NGC 5954 in Serpens Caput

June 2019 - Galaxy of the Month

The summer months in the UK are not really dark enough for any kind of astronomical observing, let alone galaxy observing. As such I have chosen a relatively bright pair of galaxies in NGC 5953 and NGC 5954, collectively known as Arp 91 for this month’s column.

This pair of galaxies in Serpens Caput was first discovered by William Herschel in 1784. William Herschel was able to split the pair and recorded it as a double nebula.

Both NGC 5953 and NGC 5954 are individually classified as active galaxies with NGC 5953 being classified as a Seyfert type 2 with a circumnuclear ring and NGC 5954 being classified as a LINER.

The interaction between the pair has caused areas of star formation in the nuclear regions and as such it is unclear that the nuclei are truly active or we are just seeing the emission from massive star burst regions in the centre. Its interacting nature also meant it was included in the earlier catalogue of interacting galaxies by Vorontsov-Velyaminov as VV 244.

It is believed that the pair lies at about 96 million light-years from us. Both of the galaxies in the pair appear to have the same mass but NGC 5953 is the more disrupted of the pair with tidal tails and plumes surrounding it.

Arp 91 is included in the galaxy group LGG 400 which also includes NGC 5951 and NGC 5962 as well as UGC 9902. This classification may be based on spatial coincidence rather than redshift data.

Visually because of the closeness of the pair this is going to require high power to split. Night Sky Observer's Guide (NSOG) thinks the group is a target for 16/18” telescopes but it should be visible as a nebulous patch in much smaller telescopes, after all William Herschel’s 18.7” speculum metal telescope was probably equivalent to a modern 11-12”, although I am sure that some would claim they can see all the H2500 with much smaller instruments. However observations with a 14” from UK skies suggest that both NGC 5954 and 5953 are both small and faint.

About 16’ away from the Arp 91 pair is the edge on spiral NGC 5951 and all three galaxies will fit in a medium power field. NGC 5951, as it is an edge on galaxy, may also require a telescope in the 40-cm class to see.

The galaxies did make it as an Object of the Week (OOTW) at the DeepSkyForum. Alvin Huey suggests in his Arp Observing Guide that with his 22” and a magnification of around 500x he scan see the extension between NGC 5953 and NGC 5954.

Interestingly in ST4I the group is listed as the NGC 5953, 5954, 5951 Trio. I have no idea where this naming came from, although it does appear in Alvin Huey’s listing of Galaxy Trio’s, it is not as far as I can see in Miles Paul’s listing of triples.

Owen Brazell - Galaxy Section Director