Peaceful It Is Not, But Universe May Be Less Violent Than Once Thought

June 1, 2004

GAINESVILLE, Fla. — A new analysis of Hubble Space Telescope data has uncovered evidence the universe experienced far fewer major collisions among galaxies than previously thought.

A team of scientists led by a University of Florida astronomer analyzed a sample of galaxies located 100 million light years away and discovered the number of violent encounters between large galaxies is one-tenth the number that earlier studies had suggested.

Although theoretical models had predicted fewer collisions were involved in the evolution of the universe, these are the first observational measurements confirming those assumptions, said UF astronomer Alister Graham, who will present the results Tuesday at the American Astronomical Society meeting in Denver. The research was funded by NASA through a grant from the Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy.

“The new result is in perfect agreement with popular models of hierarchical structure formation in our universe,” said Graham, a faculty scientist at the University of Florida and the Australian National University in Canberra. “Galactically speaking, things appear a little safer out there.”

For years, astronomers have known the collision and merger of galaxies resulted in the formation of larger galaxies. The biggest of these new-formed galaxies appear largely devoid of stars at their cores, a phenomenon believed to result from the damage caused by “supermassive” black holes from the smaller galaxies as they merge at the center of the new galaxy. These huge black holes, a billion times heavier than the sun, act like a giant gravitational slingshot, ejecting stars away from the galaxy cores. These black holes also have been known to devour stars that venture too near.

Together with Peter Erwin, of the Spanish Instituto de Astrofiscia de Canarias in the Canary Islands, and Ignacio Trujillo, of the Max Planck Institute for Astronomy in Germany, Graham used this deficit of core stars as a gauge to determine the number of collisions that created the large galaxies.

Prior measurements in similarly sized elliptical galaxies suggested they had been formed by eight to 10 major collisions not involving the formation of new stars, but Graham’s team arrived at a much different conclusion.

Using images from Hubble’s Wide Field Planetary Camera 2, the researchers were able to examine galaxies whose cores had not been depleted of stars. The technique allowed the astronomers to observe what the central stellar distributions looked like before any major collisions had occurred, enabling them to better reconstruct the loss of stars from the galaxies which had partially depleted cores.

In addition, by considering the overall galaxy structure, they were able to more accurately measure the mass and size of the galaxies’ centrally depleted regions, which typically ranged from 300 to 900 light years across. The result: The mass of the deficit of stars at the galaxies’ centers, on average, equaled rather than exceeded the mass of the black hole. “If there had been 10 mergers, we would have found a stellar deficit 10 times the mass of the central black hole,” Graham said.

“It’s important to realize that many galaxies have large central black holes but no depleted cores. It is therefore not the case that every black hole is formed by simply gobbling up its surrounding stars. Instead, we are observing the demolished cores of galaxies after the union of two massive cosmic wrecking balls,” he said.

Although small satellite galaxies have been captured by our galaxy, the Milky Way, it has not experienced a recent major merger, Graham said. If it had, the plane of its disk, visible as a faint wide ribbon in the night sky, would have been scattered and dispersed across the heavens. Such a fate is expected in about 3 billion years when the Milky Way collides with a neighboring spiral galaxy, Andromeda, he said.

Graham said he plans to expand his research by applying his method of analysis to more galaxies, and also will use Hubble’s Advanced Camera for Surveys, which will provide a wider field of view and enhanced sensitivity.

“This work nicely quantifies the amount of ‘damage’ that supermassive black holes do to galaxy cores during galaxy mergers,” said astrophysicist David Merritt, a professor at the Rochester Institute of Technology. “Previous work in this area had been hampered by a lack of knowledge of the initial (pre-merger) state. The new results are nicely consistent with the merger paradigm for galaxy formation, and with the observed masses of SMBHs (supermassive black holes) in galaxies. This work should motivate further simulations of galaxy mergers in order to pin down the precise effects of SMBHs on galaxy luminosity profiles.”