Evidence from observations and computer simulations supports a picture of galaxy growth that isn't dominated by the rough-and-tumble crashes of big galaxies.
To make a giant cookie, you would probably start by grabbing two clumps of cookie dough and smooshing them together. Astronomers long thought the same went for galaxies: want to build a big galaxy? Merge two galaxies together. Voilà.
But astronomers are finding that major mergers aren’t the fundamental galaxy builders they once seemed. Last year, a study of more than 400 Milky-Way-mass galaxies (i.e. those with tens of billions of solar masses) showed that, contrary to theory, disk galaxies grow their central bulges simultaneously with their disks, and not via mergers.
The less-is-more theme with mergers continued at the winter meeting of the American Astronomical Society. Sugata Kaviraj (University of Hertfordshire and University of Oxford, UK) presented his team’s study of 80 massive galaxies whose light left them more than 10 billion years ago, corresponding to a redshift of about 2. This epoch is a popular one with astronomers, because star formation in the universe peaked around this time. And because star formation builds up a galaxy’s stellar mass (which means it builds up the galaxy), astronomers can track galaxy growth by studying star formation.
Kaviraj and his colleagues looked for signs of a major merger — such as extended tidal features or double cores — in these early galaxies. (They defined a major merger as one in which the two original galaxies had about the same mass.) But they found that at most 27% of star formation activity during this period arose thanks to such mergers. After comparing the star formation in non-interacting galaxies with that in galaxies suffering mergers, the team concluded that the mergers’ actual contribution to cosmic star formation in this era might be more like 15%.
In a separate study of about 330 newborn elliptical galaxies, Kaviraj’s team also found that more than 50% of the blue (a.k.a. star-forming) ball-shaped galaxies don’t seem to be driving their star formation via major mergers. That matches up with theoretical work suggesting these galaxies can form without such a clash.
Overall, it looks like galaxies power most of their star formation — and therefore their growth — with minor mergers and gas accreted from their environs. The Milky Way is a prime case, as it shows no signs of a recent major merger. Instead, it’s beefed itself up by snacking. Zhibo Ma (Case Western Reserve University) reported at the meeting that his team’s work with the SEGUE K Giant Survey, an extension of the Sloan Digital Sky Survey focusing on cool giant stars, supports this view of the Milky Way. The team distinguishes several stellar populations in the Milky Way’s halo, the large sphere of older, cooler stars that encircles the galaxy. These stellar streams suggest that our galaxy has built up its halo in an ongoing process, accreting and disrupting dwarf galaxies as it ages.
These streams have particular chemical makeups, developed when the stars were first born in whatever extragalactic clump of gas the Milky Way ripped up and ate. But the streams may be able to tell us even more about their progenitors than these compositions reveal. Heidi Newberg (Rensselaer Polytechnic Institute) and her colleagues are working on using stellar streams to figure out the dark matter content of the dwarfs our galaxy has eaten, and of our galaxy itself. They’re doing it with a citizen science project called MilkyWay@home, which uses volunteers’ computer time to create a detailed, 3D view of the Milky Way. The project is still fairly new, but it promises to provide a fascinating glimpse into how our galaxy formed.
AAS presentations (click here for meeting program):
S. Kaviraj et al. "The insignificance of major mergers in the early universe." Abstract #310.07
Z. Ma et al. "The SEGUE K Giant Survey." Abstract #336.02D (dissertation talk)
H. J. Newberg et al. "Relating Dark Matter to Tidal Streams with MilkyWay@home." Abstract #336.03
S. Kaviraj et al. "The insignificance of major mergers in driving star formation at z~2." Monthly Notices of the Royal Astronomical Society, February 11, 2013.
S. Kaviraj et al. "Newborn spheroids at high redshift: when and how did the dominant, old stars in today's massive galaxies form?" Monthly Notices of the Royal Astronomical Society, January 11, 2013.