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NOAO Newsletter - NOAO Highlights! - March 1999 - Number 57


Intracluster Light in Coma

Michael Gregg (Davis) and Michael West (St. Mary's) used the Burrell Schmidt 0.6-m Telescope to obtain deep multi-color images of the Coma galaxy cluster, with the goal of exploring the nature of its "intracluster light." It was Zwicky who first noted "an extended mass of luminous intergalactic matter of very low surface brightness" in the core of the Coma. The origin of this vast sea of stars is uncertain. Perhaps it formed outside the confines of any galaxy from primordial material left over from Coma's birth; alternatively, the intracluster light may be the accumulated debris from a Hubble time of galaxy-galaxy or galaxy-cluster tidal interactions that spilled stars into intergalactic space.

Coma cluster of galaxies
Caption: The Coma cluster of galaxies imaged with the Burrell Schmidt.

Inspection of a 75 minute R-band image obtained by Gregg and West reveals three large, low surface brightness (LSB) features (Numbers 1-3 in Figure 1). All are much larger and more prominent than the slim arc of tidal debris in the Coma cluster recently reported by Trentham and Mobasher (Number 4 in Figure 1). The most striking LSB feature is a plume of stars (Figure 2) extending at least 4.5' in the halo of NGC4874. To better analyze the LSB features, Gregg and West digitally "cleaned" portions of the images to remove superposed galaxies and stars; before and after images of LSB-1 are shown in Figure 2. At a distance of 100 Mpc, it is 130 kpc long, four times the diameter of the Milky Way. The mean R-band surface brightness is mR = 25.7 mag/arcmin2 and the integrated apparent magnitude is R = 15.60.1. About 0.6 mag fainter than the plume, LSB-2 is a circular pool of diffuse light about 40 kpc in diameter, adjacent to a small subgroup of galaxies. (Figure 3). Only a little fainter, LSB-3 has a chaotic appearance (Figure 3); a bridge of light connects this object to the outskirts of the Coma member galaxy NGC4911, where there is additional low surface brightness material (inset, Figure 3). NGC4911 and LSB-3 happen to fall along a bright ridge in the X-ray distribution in Coma; LSB-3 may have originated during the same dynamical interaction that has brightened the X-ray emission. The colors of these LSB objects are consistent with those of early type galaxies, but the errors of 0.2mag leave open the possibility that the material contains young stars.

Coma cluster of galaxies
Figure 1: R-band image of the Coma cluster core showing the locations and approximate sizes of the low surface brightness features (heavy solid circles labeled 1-4) discussed here. The image is 37' 33'; north is up, east to the left. The thin solid line is one X-ray contour from ROSAT observations showing the extended ridge which includes LSB-3.

Coma cluster of galaxies
Figure 2: The plume feature before and after removing foreground objects.

The four LSB objects are most easily understood as transient features generated by galaxy-galaxy or galaxy-cluster tidal interactions. They provide strong evidence of ongoing galaxy destruction in the Coma cluster. The crossing time in the cluster core is short, 1 to 2 billion years, so they will soon disperse, augmenting the population of intergalactic stars and perhaps globular clusters; evidence of such components are now being detected in nearer clusters such as Virgo. If we are viewing Coma at a typical epoch, then the presence of it at least four large transient star piles indicates that a substantial amount of material could have been liberated from galaxies over the cluster's lifetime. Integrated over 10 to 20 billion years, the material added to the intracluster light could amount to mR ~22, roughly 20% of the luminosity of NGC4874. As the interaction rate was almost certainly higher in the past, perhaps the intracluster light can be completely accounted for through gradual accumulation over a Hubble time. The LSB objects now in Coma provide a vivid snapshot of this process in action.

Coma cluster of galaxies
Figure 3: LSB Objects LSB-2 (upper) and LSB-3 after removing foreground stars and galaxies. Each image is 400 arcsec2. The bright object to the northwest of feature LSB-3 is NGC4911, a face-on spiral. The inset in the lower right is a lower contrast display of NGC4911 showing the low surface brightness material encircling its disk and linking it to LSB-3.

Less spectacular gravitational encounters should be much more common; Gregg and West have embarked on a follow-up project using the KPNO Mosaic imager on the 0.9-m telescope to search for smaller bits of tidal debris.


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