Shakhbazian 1: Very Dense Galaxy Cluster
Shakhbazian 1, Cluster of Galaxies
RA: 10h 55m 6 sec Size: 50" X 100" cluster diameter,
DEC: +40° 27' 19" m = +16.3 to +19.3 for individual cluster members
In 1957, the Armenian astronomer Shakhbazian discovered a small compact group of compact galaxies clustered tightly in a portion of the constellation Ursa Major. (Shakhbazian has catalogued 377 compact groups of galaxies) No investigations were done on this group until 16 years later in 1973, when Lick Observatory astronomers Lloyd Robinson and Joe Wampler examined the cluster more closely. They discovered that the cluster members lie at the same distance from us, and a few other surprising facts.
Robinson and Wampler thought this group was very unusual due to its very low mass to light ratios (M/L) and its low velocity dispersions among cluster members - making it one of the most dense clusters known. They found individual galaxies had nearly the same redshifts and thus were all moving very slowly within the cluster. Halton Arp, et. al. in 1973 also studied this cluster and found the same results as did Robinson and Wampler, that is the size of the cluster was 175 kpc X 350 kpc and a distance of 700 Mpc (Hubble constant = 50 km/sec/Mpc). Arp also concluded that the cluster contained high luminosity galaxies (for their estimated masses) with no evidence of young stars present. This is similar to globular clusters around our Milky Way. Thus Arp suggested that this galaxy cluster was nothing more than giant size globular clusters !
A number of years later, with better instrumentation on the 5-meter Palomar telescope, Robert Kirshner and Eliot Malumuth re-derived the M/L ratios for the 20 observed cluster members. They found them to be much more normal for the individual type of galaxy, however the compactness of the cluster was still very high. Normally a dense cluster of galaxies would eventually result in a number of galactic cannibalism events, but there was no direct observational evidence of this with Shakhbazian 1. The density of this cluster was found to be similar to that of the center of the Coma cluster. With the new observations and newly derived mass, mass density, luminosity and velocity parameters of this cluster, it has been postulated that
Shakhbazian 1 Cluster of galaxies. Left image from the Palomar Digitized Sky Survey on CD ROM. Right image from Halton Arp et. al. PASP, 1973. 200 inch Palomar plate, 45 minute exposure.
the cluster members are very efficient in giving out light, thus containing almost exclusively stars with very little gas. Thus Kirshner and Malumuth accepted that Shahkbazian 1 cluster consists of ordinary, old galaxies.
They also calculated the collision time for cluster members and estimated it based upon the cluster density, cross section of the cluster and the velocity dispersions measured, to be 150 million years. Thus the probability of galaxy collisions within this cluster was very high. Why is this cluster so compact ? One postulate is that if the initial gas cloud in which the galaxies formed rotated slowly, it could shrink and become unusually small. This same principle can be applied to individual compact galaxies.
This object is definitely a challenge object to observe. While the individual galaxies are just under an arc second in diameter and appearing “star-like”, the visual magnitudes of cluster members are very faint ranging from m = +16.0 to m = +19.3. Surely only the largest amateur telescopes under excellent skies could glimpse this cluster. On the other hand, CCD cameras should try to image this cluster. It is well placed at high on the meridian in the spring and summer months.
REFERENCES:
Kirshner, R.P., Malumuth, E.M., 1980, Dynamics of the Shakhbazian 1 Group of Galaxies, Astrophysical Journal, 236, p. 366-372.
Murdin, P., Allen, D., 1979, Catalogue of the Universe, Book Club Associates, Cambridge University Press, London, p. 13.
Robinson, L.B., Wampler, E.J., 1973, Shakhbazian 1: A Distant Cluster of Compact Galaxies, Astrophysical Journal, 179, L135-L139.