Very small galactic bulge could change ideas of galaxy formation

COLUMBUS, Ohio — The nearby galaxy M33 has a much
smaller central bulge than astronomers had previously thought —
or perhaps no bulge at all, according to astronomers at Ohio
State University. The finding may expand current theories of how
spiral galaxies form.

Galaxies such as M33 are called spiral galaxies because
pinwheel-shaped arms of gas, dust, and stars extend directly out
from a spherical nucleus of stars at the center, giving the galaxy a
flattened disk shape. The spherical nucleus is called a bulge,
because it normally bulges out from the disk.

Using some of the first images
from the 8-meter Gemini
North telescope on Mauna
Kea in Hawaii, the Ohio State
astronomers examined the
innermost 80 parsecs (1520
trillion miles) of M33 — where
other astronomers previously
claimed finding the bulge of
this spiral galaxy. They
presented their results June 4
at the meeting of the
American Astronomical
Society in Pasadena.

Instead of the old stars that normally
populate a galactic bulge, the astronomers
found evidence of both young and
intermediate-age stars. The density of stars
in the region more closely resembles a
galactic disk than a bulge — as if the disk
extended to the very core of M33, said
Andrew Stephens, doctoral candidate in
Ohio State’s Department of Astronomy.
Stephens did the work with Jay Frogel,
professor of astronomy.

“This finding makes us question the role of a bulge in spiral galaxy
formation,” Stephens said. “If M33 doesn’t have a bulge at all,
then how exactly did it form? If it has young stars in its bulge,
what triggered their formation?”

While a typical galactic disk is made up of stars of all ages, the
bulge normally contains old stars which date from the time the
galaxy formed. This is one reason that studying bulges can tell
astronomers about how galaxies form and evolve, Stephens
explained.

According to current theory, spiral galaxies begin as a giant
rotating mass of gas and dust, which starts out in a roughly
spherical shape before the edges flatten out into a disk and create
the spiral arms. The original spherical shape lives on in an outer
region of a galaxy known as the “halo” and, to a lesser extent, in
the bulge.

With the aid of “Hokupa`a”, an adaptive optics instrument on loan
to Gemini from the University of Hawaii Institute for Astronomy,
the Ohio State astronomers recorded images of M33 in three
infrared wavelengths and combined them to form a “color” image.
The adaptive optics system corrects for turbulence in the Earth’s
atmosphere, and allows astronomers to achieve much better
resolution than was previously available from the ground.

These high-resolution images revealed bright red stars indicative
of an intermediate-age population, and even bright blue young
stars, both of which should not be present in a bulge, Stephens
said.

Previous attempts to study the bulge at the center of M33 had
yielded contradictory results, Stephens said. Spectroscopic
studies indicated the presence of a young population, but
ground-based images didn’t have enough resolution to distinguish
between the many different types of stars crowded into the center
of the galaxy.

Seven countries — Argentina, Australia, Brazil, Canada, Chile,
the United Kingdom, and the United States — share financing for
the $192-million Gemini project. The Association of Universities
for Research in Astronomy, Inc. (AURA) manages the Gemini
Observatory under a cooperative agreement with the National
Science Foundation. Ohio State is a member of AURA.

EDITOR’S NOTE: This story is embargoed until Monday,
June 4, 2001, 10:00 a.m. PDT, to coincide with presentation
at the American Astronomical Society meeting in
Pasadena. #

Contact: Andrew Stephens, 614-292-3433;
Stephens@astronomy.ohio-state.edu [During AAS, Stephens
can be reached at the Sheraton Pasadena at 626-449-4000.]
Written by Pam Frost Gorder, 614-292-9475;
Gorder.1@osu.edu

EDITOR’S NOTE: A digital image of the center of the
galaxy M33 is available to the media upon request at 614-292-9475, or at the following URL:
http://www.osu.edu/researchnews/archive/aasm33pics.htm