An otherwise quiet galaxy in the early universe spits out star stuff

An otherwise quiet galaxy in the early universe spits out star stuff

PASADENA, Calif. — A lucky celestial alignment has given astronomers a rare look at an early universe galaxy that is seeding its environment with the elements needed to forge future generations of stars and galaxies.

Seen as it was just 700 million years after the Big Bang, the distant galaxy has gas flowing around its edges. It is the oldest known commonplace galaxy, one that could have become something like the Milky Way, to exhibit such complex behavior, astronomer Hollis Akins said June 14 during a press conference at the meeting of the American Astronomical Society.

“These results also tell us that this exit activity appears to be able to shape the evolution of galaxies, even in this very early part of the universe,” said Akins, an incoming graduate student at the University of Texas at Austin. He and his colleagues also submitted their findings on June 14 to arXiv.org.

The galaxy, called A1689-zD1, appears in the magnified light of Abell 1689, a large cluster of galaxies that can bend and intensify, or gravitational lens, the light of the early galaxies in the universe (SN: 02/13/08; SN: 06/10/15). Compared to other galaxies observed in the early universe, A1689-zD1 does not produce many stars – only about 30 suns per year – which means the galaxy is not very bright for our telescopes. But the middle cluster amplified the light from A1689-zD1 by nearly 10 times.

Akins and his colleagues studied lensing light with the Atacama Large Millimeter/submillimeter Array, or ALMA, a large array of radio telescopes in Chile. The team mapped the intensities of a specific oxygen spectral line, a tracer for hot ionized gas, and a carbon spectral line, a tracer for cold neutral gas. The hot gas appears where the bright stars are, but the cold gas extends four times farther, which the team did not expect.

“There must be a mechanism [to get] carbon in the circumgalactic medium,” the space outside the galaxy, says Akins.

Only a few scenarios could explain this outgoing gas. Perhaps small galaxies are merging with A1689-zD1 and throwing gas further away where it cools, Akins said. Or maybe the heat from star formation is pushing the gas out. The latter would come as a surprise given the relatively low rate of star formation in this galaxy. While astronomers have seen gas escaping into other galaxies in the early universe, these galaxies are bustling with activity, including converting thousands of solar masses of gas into stars each year.

X-ray picture of Galaxy A169-zD1
Galaxy A169-zD1 (pictured, in radio waves) exists in the first 700 million years of the universe.ALMA/ESO, NAOJ and NRAO; H. Akins/Grinnell College; B.Saxton/NRAO/AUI/NSF

The researchers again used the ALMA data to measure the movements of cold neutral gas and hot ionized gas. The hot gas showed greater overall motion than the cold gas, implying that it is being pushed from the center of A1689-zD1 toward its outer regions, Akins said at the news conference.

Despite the galaxy’s relatively low rate of star formation, Akins and his colleagues still believe that the 30 solar masses of stars per year heat the gas enough to push it away from the galaxy’s center. Observations suggest a more orderly bulk gas flow, which involves outflows, but researchers are analyzing gas movement in more detail and cannot yet rule out other scenarios.

They think that as hot gas flows, it expands and eventually cools, Akins said, which is why they see cooler gas flowing over the edge of the galaxy. This heavy element-rich gas enriches the circumgalactic medium and will eventually be incorporated into future generations of stars (SN: 06/17/15). Due to gravitational pull, cold gas, often with fewer heavy elements, around the galaxy also falls towards its center so that A1689-zD1 can continue to make stars.

These observations of A1689-zD1 show that this gas flow occurs not only in ultrabright extreme galaxies, but even in normal galaxies in the early universe. “Knowing how this cycle works helps us understand how these galaxies form stars and how they grow,” says Caltech astrophysicist Andreas Faisst, who was not involved in the study.

Astronomers haven’t finished learning about A1689-zD1 either. “It’s a great target for follow-up observations,” says Faisst. Several of Akins’ colleagues plan to do just that with the James Webb Space Telescope (SN: 06/10/21).

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