When it comes to scientific research, Elizabeth McGrath thinks big – really, really big. A professor of physics and astronomy at Colby College, McGrath recently presented the lecture ‘Tracing the Formation of Massive Galaxies in the Early Universe’ in Searles Science Building at Bowdoin. McGrath’s research addresses not only how galaxies form but also another major unanswered question in astrophysics: what makes galaxies stop producing stars.
To explore that question McGrath has searched beyond the local universe to identify the first massive galaxies to become inactive – effectively looking back in time at a process that was going on when the universe was less than half its current age.
The properties of those distant galaxies are challenging our understanding of massive galaxy formation and decline, McGrath said.
Previous observations in the local universe showed that “there is a correlation between the overall shape or morphology of a galaxy and the rate at which it is producing new stars,” McGrath said. Star-forming galaxies are frisbee-shaped, while galaxies that have stopped producing stars are football-shaped. So scientists used to think that a galaxy begins its existence as a frisbee, and that mergers between galaxies create the football shape and cause start formation to cease.
But now that astronomers are looking at more distant galaxies, “this dichotomy in morphology is not as apparent,” McGrath said. “We have been finding massive disk galaxies that have already managed to shut down their star formation.” The merger theory doesn’t work for these galaxies, since a merger would have disrupted the disk shape.
McGrath went on to explain a new “cold streams” theory of massive galaxy formation: streams of gas move along filaments in the cosmic web and funnel into the center of a halo of dark matter, creating a massive rotating disk. This is a gentler way to rapidly build mass and produce stars, without disrupting the galaxy’s disk structure as a merger would. “But we still don’t know how this process is ultimately halted,” McGrath said. “So we continue to look for structural clues that may give an indication as to what turns star formation off in these massive disk galaxies.”
According to McGrath, the technology of the coming decade will revolutionize our understanding of the cosmic mechanisms she studies. One source of data that will continue to be valuable is the CANDELS survey, which harnesses cameras that are highly sensitive to lengthened lightwaves caused by the expanding universe – measuring objects much farther out in space (and back in time) than previously possible.