Astronomers have long suspected that galaxies grow large due to an insatiable appetite for interstellar gas, but a new study suggests they may actually slowly graze on the star stuff instead, like vast cosmic cows.
Galaxies in the distant universe continuously ingested their star-making fuel over long periods of time, according to the study, which used observations made by NASA's Spitzer Space Telescope. The find goes against prevailing theory, which holds that the galaxies tended to devour their fuel in quick bursts after run-ins with other galaxies, researchers said.
"Our study shows the merging of massive galaxies was not the dominant method of galaxy growth in the distant universe," said principal investigator Ranga-Ram Chary, of NASA’s Spitzer Science Center at Caltech in Pasadena, Calif., in a statement. "We’re finding this type of galactic cannibalism was rare. Instead, we are seeing evidence for a mechanism of galaxy growth in which a typical galaxy fed itself through a steady stream of gas, making stars at a much faster rate than previously thought."
Questioning long-held theory
Galaxies like our own Milky Way are giant collections of stars, gas and dust. They grow by feeding off gas and converting it to new stars. Astronomers have long wondered where distant galaxies, which formed billions of years ago, acquired this stellar fuel.
The most favored theory, researchers said, was that galaxies grew by merging with other galaxies, feeding off gas stirred up in the collisions.
Chary and his team looked into this question, using Spitzer to survey more than 70 remote galaxies that existed 1 to 2 billion years after the Big Bang (the event that gave birth to our universe about 13.7 billion years ago).
A surprising find
To the surprise of the astronomers, these galaxies were blazing with so-called "H alpha" radiation. This radiation comes from hydrogen gas that has been hit with ultraviolet light from stars. High levels of H alpha indicate that stars are forming vigorously.
Seventy percent of the surveyed galaxies show strong signs of H alpha radiation. By contrast, only 0.1 percent of galaxies in our local universe possess the signature, researchers said.
Previous studies using ultraviolet-light telescopes found about six times less star formation than Spitzer, which sees infrared light. Scientists think the discrepancy may be due to large amounts of dust, which obscures many wavelengths of light but lets infrared radiation through.
The team's findings suggest that the grazing galaxies fed steadily over periods of hundreds of millions of years, creating an unusual number of plump stars — some of them up to 100 times the mass of our sun.
"This is the first time that we have identified galaxies that supersize themselves by grazing," said Hyunjin Shim, also of the Spitzer Science Center and lead author of the paper, which will appear in the Aug. 1 issue of the Astrophysical Journal. "They have many more massive stars than our Milky Way galaxy."