The galaxy merger rate is one of the fundamental measures of galaxy evolution, yielding clues to how galaxies bulked up over time through encounters with other galaxies. And yet, a huge discrepancy exists over how often galaxies coalesced in the past. Measurements of galaxies in deep-field surveys made by NASA's Hubble Space Telescope generated a broad range of results: anywhere from 5 percent to 25 percent of the galaxies were merging.
Galaxies grow mostly by acquiring small amounts of matter from their surroundings. But occasionally galaxies merge with other galaxies large or small. Collisions between big galaxies can change rotating disk galaxies like the Milky Way into featureless elliptical galaxies, in which the stars are moving every which way.
In order to understand how galaxies have grown, it is essential to measure the rate at which galaxies merge. In the past, astronomers have used two principal techniques: counting the number of close pairs of galaxies about to collide and by counting the number of galaxies that appear to be disturbed in various ways. The two techniques are analogous to trying to estimate the number of automobile accidents by counting the number of cars on a collision course versus counting the number of wrecked cars seen by the side of the road.
However, these studies have often led to discrepant results. "These different techniques probe mergers at different 'snapshots' in time along the merger process", Jennifer Lotz stated. "Studies that looked for close pairs of galaxies that appeared ready to collide gave much lower numbers of mergers (5%) than those that searched for galaxies with disturbed shapes, evidence that they're in smashups (25%)."
In the new work, all the previous observations were reanalyzed using a key new ingredient: highly accurate computer simulations of galaxy collisions. These simulations, which include the effects of stellar evolution and dust, show the lengths of time over which close galaxy pairs and various types of galaxy disturbances are likely to be visible. Jennifer Lotz's team accounted for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed the effects of different orbits for the galaxies, possible collision impacts, and how the galaxies were oriented to each other.
The simulations were done by T. J. Cox - now at Carnegie Observatories in Pasadena, Patrik Jonsson - now at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and Joel Primack at the University of California, Santa Cruz - UCSC, using small supercomputers at UCSC and the large Columbia supercomputer at NASA Ames Research Center. These simulations were "observed" as if through Hubble Space Telescope by Jennifer Lotz in a series of papers with T.J. Cox, Patrik Jonsson, and Joel Primack that were published over the past three years. A key part of the analysis was a new way of measuring galaxy disturbances that was developed by Jennifer Lotz, Joel Primack, and Piero Madau in 2004. All this work was begun when Jennifer Lotz was a postdoc with Joel Primack, and T.J. Cox and Patrik Jonsson were his graduate students.
"Viewing the simulations was akin to watching a slow-motion car crash", Jennifer Lotz stated. "Having an accurate value for the merger rate is critical because galactic collisions may be a key process that drives galaxy assembly, rapid star formation at early times, and the accretion of gas onto central supermassive black holes at the centers of galaxies."
To figure out how many encounters happen over time, Jennifer Lotz needed to understand how long merging galaxies would look like "wrecks" before they settle down and begin to look like normal galaxies again.
That's why Jennifer Lotz and her team turned to highly detailed computer simulations to help make sense of the Hubble photographs. The team made simulations of the many possible galaxy collision scenarios and then mapped them to Hubble images of galaxy interactions.
Creating the computer models was a time-consuming process. Jennifer Lotz's team tried to account for a broad range of merger possibilities, from a pair of galaxies with equal masses joining together to an interaction between a giant galaxy and a puny one. The team also analyzed different orbits for the galaxies, possible collision impacts, and how galaxies were oriented to each other. In all, the group came up with 57 different merger scenarios and studied the mergers from 10 different viewing angles. The simulations followed the galaxies for 2 billion to 3 billion years, beginning at the first encounter and continuing until the union was completed, about a billion years later.
"Our simulations offer a realistic picture of mergers between galaxies", Jennifer Lotz stated. In addition to studying the smashups between giant galaxies, the team also analyzed encounters among puny galaxies. Spotting collisions with small galaxies are difficult because the objects are so dim relative to their larger companions.
"Dwarf galaxies are the most common galaxy in the universe", Jennifer Lotz stated. "They may have contributed to the buildup of large galaxies. In fact, our own Milky Way galaxy had several such mergers with small galaxies in its recent past, which helped to build up the outer regions of its halo. This study provides the first quantitative understanding of how the number of galaxies disturbed by these minor mergers changed with time."
Jennifer Lotz compared her simulation images with pictures of thousands of galaxies taken from some of Hubble's largest surveys, including the All-Wavelength Extended Groth Strip International Survey (AEGIS), the Cosmological Evolution Survey (COSMOS), and the Great Observatories Origins Deep Survey (GOODS), as well as mergers identified by the DEEP2 survey with the W.M. Keck Observatory in Hawaii. She and other groups had identified about a thousand merger candidates from these surveys but initially found very different merger rates.
"When we applied what we learned from the simulations to the Hubble surveys in our study, we derived much more consistent results", Jennifer Lotz stated.
Her next goal is to analyze galaxies that were interacting around 11 billion years ago, when star formation across the universe peaked, to see if the merger rate rises along with the star formation rate. A link between the two would mean galaxy encounters incite rapid star birth.
"The new paper led by Jennifer Lotz for the first time makes sense of all the previous observations, and shows that they are consistent with theoretical expectations", stated Joel Primack. "This is a great example of how new astronomical knowledge is now emerging from a combination of observations, theory, and supercomputer simulations." Joel Primack now heads the University of California High-Performance AstroComputing Center (UC-HiPACC), headquartered at the University of California, Santa Cruz.
The simulated galaxy merger images have been gathered into an archive called Dusty Interacting Galaxy Gadget-Sunrise Simulations (DIGGSS) and placed in the Multimission Archive at STSci (MAST), the on-line archive otherwise used only for images from space telescopes.
This research was funded by grants from NASA and NSF, and Hubble Space Telescope and Spitzer Space Telescope Theory Grants.
The paper by Jennifer Lotz et al., entitled "The Major and Minor Galaxy Merger Rates at z <1.5", is on-line at <a href="http://adsabs.harvard.edu/abs/2011arXiv1108.2508L">http://adsabs.harvard.edu/abs/2011arXiv1108.2508L</a>. Besides Jennifer Lotz, T.J. Cox, Patrik Jonsson, and Joel Primack, the other co-authors are Darren Croton at Swinburne University in Melbourne, Australia, Rachel Somerville - now at Rutgers University, and Kyle Stewart at Jet Propulsion Laboratory.