Big collisions 'make galaxies denser'
Using gravitational "lenses" in space, astronomers have discovered that the centres of the biggest galaxies are growing denser, which is an evidence of repeated collisions and mergers by massive galaxies with 100 billion stars.
"We found that during the last 6 billion years, the matter that makes up massive elliptical galaxies is getting more concentrated toward the centres of those galaxies. This is evidence that big galaxies are crashing into other big galaxies to make even bigger galaxies," Adam Bolton, principal author of the study, said.
"Most recent studies have indicated that these massive galaxies primarily grow by eating lots of smaller galaxies.
"We're suggesting that major collisions between massive galaxies are just as important as those many small snacks," he said.
The new study by researchers from the University of Utah was conducted by Bolton's team from the Sloan Digital Sky Survey-III using the survey's 2.5-meter optical telescope at Apache Point, N.M., and the Earth-orbiting Hubble Space Telescope.
The telescopes were used to observe and analyze 79 "gravitational lenses," which are galaxies between Earth and more distant galaxies. A lens galaxy's gravity bends light from a more distant galaxy, creating a ring or partial ring of light around the lens galaxy.
The size of the ring was used to determine the mass of each lens galaxy, and the speed of stars was used to calculate the concentration of mass in each lens galaxy.
Bolton conducted the study with three other University of Utah astronomers - Joel Brownstein, Yiping Shu and Ryan Arneson - and with members of the Sloan Digital Sky Survey - Christopher Kochanek (Ohio State University), David Schlegel (Lawrence Berkeley National Laboratory), Daniel Eisenstein (Harvard-Smithsonian Center for Astrophysics), David Wake (Yale University), Natalia Connolly (Hamilton College), Claudia Maraston (University of Portsmouth) and Benjamin Weaver (New York University).
The new study deals with the biggest, most massive kind of galaxies, known as massive elliptical galaxies, which each contain about 100 billion stars. Counting unseen "dark matter," they contain the mass of 1 trillion stars like our Sun.
"They are the end products of all the collisions and mergers of previous generations of galaxies, perhaps hundreds of collisions," Bolton said.
Despite recent evidence from other studies that massive elliptical galaxies grow by eating much smaller galaxies, Bolton's previous computer simulations showed that collisions between large galaxies are the only galaxy mergers that lead, over time, to increased mass density on the centre of massive elliptical galaxies.
When a small galaxy merges with a larger one, the pattern is different. The smaller galaxy is ripped apart by gravity from the larger galaxy. Stars from the smaller galaxy remain near the outskirts -- not the centre - of the larger galaxy.
"But if you have two roughly comparable galaxies and they are on a collision course, each one penetrates more toward the centre of the other, so more mass ends up in the centre," Bolton says.
Other recent studies indicate stars are spread more widely within galaxies over time, supporting the idea that massive galaxies snack on much smaller ones.
"We're finding galaxies are getting more concentrated in their mass over time even though they are getting less concentrated in the light they emit," Bolton said.
Bolton believes that large galaxy collisions explain the growing mass concentration, while galaxies gobbling smaller galaxies explain more starlight away from galactic centres.
"Both processes are important to explain the overall picture," Bolton said.
"The way the starlight evolves cannot be explained by the big collisions, so we really need both kinds of collisions, major and minor -- a few big ones and a lot of small ones," he added.
The new study also suggests the collisions between large galaxies are "dry collisions" - meaning the colliding galaxies lack large amounts of gas because most of the gas already has congealed to form stars - and that the colliding galaxies hit each other "off axis" or with what Bolton calls "glancing blows" rather than head-on.
The study has been published in The Astrophysical Journal.