Galactic Collisions Between Big Galaxies Form Even Bigger Galaxies

Brett Smith for redOrbit.com – Your Universe Online

When massive galaxies belly-up to the interstellar buffet, their centers expand as they gobble up other galaxies that contain billions and billions of stars, according to a new report from University of Utah astronomers.

“We found that during the last 6 billion years, the matter that makes up massive elliptical galaxies is getting more concentrated toward the centers of those galaxies. This is evidence that big galaxies are crashing into other big galaxies to make even bigger galaxies,” said lead author Adam Bolton, a professor at the university.

“Most recent studies have indicated that these massive galaxies primarily grow by eating lots of smaller galaxies,” he adds. “We’re suggesting that major collisions between massive galaxies are just as important as those many small snacks.”

To examine the mechanics of these intergalactic interactions, the team used the 2.5-meter optical telescope at Apache Point, N.M., and the Earth-orbiting Hubble Space Telescope, according to their report published in The Astrophysical Journal.

Images from these telescopes were used to examine 79 “gravitational lenses,” which are galaxies located between Earth and other galaxies. These galaxies bend light from the more distant galaxy and create a ring or partial ring of light around itself. Astronomers can analyze these rings to determine the mass and density of the lens galaxy.

Analysis of these lenses showed that massive galaxies with denser cores only form through the collisions of large galaxies and not the “eating” of smaller galaxies by larger ones.

“If you have two roughly comparable galaxies and they are on a collision course, each one penetrates more toward the center of the other, so more mass ends up in the center,” Bolton said.

Large and small galaxies do interact and Bolton said these interactions result in stars that are more evenly spread throughout the resulting combination.

“Both processes are important to explain the overall picture,” he 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.”

The report concedes that other mechanisms could be involved in the denser cores of larger galaxies, but Bolton refutes each one of these theories. For instance, one theory posits that gas is collapsing to form stars– thereby increasing the concentration of stars in a galaxy. Bolton argues that this scenario is implausible because the observed galaxies are too old for new star formation.

Another theory says that gravitational forces from the largest galaxies strip its neighbors of their outer stars, leaving more mass in galactic centers. According to Bolton, his new study’s observations could not have resulted from this process.

A third theory suggests that dark matter distorts astronomers’ observations of both older and younger galaxies making the centers more difficult to measure. Bolton said the differences in measurements that could be caused by dark matter are relatively small and would not explain the densities of matter observed in the lens galaxies.

The new study is part of a much larger astronomical study titled Sloan Digital Sky Survey, or SDSS-III.

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