Hickson 93 in Pegasus

September 2018 - Galaxy of the Month

The discovery history of the objects in the quintet of galaxies catalogued as Hickson 93 is a run through of some of the great visual observers of the 19th Century.

The brightest galaxy in the group, NGC 7550, was discovered by William Herschel in 1784. The next brightest galaxy in the group, NGC 7547, was discovered by his son John when revisiting his father’s observations.

Bindon Stoney, one of Lord Rosse’s assistants, discovered NGC 7549 in 1850 using the 72” telescope at Birr. Lord Rosse and his team also glimpsed another galaxy in the area which was added to the NGC as number 7553. Its position was not well defined and this galaxy was later catalogued as CGCG 454-14 by Zwicky from the POSS plates. (CGCG stands for Catalogue of Galaxies and Clusters of Galaxies). This object is almost certainly the one seen by the Birr observers.

The last of the group to be discovered visually was NGC 7558 by Albert Marth using Lassell’s 48” reflector in 1864, it may however have also been seen by the Rosse team 14 years earlier as they described a fifth nebula in this group.

The discovery history gives an idea of the challenge required to see all the galaxies in this group.

Halton Arp in his famous catalogue of peculiar galaxies added the pair NGC 7549 and NGC 7550 as Arp 99. Arp thought he could see indications of an interaction between the two due to the shape and length of the spiral arms on the opposing sides of NGC 7549. There has been some debate as to whether Arp meant NGC 7549 or NGC 7547 as the spiral component of Arp 99 however it seems fairly clear from his description that it is NGC 7549 that should be the other component of the pair. Deep images also show shells of material around NGC 7550 and NGC 7547. It is likely however that NGC 7547 is part of a physical triple system with the other two.

The group is interesting as it seems to consist of three spirals, one lenticular and one elliptical and many of the galaxies are obviously interacting. It is not often you see lenticular galaxies outside large galaxy groups however Hickson groups are often thought to be conglomerations within large loosely bound galaxy groups. In this case the group has a listing as WBL 700, although only 4 out of the 5 galaxies in HCG 93 are listed in this group in NGC 7547, NGC 7549, NGC 7550 and NGC 7553 (CGCG 454-15 on the chart). NGC 7558 may just be a line of sight object and not a physical member of the group, although to be fair Hickson did recognise this.

To see more than the two brightest members of the HCG 93 group is going to be a challenge, except from very dark skies, and even then, a telescope with an aperture of at least 40cm is likely to be needed. The galaxy group will benefit from using medium to high power when observing it, as well as keeping any extraneous light from reaching the eye, in order to find the fainter members.

If Hickson 93 is not enough of a challenge then less than half a degree south east is the galaxy group Hickson 94. This will be much more challenging. The galaxy cluster Abell 2572 is also close by. Although ACO 2572 is not a particularly rich cluster it does contain four NGC galaxies. The whole area is worth spending some time in. There is a nice drawing of the group at SkyInspector.

Owen Brazell - Galaxy Section Director