Clumpy Structure Makes Disk Galaxies Look Alike Over Time
September 12, 2013

Clumpy Structure Makes Disk Galaxies Look Alike Over Time

redOrbit Staff & Wire Reports - Your Universe Online

Researchers at Iowa State University and IBM have identified why virtually all disk galaxies grow out of their irregular, clumped appearance, and why their older stars acquire the same smooth look as they fade from a bright center to a faint edge.

The astronomers say that whether these young disk galaxies are big, small, isolated or crowded in a cluster, the reason they all eventually look alike is due to their clumpy structure, which is likely responsible for both their dissolution and the smooth universal brightness profile.

“In galaxy disks, the scars of a rough childhood, and adolescent blemishes, all smooth away with time,” wrote Iowa State professor of physics and astronomy Curtis Struck in a research summary about the study.

Struck, who studies galaxy evolution and authored the 2011 book "Galaxy Collisions," said a few explanations have been proposed, but they cover only certain types of galaxies.

There hasn’t been an explanation for the nearly universal and exponential fade in the brightness of the lookalike disk galaxies, he said. Struck and Bruce Elmegreen, a research scientist at IBM’s Thomas J. Watson Research Center in Yorktown Heights, New York, built computer models simulating galaxy evolution, and believe the answer lies in the gravitational pull of the irregular, clumped structure of younger galaxies.

The astronomers used the simplest possible galaxy model that still includes all the essential ingredients – a razor-thin disk and orbiting stars subject to the gravity of the massive clumps.

“We focused on the clumps,” Struck said. “We thought the clumpy structure of young galaxy disks may be responsible for both its own erasure and the smooth universal brightness profile.”

Struck said the models confirmed this theory.

The gravity of those clumps of interstellar gases and new stars change the orbits of nearby stars. In some cases, the changes are significant, scattering stars far away from their original and nearly circular orbits. Over time – a lot of time – that scattering from circular to slightly elliptical orbits produces the smooth fade in brightness from the center of a galaxy to its edge.

“This process takes a few hundred million years to a few billion years,” Struck said.

The researchers said the findings match data coming from the Hubble Space Telescope and large ground-based telescopes, tools that allow astronomers to see distant galaxies in their young and clumpy structure.

However, more research is needed to explain the mystery of the smooth, steady fade of older disk galaxies, Struck said.

While the two astronomers plan to add more physical processes to their models to investigate how additional complexities affect star scattering, Struck said the current models have provided a good explanation for the universal appearance of older disk galaxies.

“If there is some disturbance, some clumpiness in the galaxy, you eventually get this smooth profile,” he said.

The findings of this study were published online September 11 in The Astrophysical Journal Letters.