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.
The discovery of these “faint extended clusters” was made by Jean
Brodie, professor of astronomy and astrophysics at the University of
California, Santa Cruz, 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.
#####
Editor’s note: Reporters may contact Brodie at (831) 459-2987 or
brodie@ucolick.org.
This release is available electronically at the following web site:
http://press.ucsc.edu.
Images can be downloaded from the web at
http://www.ucsc.edu/news_events/download/.
