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RSAA News of the Month: September 2004

A Slightly Less Violent Universe?

 

A new analysis of Hubble Space Telescope data has uncovered evidence that the Universe has experienced far fewer collisions among galaxies than previously thought. RSAA astronomer Alister Graham analyzed a sample of galaxies located 100 million light years away and discovered that the number of violent encounters between large galaxies is around one-tenth of the number that earlier studies had suggested. Alister received his PhD from ANU in 1998 and has just returned to take up a position as Research Fellow at Stromlo.

Although theoretical models had predicted that fewer collisions were involved in the evolution of the universe, these are the first observational measurements confirming those theories. “The new result is in perfect agreement with popular models of hierarchical structure formation in our universe,” says Graham. “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, formed as the black holes from the cores of the smaller galaxies merge near the center of the new galaxy. These huge black holes, billions of times heavier than the sun, act like giant gravitational slingshots, throwing stars away from the galaxy cores. They can also devour stars that venture too close. The result is a galaxy with far fewer stars in the core.

Hubble Space Telescope image of colliding galaxies in the constellation of Canis Major.

NGC 2207 and IC2162 will eventually merge to form a giant elliptical galaxy.
The merging supermassive black holes at the cores of the galaxies will evacuate a region hundreds of light-years across.

Image: Space Telescope Science Institute/NASA

Graham used this deficit of core stars to gauge the number of collisions that created the large galaxies. Earlier measurements in similarly sized elliptical galaxies suggested they had been formed by eight to ten major collisions not involving the formation of new stars, but Alister arrived at a much different conclusion.

Using images from Hubble’s Wide Field Planetary Camera 2, he was able to examine galaxies whose cores had not been depleted of stars. This provided an important insight into what the central stellar distributions looked like before any major collisions had occurred, and to better judge the effects caused by black holes generated during the mergers. In addition, by considering the overall galaxy structure, well outside the core region, he was able to more accurately measure the mass and size of the galaxies' depleted central 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 ten mergers, we would have found a stellar deficit ten times the mass of the central black hole,” Alister says. “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."

Two images of a pair of colliding galaxies, NGC 4038/4039 in the constellation of Corvus.

Left: Image from the Siding Spring 40" telescope.
It shows the two disrupted spiral galaxies and the long tails of stars
thrown out of the galaxies by the gravitational tidal forces produced in the merger.
These tails give the system it's popular name, the "Antennae".

Right: Image is from the Hubble Space Telescope Wide Field Planetary Camera 2.
It shows the tidal wreckage in the central region of the merger. The cores of the merging galaxies are in the bright orange blobs.
Dust lanes loop between the cores. Compression of the gas and dust clouds has triggered a burst of star formation,
producing the strand of bright blue stars.

Images: Bessell & Sutherland, RSAA (left); Space Telescope Science Institute/NASA (right)

Although small satellite galaxies have been captured by our galaxy, the Milky Way, it has not experienced a recent major merger. 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 three billion years when the Milky Way collides with a neighboring spiral galaxy, M31 in Andromeda.

Alister plans to expand his research by applying his method of analysis to more galaxies. He hopes to 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 supermassive black holes in galaxies. This work should motivate further simulations of galaxy mergers in order to pin down the precise effects of supermassive black holes on galaxy luminosity profiles."

The research was conducted during Alister's term on the faculty of the University of Florida, Gainsville, USA. It 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.

Initial results were reported at the June meeting of the American Astronomical Society, with the final analysis appearing in the September 20 edition of the Astrophysical Journal Letters.

For previous Monthly News items, click here.