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News & Public Affairs
University of Florida

Contact Information:

Writer:
Aaron Hoover, (352) 392-0186, ahoover{at}ufl.edu

Source:
Alister Graham, (352) 392-2052, Ext. 223, graham{at}astro.ufl.edu

June 16, 2003

UF ASTRONOMERS: UNIVERSE SLIGHTLY SIMPLER THAN EXPECTED

GAINESVILLE, Fla. -- The universe just became a little less
mysterious.

Using images from the Hubble Space Telescope, astronomers at the
University of Florida have concluded that two of the most common
types of galaxies in the universe are in reality different versions
of the same thing. In spite of their similar-sounding names,
astronomers had for decades considered "dwarf elliptical" and "giant
elliptical" galaxies to be unique. The findings, which appear in this
month's edition of The Astronomical Journal, fundamentally alter
astronomers' understanding of these important components of the
universe, making it easier to understand how galaxies form in the
first place. 

"This helps to simplify the universe because we replace two distinct
galaxy types with one," said Alister Graham, a UF astronomer and lead
author of the paper. "But the implications go beyond mere
astronomical taxonomy. Astronomers had thought the formation
mechanisms for these objects must be different, but instead there is
a unifying construction process." 

Galaxies, the building blocks of the visible universe, are enormous
systems of stars bound together by gravity and scattered throughout
space. There are several different types, or shapes. For example, the
Milky Way galaxy, in which the Earth resides, is a "spiral" galaxy,
so named because its disk-like shape has an embedded spiral arm
pattern. Other galaxies are known as "irregular" galaxies because
they do not have distinct shapes. But together, dwarf and giant
elliptical galaxies are the most common. 

For the past two decades, astronomers have considered giant
elliptical galaxies, which contain hundreds of billions of stars, and
dwarf elliptical galaxies, which typically contain less than one
billion stars, as completely separate systems. In many ways it was a
natural distinction: Not only do giant elliptical galaxies contain
more stars, but the stars also are more closely packed toward the
centers of such galaxies. In other words, the overall distribution of
stars appeared to be fundamentally different.

Graham, a postdoctoral research associate, and Rafael Guzmán, a UF
associate professor of astronomy, decided to take a second look at
the accepted wisdom. The pair analyzed images of dwarf elliptical
galaxies taken by the Hubble Space Telescope and combined their
results with previously collected data on over 200 galaxies. The
resulting sample revealed that the structural properties of the
galaxies varied continuously between the allegedly different dwarf
and giant galaxy classes -- in other words, these two types were just
relatively extreme versions of the same object.

Sidney van den Bergh, former director and researcher emeritus at the
Dominion Astrophysical Observatory at the National Research Council
of Canada in Victoria, said Graham and Guzmán's result puts to rest
a "very puzzling" question. 

"In astronomy, like in physical anthropology, there is a deep
connection between the classification of species and their
evolutionary connections," van den Bergh said. "The bottom line is
that the new work of Graham and Guzmán has made life a little bit
simpler for those of us who want to understand how galaxies are
formed and have evolved."

Graham and three colleagues expand on his and Guzmán's conclusions
with a separate article that appears in the same issue of The
Astronomical Journal. 

In recent years, Graham said, a number of studies had revealed that
the innermost centers of giant elliptical galaxies -- the inner 1
percent -- had been scoured out or emptied of stars. Astronomers
suspect that massive black holes are responsible, gravitationally
hurling away any stars that ventured too near and devouring the stars
that came in really close. This scouring phenomenon had tended to dim
the centers of giant elliptical galaxies, which ran counter to the
trend that bigger galaxies tend to have brighter centers. The dimming
phenomenon was also one reason astronomers had concluded dwarf and
giant galaxies must be different types.

Building on recent revelations showing a strong connection between
the mass of the central black holes and the properties of their host
galaxies, Graham and his colleagues introduced a new mathematical
model that simultaneously describes the distribution of stars in the
inner and outer parts of the galaxy. 

"It was only after allowing for the modification of the cores by the
black holes that we were able to fully unify the dwarf and giant
galaxy population," Graham said.

Peter Erwin and Andres Asensio Ramos of the Instituto de Astrofísica
de Canarias in Spain, and Ignacio Trujillo of the Max-Planck Institut
für Astronomie in Germany, worked with Graham to support his
conclusions. Both research projects were funded in part by NASA and
the American Astronomical Society.

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