Astronomers were surprised to see hot gas pushed so far from the galaxy's core.
PASADENA, California — A fortunate celestial alignment has provided astronomers with a rare glimpse of an early universe galaxy that is seeding its surroundings with the elements required to form subsequent generations of stars and galaxies.
The distant galaxy has gas flowing over its edges, as seen 700 million years after the Big Bang. It is the earliest-known ordinary galaxy, one that could have evolved into something like the Milky Way, to exhibit such complex behavior, according to astronomer Hollis Akins during a news conference at the American Astronomical Society meeting on June 14.
"These findings also indicate that outflow activity appears to be able to shape galaxy evolution, even in this 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 to arXiv.org on June 14.
A1689-zD1 appears in light magnified by Abell 1689, a large galaxy cluster capable of bending and intensifying, or gravitationally lensing, light from the universe's earliest galaxies (SN: 2/13/08; SN: 10/6/15). In comparison to other observed galaxies in the early universe, A1689-zD1 doesn't produce many stars — only about 30 suns per year — which means the galaxy isn't visible to our telescopes. However, the intervening cluster magnified A1689-zD1's light nearly tenfold.
Akins and colleagues used the Atacama Large Millimeter/submillimeter Array, or ALMA, a large network of radio telescopes in Chile, to study the lensed light. The researchers measured 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 do, but the cold gas extends four times as far, which surprised the team.
"There has to be some mechanism [to get] carbon out into the circumgalactic medium," Akins says of the space outside the galaxy.
Only a few scenarios could account for the escaping gas. A1689-zD1 may be merging with small galaxies, flinging gas further out where it cools, according to Akins. Perhaps the gas is being pushed out by the heat of star formation. The latter would be surprising given the galaxy's relatively low rate of star formation. While astronomers have observed outflowing gas in other early-universe galaxies, those galaxies are active, converting thousands of solar masses of gas into stars each year.
The ALMA data was once again used by the researchers to measure the motions of both the cold neutral and hot ionized gases. The hot gas moved faster than the cold gas, implying that it is being pushed from the center of A1689-zD1 to its outer regions, according to Akins at the press conference.
|Galaxy A169-zD1 (shown in radio waves) existed in the first 700 million years of the universe. H. AKINS/GRINNELL COLLEGE; B. SAXTON/NRAO/AUI/NSF; ALMA/ESO, NAOJ, AND NRAO
Despite the galaxy's relatively low rate of star formation, Akins and his colleagues believe the 30-solar-mass stars formed each year heat the gas enough to push it out of the galaxy's center. The observations point to a more orderly bulk flow of gas, implying outflows; however, the researchers are still analyzing the gas movement in greater detail and cannot rule out alternative scenarios.
They believe that when hot gas flows out, it expands and eventually cools, which is why they see colder gas flowing over the galaxy's edge, according to Akins. This heavy-element-rich gas enriches the intergalactic medium and will eventually be incorporated into subsequent generations of stars (SN: 6/17/15). Because of gravity's pull, cool gas, often with fewer heavy elements, fall toward the galaxy's center, allowing A1689-zD1 to continue making stars.
These A1689-zD1 observations show that this flow of gas occurs not only in super bright, extreme galaxies, but also in normal galaxies in the early universe. "Knowing how this cycle works helps us understand how these galaxies form stars and grow," says Caltech astrophysics professor Andreas Faisst, who was not involved in the research.
Astronomers are still learning about A1689-zD1. "It's an excellent target for follow-up observations," says Faisst. Akins' colleagues intend to do the same with the James Webb Space Telescope (SN: 10/6/21).
H. Akins et al. ALMA reveals extended cool gas and hot ionized outflows in a distant star-forming galaxy. American Astronomical Society meeting, Pasadena, Calif., June 14, 2022.
H. Akins et al. ALMA reveals extended cool gas and hot ionized outflows in a typical star-forming galaxy at z=7.13. arXiv: 2206.06939. Submitted June 14, 2022.