Hickson 93 - Arp 99 Galaxy Group

Paul Hickson, Atlas of Compact Groups of Galaxies(1993):

"This group (Arp 99) consists of an accordant quartet plus a fainter high-redshift galaxy (e). The bright spiral galaxy (b) has two long plumes which result from tidal interaction. It is a radio and infrared source. The bright elliptical galaxy (a) is also a radio source."

Uppsala General Catalogue Of Galaxies(UGC, 1973):

"In Arp's class 'spiral galaxies with elliptical galaxy companions on arms'; '(a direct) connection not seen, but note difference in arms toward and away from eliptical galaxy.' (Arp)"

Hickson 93 is one of 100 "compact galaxy groups" in Paul Hickson's Atlas. Centered on giant elliptical galaxy NGC 7550, at least 4 of the 5 galaxies are gravitationally bound and strongly interacting. The fifth galaxy has a much higher red shift and thus may be at nearly twice Hickson 93's estimated 265 million light year distance. (For more on this, see Kopernik's web page on Stephan's Quintet.) As noticed by Arp spiral galaxy NGC 7549 is highly distorted with long plumes of stars and gas pulled from its spiral arms. Recent observations(1995) with the ROSAT X-ray satellite only detected NGC 7550 and was unable to detect any hot gas of the type that often lies at the center of galaxy clusters. Some astronomers believe that compact groups such as Hickson 93 will eventually combine into a single giant elliptical galaxy.

The NGC 7553 mystery:

There is no object at the coordinates listed in Dreyer's New General Catalog(NGC) for NGC 7553. The later Revised NGC(RNGC) provides coordinates that are close to the position of the dim 15th magnitude elliptical galaxy included in more modern catalogs as CGCG 454-015 or PGC 070842. However, modern star maps often portray NGC 7553 as non-existent or even worse, they show it at the wrong location. Who discovered NGC 7553 and what exactly did they see?

Lord Rosse's hired observers discovered this galaxy group using his giant 72-inch telescope located at Birr Castle in Ireland. However, they never provided a precise position for NGC 7553 so it is uncertain where Dreyer got his position for it. Later, in October 1877, the Rosse observers made a diagram of the field showing 4 of the galaxies. All are in their proper locations except for NGC 7553. While the nearest galaxy to their position is CGCG 454-015, it is quite possible that these visual observers could have actually seen a double star (near the upper left edge of our image) or even the star slightly to the south east, rather than the galaxy itself. It seems that we will never know if CGCG 454-015 is the object the discoverers of NGC 7553 saw. Confusion will continue when modern astronomers like Paul Hickson avoid using the NGC 7553 designation and computer based star maps continue to show it in the wrong location!

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George Normandin, KAS

January 8th, 2000