 29 percent less metal than another galaxy discovered in 2005, which was previously found to be the lowest in metals. Officially named AGC 198691, the galaxy is affectionately known as “Leoncino,” or “Little Lion, ” a nod to its location and to Riccardo Giovanelli, the Italian-born radio astronomer who led the group at Cornell University that first identified it.
“Finding the most metal-poor galaxy ever is exciting since it could help contribute to a quantitative test of the Big Bang,” Indiana University-Bloomington Professor John J. Salzer, a co-author of the new study, said in a press release. “There are relatively few ways to explore conditions at the birth of the universe, but low-metal galaxies are among the most promising.”
As a so-called “dwarf galaxy,” Little Lion is only about 1,000 light years in diameter and is composed of several million stars. (By contrast, the Milky Way contains an estimated 200 billion to 400 billion stars.) According to the researchers, the galaxy owes its blue color to the presence of recently formed hot stars. It is also surprisingly dim, with the lowest luminosity level ever seen in a system of its type.
In a particularly lucky break for scientists, the newly identified galaxy is within what’s known as the “local universe,” a region of space within some 1 billion light years away from Earth. The local universe is thought to contain millions of galaxies, only a fraction of which astronomers have catalogued so far. Little Lion’s relative proximity will make it easier to study than low-metal galaxies located further from Earth.
To estimate the abundance of metal in a galaxy, astronomers analyze the light emitted by that galaxy. In their study of Little Lion, published last week in the Astrophysical Journal, the researchers used spectrographs on two telescopes located near Tucson, Arizona: the Mayall 4-meter telescope at Kitt Peak National Observatory and the Multiple Mirror Telescope located at the summit of Mount Hopkins. After spectroscopic observations capture the light waves emitted by a galaxy, astronomers can then view the light in a manner similar to the way a prism creates a rainbow.
Salzer said his team will continue to study the newly identified galaxy, and will try to use other telescopes, including the Hubble Space Telescope, to delve deeper into this extraordinary find. “We’re eager to continue to explore this mysterious galaxy,” he said. “Low-metal-abundance galaxies are extremely rare, so we want to learn everything we can.”