In order to form, galaxies need cold gas to undergo gravitational collapse. The bigger the Milky Way, the more cold gas it takes to fuse and grow.
Huge galaxies found in the early Universe needed a lot – a store of cold molecular gases that are a total of 100 billion times the mass of our sun.
But where did these early super-sized galaxies get so much cold gas when they were trapped in a warmer environment?
In a new study, astronomers led by the University of Iowa report direct, observational evidence of streams of cold gas they believe supplied these early, massive galaxies. They discovered cold gas pipelines that cut through the hot atmosphere into the dark matter halo of an early massive galaxy and provided the materials for the galaxy to form stars.
About two decades ago, physicists working with simulations theorized that during the early Universe cosmic filaments brought cold gas and embryonic button-shaped galaxies to a halo of dark matter, where it all clumped together to form huge galaxies. The theory held that the filaments should be narrow and densely filled with cold gas to prevent them from being peeled off by the hotter surrounding atmosphere.
But the theory lacked direct evidence. In this study, scientists studied a gaseous region around a huge galaxy that was formed when the universe was about 2.5 billion years old, or just 20% of its current age. The galaxy had not been studied before, and it took the team five years to determine its exact location and distance (via the redshift). The team needed a specially equipped observatory, the Atacama Large Millimeter / Submillimeter Array, because the environment of the target galaxy is so dusty that it can only be seen in the submillimeter range of the electromagnetic spectrum.
“It’s the prototype, the first case where we’ve detected a halo-scale current feeding a very massive galaxy,” said Hai Fu, associate professor in the Iowa Department of Physics and Astronomy and lead and corresponding author of the study. “Based on our observations, such currents can fill the reservoir in about a billion years, which is much shorter than the amount of time available to the galaxy in the era we were observing.”
Crucially, the researchers found two background quasars projected at close angular distances from the target galaxy, just as the motion of Jupiter and Saturn brought them closer together when viewed from Earth during the Great Conjunction last December. Because of this unique configuration, the light from the quasars penetrating the halas of the foreground galaxy left behind chemical “fingerprints” confirming the existence of a narrow stream of cold gas.
Those chemical fingerprints showed that the gas in the streams had a low concentration of heavy elements, such as aluminum, carbon, iron and magnesium. Because these elements are formed while the star is still shining and are released into the surrounding medium when the star dies, the researchers determined that the cold gas streams must flow in from outside rather than be expelled from the galaxy itself.
“Of the 70,000 galaxies in our survey, this is the only one associated with two quasars, both close enough to investigate the halogen. In fact, both quasars are projected on the same side of the galaxy, so their light can be blocked by the same current at two different angular distances. Fu says. “So I feel extremely fortunate that nature has given us this opportunity to detect this important artery that leads to the heart of a phenomenal galaxy during its adolescence.”
The study, “A long stream of low-metal cool gas around a massive starburst galaxy at Z = 2.67,” was published online in the Astrophysical Journal February 24.
Co-authors of the study include Rui Xue, who was a postdoctoral fellow in Iowa and now a software engineer at the National Radio Astronomical Observatory; Jason Prochaska from the University of California, Santa Cruz; Alan Stockton from the University of Hawaii-Honolulu; Sam Ponnada, who graduated from Iowa last May and is a graduate of the California Institute of Technology; Marie Wingyee Lau, from the University of California, Riverside; Asantha Cooray, from the University of California, Irvine; and Desika Narayanan, from the University of Florida.
The US National Science Foundation funded the research.