Galaxy of the Month in Eridanus

• 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

• NGC 1507 in Eridanus

  December 2023 - Galaxy of the Month

  With the winter clusters and nebulae season approaching I thought for this month I would try the edge on galaxy NGC 1507 in Eridanus for the GOM. NGC 1507 was first discovered by William Herschel in January 1785 and he described it as very faint.

  It is classified as a barred spiral (SB(s)m pec) and if its distance is about 40 million light-years then it is a small galaxy at perhaps 45,000 light-years in diameter. It does seem a little distorted and from the colours would appear to have a lot of star formation going on, including some very large star clusters. NGC 1507 appears to have no distinct nucleus. It was included in Markarians catalogue of blue sources as Mrk 1080. If the galaxy has been distorted by a merger or interaction this would be odd as the galaxy appears to lie in a void with no other galaxies nearby. There is a nice amateur image showing its blobby nature on Gary Imm's AstroBin. Unfortunately, there does not seem to be much other research on this object.

  Observationally NGC 1507 appears in the Astronomical League's Herschel II list. It does arise above the 30 degree altitude line as seen from the southern UK so should be visible but, as always with this type of galaxy, observe it when it is on or near the meridian. As it is quite small I would suggest a medium to high power when observing it.

  Steve Gottlieb records it as being fairly faint with a 13.1” and the The Night Sky Observer's Guide (NSOG) Vol 1 suggests that it should be visible in 20-25cm telescopes as a faint streak. They also suggest that 30-37cm should show some sign of mottling, however I suspect that under typical UK skies this may require 40cm or more to see as most of the observations in the NSOG were made in high dry sites. Luginbuhl and Skiff (L&S) suggests that it can just be seen in 15cm, although again this was from a premium observing site. They also suggest that 30cm should show some of the condensations in the galaxy.

  Owen Brazell - Galaxy Section Director

• NGC 1723 in Eridanus

  December 2021 - Galaxy of the Month

  NGC 1723, along with NGC 1721, NGC 1725 and NGC 1728, forms a nice quartet of galaxies in Eridanus. Perhaps surprisingly they were not found by William Herschel. NGC 1723 was discovered by Tempel using a 11” refractor in 1882 and the other trio by Barnard in 1885 using the 6” refractor at Vanderbuilt university, and independently by Lewis Swift using a 16” refractor a month later.

  Although the group only just rises above one air mass from the southern UK (it is just below Rigel) it should not be that difficult to observe, although to be fair the discoverers all regarded the objects as quite faint.

  The NGC 1721, NGC 1725 and NGC 1728 trio are also classified as VV 699 in the Vorontsov-Velyaminov interacting galaxy catalogue. Interestingly though whilst NGC 1723, NGC 1724 and NGC 1728 have similar redshifts NGC 1721 seems to have a much higher redshift. The distances to the first 3 would be around 55Mpc whilst, if the redshift velocity is a true distance indicator, NGC 1721 is at around 66 Mpc. I wonder then if NGC 1721 is purely a line-of-sight galaxy and the others are an interacting triplet. However, given that NGC 1721, NGC 1725 and NGC 1728 seem to be interacting the redshift for NCG 1721 may just reflect peculiar motions within the group rather than a true distance. The discrepancy would not be out of the range for intra group movements. Unfortunately, there are no redshift independent distance measurements to the group.

  NGC 1723 is sometimes referred to as part of this group, although it may be unrelated to the other three galaxies it is an odd galaxy with its spiral arms coming off a ring and a prominent bar. The ends of the spiral arms show some signs of distortions, although it is not clear what galaxy NGC 1723 maybe interacting with. There are a number of other faint galaxies nearby. NGC 1721 is also a spiral galaxy and appears to have signs of an interaction given the way its outer spiral arms are behaving. Neither NGC 1725 or NGC 1728 show much signs of an interaction.

  All the galaxies in the group are spirals with the possible exception of NGC 1725 which is classified as an S0, a lenticular. The NGC 1721 trio has also had the designation KTS28 in the catalogue of isolated southern triplets by Karachentseva, although of course if NGC 1723 is part of the group then this designation would be somewhat misleading. Perhaps surprisingly there has not been that much research done on the group.

  

  The group is included in the Interstellarum Field guide where it suggests they are targets for 8” telescopes but as usual I suspect that this is optimistic unless you are observing from 2000m in the Alps. The Night Sky Observer's Guide (NSOG) Vol 1 suggests the group are more likely to be challenges for 40-45cm telescopes and I suspect from the UK that this maybe more likely. The group is sufficiently tight that all the main galaxies will appear in the field of a medium to high power modern hyperwide field eyepiece. Given their altitude I suspect that trying to observe them within an hour of meridian transit and a good southern horizon may be needed.

  Owen Brazell - Galaxy Section Director

• NGC 1622 in Eridanus

  January 2019 - Galaxy of the Month

  NGC 1622, along with NGC 1618 and NGC 1625 form a nice triplet just north of the bright star Nu Eri.

  The discovery history of the group is quite interesting as they are all relatively bright galaxies. NGC 1618 was found by William Herschel in 1786, NGC 1625 by John Herschel in 1827 whilst reobserving his fathers’ discoveries and NGC 1622 by George Stoney whilst using Lord Rosse’s 72” at Birr in 1850.

  NGC 1622 was also independently discovered by d’Arrest on the 1st Jan 1862 using the 11” Refractor at Copenhagen, no New Year’s Eve’s parties for him! You would have thought that William could have picked up NGC 1625 as well, although NGC 1622 may have been to faint for him. Perhaps scattered light from Nu Eri impacted his observations.

  John Herschel originally thought that the discoveries by Stoney and d’Arrest were two separate objects and added them into the GC as such (numbers 881 and 878). Dreyer spotted the error and merged both into NGC 1622.

  The galaxies may form a physical system as they all have similar redshifts. All three are spiral galaxies with an inclination close to edge on, with the exception of NGC 1618 which is rather wider open.

  None of the galaxies show any obvious signs of interactions, although the outer disk of NGC 1622 does appear to show quite a strong warp. NGC 1625 also seems to show a slight warp in its disk and also appears to have a galaxy superimposed on its disk. Given the radial velocity of the object it may well be a dwarf galaxy associated with NGC 1625. The group would appear to be about 220 million light years away.

  All the galaxies in the group appear to be barred spirals which is relatively unusual. Of the three it would appear that NGC 1618 is the one most actively forming stars at this time.

  The group is tight enough that it will fit in the field of a modern hyperwide eyepiece at medium power. The Night Sky Observer's Guide (NSOG) suggests that it is a tough target for 8-10” telescopes but should be easy in 14-18” ones.

  I suspect the main challenge seeing the galaxies here is going to be keeping the 4th magnitude star Nu Eri out of the field whilst looking for the galaxies, a similar challenge to that with NGC 404 and Mirach in And. The galaxies are quite faint and may well require a telescope in the 40-cm plus category to show well, especially from UK skies.

  If these are not enough of a challenge then the compact group Hickson 30 lies only 20’ north of NGC 1622. Hickson 30 contains no NGC galaxies, although the brightest galaxies are around 14th magnitude so it should be visible in larger scopes.

  Owen Brazell - Galaxy Section Director

• December 2013 - Galaxy of the Month

  NGC 1300 in Eridanus

  

  Image Courtesy of R Jay GaBany, USA. For more images from R Jay GaBany please visit the Cosmotography website. You can click on image for a high resolution version.

  Most of the galaxies we have covered so far in the galaxy of the month column have been primarily aimed at northern observers. This month’s object is perhaps aimed more at mid northern observers. NGC 1300 lies in the constellation of Eridanus and was discovered by John Herschel in 1835 using the 18.7" reflector from his observatory at Feldhausen on the Cape.

  It is a classic barred spiral that has, amongst other things, the distinction of being one of the largest images of a single galaxy made by the Hubble Space Telescope. It is often touted as the finest example of a barred spiral, although many would suggest the great Fornax spiral NGC 1365 is a better example.

  The galaxy is unusual for a spiral in that it does not appear to contain a black hole at the centre. The main features of the galaxy as can be seen is that it is a barred spiral, however detailed observations of centre disk of the galaxy suggest it shows its own grand design spiral pattern which does not necessarily tie up with the larger pattern. This structure is actually not that unusual in some of the larger barred spirals.

  The NGC 1300 lies about 69 million light years away and appears to be part of the Eridanus galaxy group (Or Fornax II cluster) which is in turn a sub group of the Eridanus Cloud. This is a loose cluster of galaxies that appears to include about 200 galaxies that are condensing out of the general Hubble flow. The Eridanus Cloud also includes as a separate subgroup the better known Fornax cluster. The majority of galaxies (70%) in the Eridanus cloud appear to be either spiral or irregular. The size of NGC 1300 at a suspected 110,000 light years puts it at about the size of our Milky Way galaxy.

  Observationally from northern latitudes NGC 1300 is going to be a challenge as it is always going to lie low on the southern horizon. Using a 15" telescope shows a nice bar with a bright core surrounded by a haze even though the altitude of observation was only about 19 degrees. Observations with larger telescopes and from more southerly location suggest that it will show nice spiral arms along with some detail in the spiral arms. In the same medium power field of view there is also the tight spiral galaxy NGC 1297 which is also part of the same group.

  Owen Brazell - Galaxy Section Director