DSS image of the centre of the Virgo Cluster, showing the locations of the 2 HST WFPC2 fields in which red giant stars have been found by Ferguson, Tanvir & von Hippel (1998; left) and our group.
In 1951, Fritz Zwicky noted that the Coma cluster of galaxies appeared to very faint, diffuse light originating from between the galaxies and was thought to be due to the presence of intergalactic stars ; stars that are not associated with any paticular galaxy. Further photographic observations showed this diffuse light existed in other clusters, although deriving actually how much intracluster light was present proved extremely difficult, due to the extreme faintness of the light -- typically, the intracluster light is about 1% the brightness of the sky background at a dark site. However, recent efforts by a number of research groups have been able to probe this diffuse light in distant Abell clusters.
With the nearest large galaxy cluster (the Virgo cluster) located 15 Mpc away, the Hubble Space Telescope was required to image individual stars (first seen by Ferguson, Tanvir & von Hippel, 1998) associated with the intracluster light. Over the past few years, a few groups have also discovered intracluster planetary nebulae, the eventual end state of most stars. Furthermore, intergalactic globular clusters have now been observed in at least 2 galaxy clusters. All of these objects are tracers of the stellar populations that are removed (presumably from tidal interactions) from galaxies. There are now a number astronomers that are searching for intracluster stars, planetary nebulae, and globular clusters in galaxy clusters and galaxy groups. And from all of the observations, it is clear that intracluster stars are very common in galaxy clusters; they comprise roughly 10 - 20% of the TOTAL stellar light in the Virgo cluster and other rich clusters. These objects are all likely to have been removed from galaxies via gravitational forces.Studies of the intracluster stellar populations are very important for formation scenarios for galaxies and galaxy clusters --
While at Penn State, I began work on a large scale project to study both intracluster stars and planetary nebulae with Robin Ciardullo, John Feldmeier , George Jacoby, and Steinn Sigurdsson The first aspect of this work was a study of the intracluster red-giant-branch (RGB) stars in the Virgo cluster using the Hubble Space Telescope. We have found several hundred individual IC-RGB stars in 2 WFPC2 fields (below), and analysis of the luminosity function of those stars suggests they are somewhat metal-rich (from 0.2 to 0.5 solar metallicity), and were likely stripped from intermediate luminosity spiral or elliptical galaxies.
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DSS image of the centre of the Virgo Cluster, showing the locations of the 2 HST WFPC2 fields in which red giant stars have been found by Ferguson, Tanvir & von Hippel (1998; left) and our group. |
| Luminosity functions of intracluster red giant stars (blue), and red giant stars stars in the Virgo dwarf galaxy VCC 1104 (red; from Harris et al. 1998). The intracluster stars are fainter than expected, and we surmise that this effect is due to a difference in the chemical compotion of the intracluster stars from that of the stars in the dwarf galaxy. |  |
From these observations, we find that 10 - 20% of the total light in the Virgo cluster comes from the intracluster stars; other groups have reached similar conclusions based on direct studies of the diffuse light and intracluster planetary nebulae. For the details, our paper is here .
More recently, we have been searching for both intracluster planetary nebulae and red giant branch stars in the smaller M81 group of galaxies(below). Some preliminary results on the planetary nebulae (led by John Feldmeier) is presented here , and results from the RGB star survey were presented at IAU Symposium 217 in the summer of 2003 (PS -- PDF) The images below show what the group looks like in the optical (right; boxes show where we are looking for PN[blue] and RGB stars [red]) and in the radio (left; 21 cm map from Yun et al 1994) -- the latter shows that hydrogen gas has been ripped from all of the primary galaxies after recent passages 200-300 million years ago. We are looking to see if stars have also been ripped out, or are insead forming inside the gas...
In our search of the M81 group for red giant branch stars, we (undergraduate student Megan DeCesar, myself, John Feldmeier, Robin Ciardullo and Denise Hurley-Keller) have also found a number of younger BLUE stars that lie within the streams of hydrogen gas that permeate the M81 system. Some of the `clumps' of these stars may be local equivalents of `tidal dwarf galaxies' -- see our press release from the Jan 2004 AAS meeting.
I am continuing to study various aspects of `intergalactic' stellar populations; our group has recently been awarded time (37 orbits) on the HST (PI: Robin Ciardullo) to derive colours for the RGB stars in the Virgo Cluster, form which we will be able to directly measure the chemical composition of the stars, and gain further insight on just what kinds of galaxies they originally formed in. Stay tuned!
Papers :
Intracluster Red Giant Stars in the Virgo Cluster , Durrell, P.R., Ciardullo, R., Feldmeier, J.J., Jacoby, G.H., & Sigurdsson, S. 2002, ApJ, 570, 119 NASA ADS link
More on intracluster stars :
-- John Feldmeier's web page at CWRU - searching for intracluster planetary nebulae
-- the
The PoPSIcL Project at CWRU - looking for intracluster light in galaxy clusters
-- more intracluster
star links
-- Intracluster Light in the Coma Cluster
-- intergalactic globular clusters -- work being done by Mike West, Andres Jordan, Pat Cote, and others...
