"As we look at distant galaxies, we see how they looked when their light left for Earth," said lead author Casey Papovich, according to a press release announcing the paper's publication in The Astrophysical Journal. "Because the galaxies are billions of light-years distant, we can see how they looked billions of years in the past."
Papovich, Texas A&M postdoctoral researchers Vithal Tilvi and Ryan Quadri and roughly two dozen astronomers around the world spent a year studying distant galaxies similar in mass to the ancestor of our Milky Way. The galaxies chosen were found in two deep-sky surveys of the universe, the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and the FourStar Galaxy Evolution Survey (ZFOURGE).
"Most stars today exist in galaxies like the Milky Way, so by studying how galaxies like our own formed, we've come to understand the most typical locations of stars in the universe," said Papovich, according to the press release. "We now have the best picture of how galaxies like our own formed their stars."
"When we calculate the star-formation rate of a Milky Way-like galaxy in the past and add up all the stars it would have produced, we find the mass growth we expected," Papovich said, according to the press release. "We also know that most of these stars formed inside the Milky Way, rather than in other smaller galaxies that later merged with our own. The whole picture hangs together."
Papovich notes that our sun is one of the more recent stars, born roughly 5 billion years ago at a point when star formation in the Milky Way had slowed to a crawl. The sun's late appearance actually may have been a good thing - allowing for growth of the planets in our solar system.
The team's multiwavelength study, supported by funding from both NASA and the National Science Foundation, spans ultraviolet to far-infrared light, combining observations from NASA's Hubble and Spitzer Space Telescopes, the European Space Agency's Herschel Space Observatory, and ground-based telescopes, including the Magellan Baade Telescope at the Las Campanas Observatory in Chile. The Hubble images from the CANDELS survey also provided structural information about galaxy sizes and how they evolved. Far-infrared light observations from Spitzer and Herschel helped the astronomers trace the star-formation rate.
Reference:
"ZFOURGE/CANDELS: On the Evolution of M* Galaxy Progenitors from z=3 to 0.5," Casey Papovich et al., Astrophysical Journal, 2015 April 9 [https://iopscience.iop.org/0004-637X/803/1/26/article, preprint: https://arxiv.org/abs/1412.3806].
