NGC 1888 and Arp 123 in Lepus

December 2019 - Galaxy of the Month

This interactive image of the NGC 1888 was provided by the Pan-STARRS1 Surveys using Aladin Sky Atlas. We also have a finder chart that should help you identify these galaxies.

The galaxy pair NGC1888/1889 in Lepus was first discovered by William Herschel in 1785, although he only saw one of the pair, NGC 1888. It took Bindon Stoney using Lord Rosse's 72" in 1851 to discover the smaller galaxy in the pair which became NGC 1889.

Arp catalogued the pair as Arp 123 in his group of "Ellipticals close to and perturbing spirals". NGC 1888 does look as it is being distorted by an encounter with extended spiral arms. The galaxy does have a spiral arm on the opposite side to NGC 1889 which contains lots of young blue stars. Vorontsov-Velyaminov also included it in his extended catalogue of interacting galaxies as VV 1138. The pair also hosted the Type Ia supernova SN 2018yu.

The galaxies lie at a distance of perhaps 110 million light years from us.

Close by and in the same high power field is the edge on galaxy MCG-2-14-15, also known as RFGC 973, which at magnitude 14.5 should be visible in larger telescopes. For those with very large telescopes there is a third edge on galaxy called LEDA 147414 (PGC 147414), but at mag 16.5 this is likely to require very large telescopes from high dark sites to find.

Unfortunately for UK observers Arp 123 never rises that high, barely reaching the 1 airmass line at 30 degrees. Nevertheless it was visible in my 15” telescope and in an 18” at the October 2019 Haw Wood star party in Suffolk at the high altitude of maybe 70m. The MCG galaxy was not seen but the night was not the most transparent. The group did make the DeepSkyForum (DSF) Object of the Week in 2012.

NGC 1888 is classified as SB(s)c pec and NGC 1889 as cD pec. Arp gave them different classifications as Sc and E0. The pair are almost certainly in the early throws of a merger. MCG -2-14-15 is well in the background and not associated with the main pair.

Perhaps surprisingly given this is an Arp pair there is not much in the literature on them. The pair makes both the Night Sky Observer's Guide (NSOG) Volume 1 and the Webb Society Deep-Sky Observer's Handbook (WSDSOH) Volume 4, which suggests that a 16” is good whereas the NSOG suggests that a 30cm telescope will show the pair. Both suggest high power is needed to split them.

Owen Brazell - Galaxy Section Director