Electronic,
Starry bulges yield secrets to galaxy growth (News release number: STScI-1999-34)
(Oct 6, 1999)
Abstract
News Release Number: STScI-1999-34 The full news
release story: NASA's Hubble Space Telescope is
uncovering important new clues to a galaxy's birth and
growth by peering into its heart — a bulge of
millions of stars that resembles a bulbous center yolk
in the middle of a disk of egg white. Hubble
astronomers are trying to solve the mystery of which
came first: the stellar disk or the central bulge? Two
complementary surveys by independent teams of
astronomers using Hubble show that the hubs of some
galaxies formed early in the Universe, while others
formed more slowly, across a long stretch of time.
Hubble confirms that the evolutionary paths of bulges
and disks are connected. The central bulge stabilizes a
galaxy's development and largely controls the ebb and
flow of star birth in the core. The central bulge holds
secrets as to how and when a galaxy formed. Before
Hubble, astronomers had detailed information only about
the complex core of our galaxy, which has a small bulge
peppered with massive young star clusters and a
telltale bar structure funneling gas to the center.
Hubble allows astronomers to see bright star clusters,
bars and other structures deep inside the bulges of
other galaxies. A group led by Reynier Peletier from
the University of Nottingham, in the United Kingdom,
has confirmed that the central bulges of more tightly
wound spirals were all created at more or less the same
time in the early universe. A second team, led by C.
Marcella Carollo of Columbia University in New York,
surveyed galaxies that have small bulges and bar-like
structures that bisect the nucleus like the slash
across a no-smoking sign. They found that the bulges in
these galaxies grew more recently, through markedly
different processes happening within the galaxy's disk.
Both surveys used Hubble's precise resolution to peer
into bulbous hubs of more than 200 neighboring
galaxies, out to a distance of 100 million light-years.
Using Hubble's visible-light and infrared cameras to
penetrate deep into the cores of the galaxies,
astronomers were able to untangle the stars' true
colors — a measure of age — from their apparent
colors, which are made redder by interstellar dust.
Peletier's team used Hubble to look into the center of
20 spiral galaxies that have large bulges. The team
found that elliptical bulges of stars formed over a
relatively brief period very early in the young
universe. This could have happened through the collapse
of a single cloud of hydrogen or merger of primeval
star clusters. Äpparently everywhere in the universe
these intermediate-sized galaxies must have started
forming early on," reports Peletier in a paper to be
published in the Monthly Notices of the Royal
Astronomical Society. "The bulges of early spiral
galaxies are old, and at least the outer parts of their
disks are considerably younger." Carollo's team found
that in a different class of spiral galaxy, a small
bulge probably formed early on, but was later fed by
gas flowing into the galaxy's core, likely along a
bar-like structure caused by instabilities in the
surrounding disk of stars. The gas fueled the birth of
new stars, and the bulge inflated like a beach ball as
brilliant star clusters populated the center. Carollo's
results, to be published in the Astrophysical Journal,
show young and old stars in the bulge. The researchers
say that these types of bulges can continue to grow in
galaxies in the present universe, but it is unlikely
that they will ever become as big as those giant bulges
that formed when the universe was young. The Space
Telescope Science Institute is operated by the
Association of Universities for Research in Astronomy,
Inc. for NASA, under contract with NASA's Goddard Space
Flight Center, Greenbelt, MD. The Hubble Space
Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).
CONTACT Don Savage NASA Headquarters, Washington, DC
(Phone: 202/358-1547) Nancy Neal Goddard Space Flight
Center, Greenbelt, MD (Phone: 301/286-0039) Ray Villard
Space Telescope Science Institute, Baltimore,
?? BibTeX string too long for field ``abstract''.
MD
(Phone: 410-338-4514)
Lars Lindberg Christensen
Space Telescope-European Coordinating Facility, Garching, Germany
(Phone: +49-89-3200-6291)
(Email: lars@eso.org)
