NGC 470 in Pisces

October 2022 - Galaxy of the Month

Our galaxy of the month this month is the tight triplet of galaxies in Pisces around NGC 470. I must thank Mark Stuart for recommending this group to me. The triplet includes NGC 467, NGC 470 and NGC 474. All three galaxies were found by William Herschel. NGC 470 and NGC 474 in 1784 and then NGC 467 a year later in 1785.

NGC 470 is a disturbed system and was included by Vorontosv-Velyaminov as number 948 in his extended catalogue. NGC 474 is also a disturbed system and Arp included it as Arp 227. It is not clear whether Arp meant Arp 227 to include both NGC 470 and NGC 474, or just NGC 474.

Arp 227 is a shell galaxy and appears to be in a physical pair with NGC 470. The pair lie at about 100 million light-years from us. NGC 467 is also a shell galaxy but is much further away than the others, although some older sources do suggest it is gravitationally part of the NGC 474 group. NGC 467 is also suggested to be a lenticular galaxy.

Shell galaxies are relatively rare, with maybe 10-20% of galaxies in this class, so to get two in the same field is unusual. Shell galaxies come about when another galaxy has been digested and the stars thrown out from the interaction form these shells. It would appear that NGC 474 ingested a spiral galaxy in two phases, the first pass being about 1.3 billion years ago and the final pass about 900,000 million years ago. It is not clear if NGC 470 is gravitationally interacting with NGC 474 as well.

NGC 470 has some very chunky spiral arms and there are some suggestions of plumes in deep images. NGC 470 shows up strongly in the GALEX UV images which suggests that there is a lot of star formation going on in it. The other two galaxies barely register in the UV.

Hubble took a nice image of NGC 474. NGC 474 would appear to be about 2.5 times the size of our Milky Way at about 250,000 light-years across. There is also a Canada France Hawaii Telescope (CFHT) image of both NGC 470 and NGC 474. NGC 474 seems to be catalogued as a lenticular galaxy, although there were suggestions that it may be an elliptical.

The galaxy trio is pretty tight so all the galaxies will fit in the same field in a modern hyperwide eyepiece at medium to high power. The presence however of the 8th magnitude star HD7991 in the field may make finding NGC 467 more difficult than it might be. The Night Sky Observer's Guide (NSOG) Volume 1 suggests the whole group should be visible in 20-25cm telescopes but recommends 30-35cm for this. The NSOG also suggests that NGC 470 and NGC 474 have very bright nuclei but the rest of the galaxies are rather faint. Given the poorer skies of the UK I suspect that 40cm may be required to see much.

Owen Brazell - Galaxy Section Director