The universe just became a little simpler
June 18, 2003
Using images from the Hubble Space Telescope, astronomers 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 long considered œdwarf
elliptical' and œgiant elliptical' galaxies to be distinct objects. The
new findings, which appear in this monthÆs edition of The Astronomical
Journal, fundamentally alter astronomersÆ understanding of these
important components of the universe.
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
œspiral' galaxy, so named because its disk-like shape has an embedded
spiral arm pattern. Other galaxies are known as œirregular' 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 are more closely packed toward the centers of such
galaxies. In other words, the overall distribution of stars appeared to
be fundamentally different.
Alister Graham and Rafael Guzm›n from the University of
Florida decided to take a second look at the accepted wisdom. Expanding
on work started by Graham at the Instituto de AstrofÐsica de Canarias
(IAC) in Spain, 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 distributions of stars displaying a continuous variety of
structures between the allegedly different dwarf and giant galaxy
classes - in other words, these two types were just relatively extreme
versions of the same object. Moreover, there was one rather interesting
caveat.
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 one
reason astronomers had concluded dwarf and giant galaxies must be
different types.
Together with Ignacio Trujillo of the Max-Planck Institut f¹r
Astronomie in Germany and Peter Erwin and Andres Asensio Ramos of the
IAC, Graham addresses this phenomenon in a separate article that
appears in the same issue of The Astronomical Journal. 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. œIt 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.
œThis helps to simplify the universe slightly because we can
replace two distinct galaxy types with one,' said Graham. œBut the
implications go beyond mere astronomical taxonomy. Astronomers had
thought the formation mechanisms for these objects must be different,
but instead there must be a unifying construction process.'
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 Guzm›nÆs result puts to
rest a œvery puzzling' question.
œIn astronomy, like in physical anthropology, there is a deep
connection between the classification of species and their evolutionary
connections,' van den Bergh said. œThe bottom line is that the new work
of Graham and Guzm›n has made life a little bit simpler for those of us
who want to understand how galaxies are formed and have evolved.'
Instituto de Astrofisica de Canarias
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