- Title: The insignificance of major mergers in driving star formation at z~2.
- Authors: Kaviraj, S.; Cohen, S.; Windhorst, R. A.; Silk, J.; O’Connell, R. W.; Dopita, M. A.; Dekel, A.; Hathi, N. P.; Straughn, A.; Rutkowski, M.
- First Author’s Institutions: Blackett Laboratory, Imperial College London, London, UK and Department of Physics, University of Oxford, Oxford, UK
Major mergers – collisions between galaxies of similar masses – were once thought to be responsible for a flurry of star formation in the early Universe. Locally, such train wrecks result in dramatic enhancements in star formation during the collision, and settle into quiescent elliptical galaxies. If galaxies collided more frequently in the past, these events could explain a global star formation rate hundreds of time higher than what we see today. However, recent studies suggest initial estimates of the ubiquity of mergers suffered from observational bias (many of the bright, easy-to-see galaxies are major mergers), and may have been overly optimistic. Kaviraj et al. measure star formation rates of and visually classify distant galaxies that were selected based on (rest-frame) optical images, which highlights the underlying distribution of old stars and therefore can include the more quiescent galaxies. They conclude that major mergers actually contribute a small percentage (17-27%) of the total star formation at a redshift of 2, when the Universe was 3.3 billion years old and vigorously creating new stars.
Morphologies and star formation rates
Kaviraj et al. examined Hubble WFC3 images of 80 massive galaxies (stellar masses > 100 billion times the mass of our Sun), and categorized them as either major mergers, which contain two galaxies with mass ratios at least 1:3; disk galaxies (spirals); or compact spheroids (elliptical galaxies or their precursors). To find the star formation rate of each galaxy, the authors employed a technique called population synthesis modeling to replicate their star formation histories. This method (also described in this astrobite) compares the observed color of a galaxy – or more technically, its energy output at each of ten available wavelengths – to the evolutionary outcomes of a set of star-forming models, and chooses the best match. The technique works because the mass of a star dictates both its color and its lifetime. If a galaxy recently had a burst of star formation, massive, blue, short-lived stars would dominate the collection; in contrast, if a galaxy had not formed new stars for a long time, only low mass, red stars would remain.
Star formation breakdown
Immediately, the authors discover that major mergers harbor a mere 27% of the star formation in the early Universe (Figure 2). They point out that this value is an upper limit to the major merger contribution: the individual galaxies in an interacting pair would form some stars even without the extra help. They compare the mean specific star formation rates, the star formation rate per unit mass, of mergers to those of normal disk galaxies, and find almost half of the star formation in these merging systems did not result from the collision!
Additionally, many of the compact spheroids lack tidal tails, the signature of a recent interaction, even though they have not settled into their final state as quiescent elliptical galaxies. This hints that elliptical galaxies can emerge without multi-galaxy interaction, again downgrading the influence of major mergers.
Clearly, major mergers cannot explain the bulk of the star formation during the Universe’s most active era. Instead, galaxy evolution seems to favor other processes like minor mergers or accretion of clumps of cold gas directly from the “cosmic web”. Still, the authors caution that this research is not finished: major mergers could be important at earlier times. Future work by both professional and citizen scientists will expand this work to a range of redshifts, and can help answer the long-standing question of what drives star formation in the early Universe, and whether or not major mergers have been relegated to the back seat.