The universe has experienced far fewer collisions among galaxies than
previously thought, according to a new analysis of Hubble Space Telescope
data by an ANU researcher. Astronomer Dr. Alister Graham, from the
Research School of Astronomy and Astrophysics, 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 earlier studies had suggested. Although theoretical models
predict that fewer collisions were involved in the evolution of the
universe, Dr. Graham’s observations are the first that confirm these
theories.
“The new result is in perfect agreement with popular models of
hierarchical structure formation in our universe,” Dr Graham said.
“Galactically speaking, things appear to be 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
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 near the centre of the new
galaxy. However, rather than requiring multiple mergers to clear away the
stars from the heart of a galaxy, Dr Graham has shown just one collision
between two galaxies is sufficient.
Using images from Hubble’s Wide Field Planetary Camera 2, Dr Graham was
able to examine galaxies 100 million light years away, whose cores had not
been depleted of stars, providing an important insight into star
distributions before any major collisions occurred. By considering the
overall galaxy structure, he was able to more accurately measure the sizes
of the depleted cores in the galaxies built by collision. The result: the
mass of the deficit of stars at the galaxies centers equaled rather than
exceeded the mass of the black hole.
“If there had been 10 mergers, we would have found a star deficit 10 times
the mass of the central black hole. Many galaxies have large central
black holes but no depleted cores. It is therefore not the case that
every black hole is formed by gobbling up its surrounding stars. Instead,
we’re observing the demolished cores of galaxies after the union of two
massive cosmic wrecking balls.”
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, Andromeda.
The research was conducted during Dr. Graham’s tenure at the University of
Florida and was funded by NASA via a grant from the Space Telescope
Science Institute in Baltimore. Dr. Graham’s research will appear in the
September 20 edition of Astrophysical Journal Letters. Thanks to
University of Florida News & Public Affairs for their assistance.
