McMahan Observatory

Obscure Globular Clusters

"Obscure" globular clusters are generally regarded as being globular clusters in the Milky Way galaxy (Galactic globular clusters) that were not included in the NGC (not the revised NGC) or the IC. Of the 153 recognized globular clusters in the Milky Way galaxy, 104 are contained in the NGC, 3 in the IC, and the remaining 46 have been discovered or recognized as globular clusters since 1949. The six most recent have been discovered or re-classified as globulars since 2000¹. Four globular clusters, M54, Arp2, Terzan 7 and Terzan 8 are probably part of the Sagittarius Dwarf Elliptical Galaxy (SagDEG), which is being tidally shredded by the Milky Way galaxy, and these clusters will become part of the Milky Way halo. Palomar 12 probably originated in SagDEG and has already been transferred to our galaxy's halo².

Most of the obscure globulars were discovered on the Palomar Observatory Sky Survey plates (POSS). The best known of these recently discovered globular clusters are the Palomars and Terzans. Small numbers of clusters have been added from other sources including the Two Micron All Sky Survey by various observers (2Mass-GC01 &2Mass-GC02, Ton 2, Liller 1, UKS 1), Andrews & Lindsay (AL3 reclassified in 2006), Arp(Arp 2), Arp-Madore (AM1 & AM4), Djorgovski (Djorg 1 & Djorg 2), European Southern Observatory by various observers (E3, ESO 452-11, ESO 280-SC06, Eridanus G.C.), Haute-Provence Observatory (HP 1), Liller (Liller 1), Lynga (Lynga 7), Ruprecht (Rup 106), Spitzer Space Telescope (Glimpse-C01), Weinberger (PYXIS G.C.), van den Berg & Hagen (BH 176) and Whiting et. al (Whiting 1).^1,3

Thirteen of the 15 Palomar clusters were discovered on the POSS plates. The other two, Palomar 7 and Palomar 9, were already cataloged as IC 1276 and NGC 6717, respectively. Palomar clusters are predominantly located far from the galactic center. One is more than 100 kpc (kilo parsecs) from the center; Three are farther than 50 kpc distant; nine more than 10 kpc and only four are less than 5 kpc distant. One Palomar cluster is located more than 100 kpc above or below the galactic plane; three more than 49 kpc and seven more than 10 kpc. The other 8 are less than 5 kpc distant from the galactic plane.³

By comparison, the Terzans are primarily located near the galactic center. Terzan 8 is the most distant at 19 kpc. All of the others are less than 3.5 kpc from the center of the Galaxy. Two, Terzan 7 and 8 are located more than 20 degrees below the Galactic plane. Terzan 3 is located at 19 degrees above, and all of the others are located within a few degrees of the plane.

The plots of metallicity vs cluster position linked below show that the clusters near the galactic center are more metal rich than ones far from the center, but there is no gradient of metallicity with distance from the center. It is a strong bimodal separation of metallicity that seems to occur around 6-8 kpc from the Galactic center. It would appear that a similar bimodal distribution exists between metallicity and distance above or below the Galactic equator. However, R_gc and Z are not independent variable in this sample of globular clusters. Except for Palomar 2, distance from the Galactic equator increases essentially linearly with distance from the Galactic center. Palomar 2 is far from the Galactic center but resides almost on the Galactic equator.

There is a large body of research that establishes a very strong relationship between the metallicity of clusters and age. In general older clusters are metal poor, because there were few heavy elements in the early gas clouds from which old stars were formed. Younger clusters are composed of younger stars that were formed from gas clouds containing higher content of heavy elements dispersed though the galaxy from supernova explosions of earlier high mass stars. Globular clusters tend to be very metal poor and very old. There are a few younger globulars, notably Terzan 7 and Palomar 12 (which were probably not formed in the Milky Way galaxy). An interesting plot is linked below that shows the relationship of metallicity to age for a number of Globular and open clusters. The globulars as a group are older than the open clusters and many appear to be as old as our Galaxy.

Prior to 2000 the most challenging known "obscure" globular cluster that could be observed or imaged by amateur astronomers was UKS-1. It is probably still the most challenging visual observing target within the capability of the largest amateur equipment. It has been reported by Barbara Wilson and Larry Mitchell that under extremely dark sky conditions it has been observed with great difficulty using a 36" aperture Newtonian. It can be imaged with difficulty using a 10" aperture telescope but is barely discernable after a 15 minute unfiltered exposure. Many hours of exposure are needed to obtain an image that yields any detail. The background of this page is an image I captured of UKS-1. The image is available on the Obscure Globular Images page.

BIBLIOGRAPHY

Ref. No.	Reference	Link
1.	Discovery Table of the Milky Way Globular Clusters; Students for the Exploration and Development of Space (SEDS) Website	1-SEDS
2.	The Sagittarius Dwarf Elliptical Galaxy, SagDEG; Students for the Exploration and Development of Space (SEDS) Website	2-SEDS
3.	CATALOG OF PARAMETERS FOR MILKY WAY GLOBULAR CLUSTERS:THE DATABASE Compiled by William E. Harris, McMaster University, February 2003	Harris
4.	Cluster Age vs. Metallicity Graph, Review of the Universe, [Star Clusters], [Distribution of Globular Clusters in the Milky Way] in website: universe-review.ca	Rev of Universe
5.	Star Clusters; Archinal, Brent A. and Hynes, Steven J.; Willmann-Bell; 2003; ISBN 0-943396-80-8	Willmann-Bell