Object of the Week April 6, 2015 – Copeland’s Septet (Hickson 57, Arp 320)

Galaxy Group (Septet)
Leo
RA (centre): 11h 37.9m
DEC: +22° 00.0′

NGC 3745 (HCG 57g)
16.2bmag

NGC 3746 (UGC 6957, HCG 57b)
15.0bmag

NGC 3748 (HCG 57e)
15.8bmag

NGC 3750 (HCG 57c, VV 282c)
14.9bmag

NGC 3751 (UGC 6601, HCG 57f)
15.0bmag

NGC 3753 (UGC 6602, HCG 57a, VV 282a)
15.5bmag

NGC 3754 (HCG 57d, VV 282b)
15.0bmag

PGC 36010 (HCG 57h)
17.0bmag

2MASX J11373896+2202269 (PGC 169872)
16.5bmag

Ralph Copeland, from January 1871 assistant of Lawrence Parsons at Birr Castle discovered during two nights in spring 1874 seven galaxies within 7' with the large 72" Leviathan reflector – largest telescope of the time. Faintest galaxy was NGC 3745 with 16.2 bmag. This brightness is interesting to make a statement about the capability of the big Speculum mirror. He misses the near and only a few tenth magnitudes fainter galaxy PGC 36010 which has a brightness of 16.5 bmag. The big problem of the mirror was the loss of reflectivity during an observing session before the change of the second, new polished mirror.

Unfortunately Dreyer made a relation fault with an from Copeland named reference star. This is because the Septet was catalogued as “nonexistent” in the RNGC.

Later on several famous catalogues pick up this group like Vorontsov-Velyaminov (galaxy trio NGC 3750/3753/3754), Hickson (eight galaxies – all NGC + PGC) and Arp. The name “Copeland’s Septet” has been introduced with the RC2 from deVaucouleurs/Corwin in 1976.

The measured redshifts of all nine (seven NGC + two other companions) named galaxies indicates a gravitational bound unit. Deep photographs also show strong signs of interacting effects. Especially the brightest member NGC 3753 shows tidal arms.

The group should be visible from 8” aperture or maybe smaller? Observers with 10” telescopes reports about separation of some galaxies. With 16.5bmag or 17.0bmag for the 2MASX galaxy it should be possible to see all nine galaxies with 16”-18” aperture. A quick and dirty look with 27” shows all nine galaxies as direct vision objects but did not show the tidal arms of NGC 3753. Bigger could show structures within the galaxies and some more background galaxies beyond 18bmag in the direct neighborhood.

labeled DSS 10'x10'
Copeland_DSSb10.jpg

200" plate of Arp Atlas
big_arp320.jpeg

16", 257x, NELM 6m3
HCG57.jpg

"GIVE IT A GO AND LET US KNOW"