Very Small Galactic Bulge Could Change Ideas Of Galaxy Formation

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.