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April 1, 2002

Astronomers discover a new class of objects in two nearby galaxies

By Tim Stephens

Astronomers searching for globular star clusters in a nearby galaxy have discovered an entirely new class of objects, unlike anything previously described. Much larger and fainter than typical globular clusters, the new objects were first detected in Hubble Space Telescope images of the lenticular galaxy NGC 1023. They may hold clues to how galaxies of this type formed.

Images from the Hubble Space Telescope show a portion of the galaxy NGC 1023 and closer views of a faint extended cluster (left inset) and a normal globular cluster (right inset). The images were taken by HST's Wide Field Planetary Camera 2. Photo: S. Larsen, NASA.
The discovery of these "faint extended clusters" was made by Jean Brodie, professor of astronomy and astrophysics, and postdoctoral researcher Soeren Larsen.

The astronomers described their most recent observations of the objects--which they refer to informally as "faint fuzzies"--in a paper submitted to the Astronomical Journal.

This latest work, based on observations at the W. M. Keck Observatory in Hawaii, confirmed and extended their initial report of the discovery, which was published in the Astronomical Journal in December 2000.

"It is no surprise that these objects had never been seen before, because they are very faint," Brodie said. "In all the data archives from the Hubble Space Telescope, there are only four galaxies for which we have good enough observations to be able to detect them."

Of those four galaxies, the researchers found evidence of faint extended clusters in two (NGC 1023 and NGC 3384), but ruled out their existence in the other two galaxies.

Astronomers have traditionally recognized two kinds of star clusters: open clusters, which contain young stars in relatively small numbers (a few dozen to thousands), and globular clusters, which typically contain hundreds of thousands of densely packed, very old stars. Globular clusters are thought to be the oldest radiant objects in the universe and are found in all types of galaxies, usually in large numbers. "We see globular clusters in every galaxy we look at," Larsen said.

The faint extended clusters seem to be about the same age as globular clusters, but they look and act very different. Whereas globular clusters are typically 15 to 20 light-years in diameter, faint fuzzies range from 50 to 100 light-years across. They are also extremely faint, while globular clusters are fairly bright objects. Another important difference is that the faint fuzzies are associated with the disk of their host galaxy, whereas most globular clusters are associated with the halo or spheroidal component, moving in random orbits around the host galaxy.

"The association with the disk is significant because it means that they probably formed in a very different way from globular clusters," Larsen said.

Based on the Hubble images, Brodie and Larsen determined the size and brightness of these unusual objects and their distribution in the galaxy. They also noted that the light from the objects has predominantly red colors, indicating that they contain relatively old stars.

To confirm these observations, and in particular to rule out the possibility that these were background objects and not part of the galaxy itself, the researchers needed to get spectra of the faint fuzzies. In a spectrum, light from an object is separated into its component wavelengths, revealing a wealth of information about its composition and motion. In December 2001, Brodie and Larsen spent two nights gathering spectra with the LRIS spectrograph on the Keck I Telescope in Hawaii.

"The spectra have confirmed everything we had speculated about these objects based on the Hubble data," Brodie said.

The Keck spectra showed that the faint fuzzies have about the same velocity as the host galaxy. In other words, they are moving away from us (due to the expansion of the universe) at the same speed as the galaxy, meaning they are not background objects but part of the galaxy itself. Furthermore, when the researchers plotted the velocities of the clusters as a function of their positions in the galaxy, they could see that the clusters are rotating around the center of the galaxy like the disk does.

The two galaxies in which the researchers have detected faint fuzzies are both lenticular galaxies. A lenticular galaxy has a disk component similar to that of a spiral galaxy, except that there are no spiral arms and most of the stars in the disk are old. In spiral galaxies, the arms of the disk are sites of very active star formation. In many respects, lenticular galaxies are more like elliptical galaxies, which are large, football-shaped galaxies with no disk and very old stars.

"At this point, we don't know how common these clusters are or if they occur exclusively in lenticular galaxies," Brodie said. "We have some speculation as to how they may have formed, but no explanation of why they are so large," she added.

NGC 1023 appears to be interacting with a nearby dwarf companion galaxy. At Keck, the researchers took spectra of some bright spots in the dwarf companion, which turned out to be clusters of very blue, very young stars. Without Hubble images of the companion, they can't tell how large these star clusters are, but presumably they are forming as a result of the gravitational interaction of the two galaxies.

"It may be that in the past, other dwarf galaxies have interacted with NGC 1023 and been drawn into the disk, giving rise to the faint extended clusters. Over time, the stars in the clusters would redden as they aged. That doesn't explain why the clusters are so big, but it is an interesting possibility," Brodie said.

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