Galaxy of the Month Archive 2025

In this series of articles we draw your attention to galaxies particularly worthly of an observer's time.

• NGC 2648 in Cancer

  February 2025 - Galaxy of the Month

  Our object this month is the galaxy NGC 2648, also known as Arp 89, in Cancer. NGC 2648 was first discovered by William Herschel in 1784 and is paired with the galaxy PGC 24469, also known as MCG +02-22-006, which is a part of Arp 89. NGC 2468 is one of the brighter galaxies in Cancer, which however is not saying much! R.J. Mitchell observing with the 72” at Birr did actually find PGC 24469 in 1857 but it was not included in the final copy of Lord Rosse’s monograph and hence did not make it into the NGC. The full story can be found on Harold Corwin's website.

  NGC 2468 and PGC 24469 are physically interacting. NGC 2648 is thought to be a retired galaxy, one in which star formation has ceased. The pair lie at about 112 million light-years from us. Arp classified it in his group Spiral galaxy with large high surface brightness companion on arm. If they are at this distance then the pair would be about 150,000 light-years across.

  Both NGC 2648 and PGC 24469 shows signs of tidal interactions with one of the spiral arms of NGC 2468 being pulled towards its companion and tidal tails coming out of PGC 24469. PGC 24469 is also suggested to be a WR (Wolf-Rayet) galaxy which suggests a lot of star formation going on as it contains a lot of short lived massive Wolf-Rayet stars. The star formation could have been triggered by the interaction with NGC 2648. NGC 2648 although classified as a spiral shows no signs of star formation going on and its colours are rather yellow, not dissimilar to a lenticular however the pair are quite isolated and there is no nearby cluster for it to have formed in the usual way as a lenticular. Interestingly despite it appearing in Arp’s catalogue it did not make it into the original VV catalogue of interacting galaxies but was added in the extended version as VV 1239.

  Steve Gottleib did not note the faint companion PGC 24469 when observing with his 17.5” but did see it as faint with the 24”. Victor van Wulfen suggested he saw both with his 14” in mountain skies. Perhaps surprisingly as an Arp object it does not appear in the The Night Sky Observer's Guide. Alvin Huey in his Arp Observing guide says that both galaxies were clearly seen at high power in his 22”.

  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.

• NGC 1407 in Eridanus

  January 2025 - Galaxy of the Month

  This month’s challenge is the group of galaxies around NGC 1407 in Eridanus. Although the two main galaxies are relatively bright in themselves their low culminating altitude, even from the southern UK, makes them challenging to find. NGC 1407, NGC 1400 and NGC 1393 were discovered by William Herschel in 1785/86. John Herschel added NGC 1383 in 1835, although this may have been from his South African sojourn.

  These galaxies form a clump within the much larger Eridanus cluster, sometimes known as the Eridanus A group. This cluster has maybe 200 members and lies perhaps 75 million light-years away (23 Mpc). Somewhat confusingly there is also the Eridanus Group which refers to this area as well. The cluster is thought to be just condensing out of the Hubble flow and is related to the nearby Fornax cluster and is sometimes known as Fornax II. The whole lot is part of the Eridanus-Fornax-Dorado filament, a structure perhaps 10 Mpc in length.

  NGC 1407 is classed as an E0 elliptical galaxy, practically circular as we view it and NGC 1400 is an SB0, a pretty round lenticular, although recent measurements suggest it may in fact be an E1 galaxy. The other galaxies in the field are mostly spirals. In the Eridanus cluster as a whole some 30% of the galaxies are elliptical or lenticular and the rest spirals and irregulars.

  The concentration of galaxies around NGC 1407 is such that with a medium power eyepiece of say 200x and a medium to large telescope you may get four or five NGC galaxies in the same field. The fainter ones will be more of a challenge than the bright ones having been found with large telescopes towards the end of the 19th century by Marth and Leavenworth.

  NGC 1407 itself is unusual in that it contains some relatively young stars which suggests that it has undergone a recent star formation episode. It has also undergone several radio outbursts and as expected contains a supermassive black hole.

  The NGC 1407 group also shows some of the characteristics of a fossil group and contains many dwarf galaxies. Fossil galaxy groups are found when many of the more massive galaxies have merged into the central brightest galaxy, here NGC 1407, and leave many dwarf galaxies. NGC 1400 itself is odd in that it displays an anomalous velocity compared to the rest of the group but it does appear to be part of the group and may be making its first approach, along with its attendant galaxies. The interstellar medium in it has probably been stripped out by RAM pressure stripping by the IGM in the cluster. It is likely, although by no means certain, that NGC 1400 and 1407 are interacting.

  The relationship of the galaxies in the Eridanus and Fornax clusters can be seen on the Atlas of the Universe website. NGC 1400 and 1407 appear in Steven O’Meara’s Secret Deep project as numbers 12 and 13 and in the Herschel 400 list. It is perhaps important to note that O’Mearas book was written some time ago and some of the information regarding the astrophysics is now out of date.

  As these galaxies do not rise very high to observe them find a site with a good dark southern horizon and try and observe them on the meridian. From lower latitudes the main galaxies should be visible in say 15cm but I suspect from the Uk maybe 22cm will be needed to see them. The The Night Sky Observer's Guide (NSOG) Volume 1 suggests that 20-25cm would be useful to see NGC 1400 and 1407 although 30-35cm would give better views. There is also a downloadable guide to the Eridanus group on the Clear Skies website.

  Owen Brazell - Galaxy Section Director