FOR RELEASE: 10:30 am EST January 10, 2006

Donna Weaver
Space Telescope Science Institute, Baltimore
(Phone: 410/338-4493)

Rogier Windhorst
Arizona State University, Tempe, Ariz.
(Phone: 480/965-7143)

RELEASE NO.: STScI-PR06-04

MONSTER BLACK HOLES GROW AFTER GALACTIC MERGERS

An analysis of the Hubble Space Telescope’s deepest view of the universe
offers compelling evidence that monster black holes in the centers of
galaxies were not born big but grew over time through repeated galactic
mergers.

The Hubble Ultra Deep Field (HUDF) studies also confirm recent computer
simulations that predict that black holes in newly merging galaxies are
enshrouded in so much dust that astronomers cannot see them feeding for
hundreds of millions to a billion years. 

"By studying distant galaxies in the Hubble Ultra Deep Field, we have the
first statistical evidence for how supermassive black holes grow as
galaxies grow," said astronomer Rogier Windhorst of Arizona State
University in Tempe, Ariz., and a member of both teams that conducted the
analysis. 

The two teams will present their results today at the 207th meeting of the
 American Astronomical Society in Washington, D.C.  

Both HUDF teams believe they are seeing two distinct phases in galaxy
evolution: the first phase representing the early-merging systems where
any central black hole is still enshrouded in dust, and the much later
"variable-object phase," in which the merged system has cleared out
enough gas for the inner accretion disk around the black hole to become
visible and emit the light which varies with time.

A team of astronomers, led by Amber Straughn of Arizona State University,
searched the HUDF for "tadpole galaxies," so-called because they have
bright knots and tails of stars caused by gravitational tidal forces which
distort galaxies during mergers. The team found about 165 tadpole galaxies,
representing about 6 percent of the 2,700 galaxies in the tadpole galaxy
study.

The conventional wisdom is that the galaxies should flicker over time as
material swirls around an accretion disk surrounding a black hole. The
material is heated and begins to glow. As it spirals down toward the black
hole, it can rapidly change in brightness. 

"To our surprise, however, these tadpole objects did not show any fluctuation
in brightness which would be expected," Straughn said. "This study of
tadpole galaxies suggests that black holes in newly merging galaxies are
enshrouded in dust such that we cannot see them accreting material."
 
A second team, led by Seth Cohen of Arizona State University studied the
brightness of about 4,600 HUDF objects over several weeks to many months.
The Hubble team found that about 45 (non-tadpole) objects, representing 1
percent of the faint galaxies in the study, fluctuated significantly in
brightness over time. This result indicates that the galaxies probably
contain supermassive black holes that are feeding on stars or gas. 

"According to the theoretical models, black holes feed about half a billion
or a billion years after the galaxy merger started," Windhorst said. “our
observations support this model by providing evidence for a time-delay
between a galaxy collision on onset of black hole accretion." The models
were produced by Lars Hernquist, Philip Hopkins, Tiziana di Matteo, and
Volker Springel of the Harvard Smithsonian Center for Astrophysics in
Cambridge, Mass.

The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. The Space Telescope Science Institute
in Baltimore conducts Hubble science operations. The Institute is operated
for NASA by the Association of Universities for Research in Astronomy, Inc.,
Washington.

For images and additional information about this research on the Web, visit:
http://hubblesite.org/news/2006/04