NGC 7603 in Pisces
September 2023 - Galaxy of the Month
As the galaxy fields of autumn roll around, I have chosen NGC 7603, also known as Arp 92, in Pisces for the galaxy of the month. First discovered in 1864 by Albert Marth using William Lassell’s 48” speculum metal mirror telescope from Malta.
Arp included it in his group of spiral galaxies with an elliptical companion. NGC 7603 appears to be interacting with the small elliptical galaxy PGC 71041 nearby, indeed there appears to be a filament connecting the two galaxies. Arp 92 is at the centre of various controversies about cosmological redshifts due to the fact there are two quasars involved with the filament, one at either end, note the dots in the accompanying SDSS image, as well which Arp and Hoyle took to mean they were connected and thus quasars were not at cosmological distances.
NGC 7603 itself is a distorted spiral galaxy. It seems that occasionally PGC 71041 is also known as NGC 7603B. NGC 7603 is classified as a Seyfert class 1 AGN, although its type seems to have varied over the last 20 years or so due to changes in its spectrum. NGC 7603 is also a very strong blue source catalogued as Mrk 530.
There are however issues with PGC 71041 being the cause of the interaction as it appears to be nearly twice as far away than NGC 7603. The fact that the spiral arm/filament overlies it appears to be totally coincidental. One other possible candidate for casing the disruption is the barred spiral NGC 7589 which appears to be at the same distance as NGC 7603. It is also possible that the plumes from NGC 7603 are the result of a merger event rather than an interaction.
There do not seem to be any high-quality images of Arp 92 taken by the HST, but there is one image on Wikipedia. This is a relatively raw/ unprocessed Hubble image as evidenced by all the cosmic ray streaks.
NGC 7603 would appear to lie at a distance of about 127 Mpc. It was also included in the extended VV catalogue of interacting galaxies as VV 1975.
It is interesting that given the usual Arp mania there are no observations of Arp 92 in The Night Sky Observer's Guide (NSOG). I note that both Steve Gottlieb and Alvin Huey felt that NGC 7603 was not too hard to find in their large instruments but the companion was very faint in 50cm class telescopes and not surprisingly no spiral structure was seen. Both NGC 7603 and NGC 7589 should appear in the same field with a medium power eyepiece, however as both are quite faint it is probably worth using the highest power eyepiece you can that fits them both in the same field to find them. In Steve Gottlieb's complete NGC observations (on Adventures in Deep Space) he also has observations of the pair with a 48” Dobsonian.
Owen Brazell - Galaxy Section Director
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Galaxy of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.