NGC 7585 in Aquarius

September 2020 - Galaxy of the Month

The small group of galaxies around NGC 7585 (NGC 7576 and NGC 7592) in Aquarius were all discovered by William Herschel. He found NGC 7585 and NGC 7592 in September 1784 but NGC 7576 had to wait until the following year in October 1785 to be picked up, although Herschel did not spot that NGC 7592 was a double galaxy.

NGC 7585 was later added by Halton Arp to his catalogue of Peculiar galaxies as Arp 223 and by Vorontsov-Velyaminov to his interacting galaxy catalogue as VV 1973. Arp did not spot that NGC 7592 was also an interacting pair although Vorontsov-Velyaminov did and it is classified as VV 731.

NGC 7585 appears to be a shell galaxy although Arp catalogued in his group of galaxies with amorphous spiral arms. The suggestion is that it is the result of the merger of two galaxies. Its current classification is fairly complex as (R’) SA0 +(s) pec. The suggestion is it is probably a lenticular galaxy.

The three galaxies in the field do not seem to be related, although NGC 7576 is also a disturbed galaxy, possibly a rare ring one. NGC 7592 is a much more distant object. NGC 7585 itself maybe 145 million light-years away or so. NGC 7576 is, by some measurements, at a similar distance. The RC2 suggests they are a non-interacting pair. NGC 7576 does show up quite brightly in the ultraviolet GALEX survey which normally suggests some kind of star formation activity. Only the nucleus of NGC 7585 shows up in the UV. Both galaxies show up well in the infrared WISE images. Hubble has looked at NGC 7585 and in the near infra-red views with the NIC instrument a strong bar appears to show up.

Perhaps surprisingly given the nature of the group not much individual research appears to have been done on them. NGC 7592 on the other had has had a lot of work done and the interacting pair have been well imaged in various wavebands by Hubble and shown to be Seyfert type AGN’s. Unfortunately I can’t find a colour processed version of the Hubble images of NGC 7592.

Observational from UK latitudes these galaxies will be a challenge as they do not rise much above the 30 degree altitude line and will be best observed when on the meridian. Although Luginbuhl and Skiff (L&S) does suggest that NGC 7585 will be visible in good skies with a 25cm Night Sky Observers Guide (NSOG) Vol 1 suggests they are more of a challenge for 30cm+. UK observations of NGC 7585 with 25cm suggest it is not very impressive. I suspect that probably 40cm plus would be needed to see NGC 7592 well and to split them perhaps 55cm plus. None of these galaxies were bright enough to make it into the Herschel I or II lists although NGC 7585 did make it into the Herschel 3 list. The NGC 7585/7576 pair are close enough together to make it into the same field of a modern hyperwide eyepiece at perhaps 260x.

The Arp 223 pair was also covered in the DeepSkyForum (DSF) Object of the Week for October 20th.

Owen Brazell - Galaxy Section Director