Galaxy of the Month Archive 2023

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

• Arp 232 in Leo

  May 2023 - Galaxy of the Month

  As the skies start to get brighter as we approach the summer season in the northern hemisphere, I have chosen the relatively bright galaxy NGC 2911, also known as Arp 232, as my galaxy of the month. The galaxy is also included in the extended Vorontsov-Velyaminov (VV) catalogue as VV 1290.

  Lying in the constellation of Leo, NGC 2911 was discovered by William Herschel in 1784. It is catalogued as a lenticular galaxy, although unusually for that type of galaxy it does contain a number of dust lanes. Arp catalogued it as an example of his galaxies in fission group, but also suggested that it might have concentric rings. NASA/IPAC Extragalactic Database (NED) suggests it is part of WBL 226, a small group of three galaxies, which also contains NGC 2914 along with UGC 5093.

  The nucleus of NGC 2911 appears to contain an AGN of the LINER type, which suggests there maybe, or have been interactions in the group. The AGN has also been classified as a type 3 Seyfert. There does not seem to be a lot of activity in the nucleus given that NGC 2911 is practically invisible in the UV images from GALEX.

  There is also some confusion in the group as an object (PGC 27167) was found that some sources include as NGC 2912. This cannot be true as studies of the original discovery notes for NGC 2912 show it cannot be this object and the object included in the New General Catalogue of Nebulae and Clusters of Stars (NGC) as NGC 2912 is now thought to be just a star.

  It is unlikely that NGC 2911 is interacting with NGC 2914. The group lies at a distance of perhaps 155 million light-years from us. Perhaps surprisingly for an Arp object there does not seem to have been a lot of research published on it. NGC 2914 was also included by Arp in his catalogue as Arp 137 so you get two Arp objects in the same field. NGC 2914 is also classified as a lenticular galaxy but this time showing what appears to be tidal tails. It may also be an example of a polar ring galaxy.

  The whole group is very compact and all three objects will fall in the field of a high power ultra-wide field eyepiece. A medium power field will also include the galaxy NGC 2919. Some sources suggest that this galaxy may also be part of the NGC 2911 group but it is probably unlikely. The Night Sky Observer's Guide (NSOG) Vol. 2 suggests that this galaxy group maybe more challenging than I suggested and it is a target for 40-45cm scopes and even with those NGC 2914 is no more than a faint patch. UGC 5093 is suggested to be an AV object with that class of telescope. Observations form the UK with a 40cm telescope would fit those notes with NGC 2911 being described as a faint oval.

  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 3432 in Leo Minor

  April 2023 - Galaxy of the Month

  As we move into the brighter spring skies, I have stayed in the constellation of Leo Minor for this month’s GOM.

  NGC 3432 was first discovered by William Herschel in 1785 and was included by Halton Arp in his atlas of peculiar galaxies as Arp 206. It was also added as VV11 in Vorontsov-Velyaminov’s catalogue.

  NGC 3432 appears to be a barred spiral galaxy seen edge-on and is interacting with its neighbour the dwarf galaxy UGC 5983. This interaction appears to be causing intense star formation in NGC 3432 as well as filaments between them. The GALEX satellite image in the UV shows that star formation is occurring throughout the galaxy. Due to the amount of dust the core of the galaxy is not well seen.

  Interestingly despite all the star formation going on NGC 3242 is not classified as a starburst galaxy. This maybe because the number of really massive stars being formed is not that high yet. It is suggested that the interaction with UGC 5983 took place very recently, say less than 400 million years ago and that is not enough for a full starburst to get going, in fact the star formation rate is unusually low at the moment. It appears that both ends of NGC 3432 are strongly warped from the gravitational attraction.

  The pair is thought to lie about 30 million light-years from us. NGC 3432 was home to an unusual transient event catalogued as SN 2000ch, initially catalogued as a type II supernova it was later shown to be an LBV like Eta Car as it underwent numerous brightening events.

  There is a Hubble image on NASA's website, although a more normal coloured image by the HST can be seen on NGC 3432’s Wikipedia page.

  Interestingly the interaction does not appear to have sparked that much star formation in UGC 5983. There is an odd condensation though in NGC 3432 off to its northern end. At about 55,000 light-years across NGC 3432 is a relatively small galaxy. UGC 5983 is only 12-15 thousand light-years across and as such is a dwarf galaxy, about the size of the LMC. Observations suggest that NGC 3432 may hide an AGN of the LINER type at its core.

  The galaxy makes it into a number of popular observing lists including the Astronomical Leagues H400 list and Stephen O’Meara’s The Secret Deep.

  The visibility of the galaxy seems to depend on the observer’s location and the companion galaxy UGC 5983 appears to be very difficult to spot so that is the challenge for larger telescope observers.

  The Night Sky Observer's Guide (NSOG) suggests that in 20-25cm telescopes the galaxy is visible as a long spindle, which is backed up by observations from the UK. In 40-45cm telescopes the suggestion is that it will show some of the mottling in the spindle. Unfortunately, there are no large telescope observations in NSOG. There are also observations of the galaxy with a wide range of telescopes on the Webb Deep-Sky Society Observer's Handbook (WSDSOH) Volume 4.

  I suspect that using a medium power may be best to see this galaxy, although if the seeing and transparency conditions are good then high power may tease out UGC 5983.

  Owen Brazell - Galaxy Section Director

• NGC 3395 in Leo Minor

  March 2023 - Galaxy of the Month

  This month’s challenge is the galaxy pair in Leo Minor, NGC 3395 and NGC 3396, also known as Arp 270 and VV 246. The pair were discovered by William Herschel in December 1785. They are part of the NGC 3430 group, also known as LGG 218, which lists about 15 members lying at about 30 Mpc.

  There are a number of other fainter galaxies in the field that were discovered by Stephan Javelle using a 76cm refractor, these being IC 2604, IC 2605, IC 2608 and IC 2612, although IC 2605 is just a bright spot in NGC 3395. IC 2612 is not associated with the group being a background galaxy.

  UV observations of the group with the GALEX satellite show that large amounts of star formation appear to be going on in both NGC 3395 and 3396 along with the nearby galaxy NGC 3430, in fact NGC 3395 is classified as having a starburst nucleus.

  The galaxies appear to be in the early stages of an interaction and are still separate, although at the calculated distance they would only be about 12 kpc apart, about a third of the distance between our Milky Way and the LMC. The galaxies appear to be overlapping and a bridge and tidal tails have been detected in the pair. The suggestion is that they have already had one close pass and the second will be due in 10 million years or so.

  NGC 3395 has been classified as a distorted spiral and NGC 3396 as Im (a Magellanic style irregular), although with its bright line spectrum it has also been classified as a Wolf-Rayet galaxy because of all the massive star formation going on. NGC 3395 was one of the galaxies that the Rosse team at thought to be spirals after observations using the 72” at Birr.

  Perhaps surprisingly I can’t find an HST image of the pair. One assumes because they have strong infra-red emission, probably caused by the strong star formation, that at some point the JWST will have a look.

  The NGC 3395/3396 pair is one of the best and brightest of the galaxies in the Arp atlas. NGC 3395 makes the Astronomical League’s H400 list, although the brighter of the pair, NGC 3396 does not.

  The galaxies are a close pair so using high power to separate them will work well. A medium power field judiciously placed will also include the low surface brightness galaxy NGC 3430 in the same field of view as the others. This pair should be considerably easier to see than the last two GOM’s.

  There are many reports of people seeing them with a 25cm (10”) scope which suggest both galaxies are easy to see. The Night Sky Observer's Guide Vol. 2 suggests that at a medium power in 40-45cm (16-18”) scopes the pair is well seen and looks like a butterfly. The nucleus of NGC 3395 is almost stellar. The pair have an entry in OOTW in the DSF forum.

  Owen Brazell - Galaxy Section Director

• Hickson 40 in Hydra

  February 2023 - Galaxy of the Month

  After last months really rather hard challenge we have something slightly easier this month in the galaxy group Hickson 40, also known as Arp 321 and VV 116, in Hydra.

  Although this may be challenging because of its altitude from northern climes it is a really nice tight group. Consisting of three spirals, an elliptical and a lenticular galaxy this is one of the tightest groups in the Hickson catalogue. All the galaxies appear to contain compact sources, suggested to be supermassive black holes, at their centres. The size of the group is so small that all five galaxies could fit in a volume less than twice the size of our Milky Way galaxy.

  Although such compact groupings are regularly found in large galaxy clusters Hickson 40 appears to be a rather isolated field group. There actually appear to be 7 galaxies in the group but only 5 are easily seen. The group is located perhaps 300 million light-years away and is in the process of undergoing a merger to form one large elliptical galaxy. This may take place in a billion years or so.

  Hubble imaged these galaxies as part of its 32 birthday study. The group was classified by Arp as part of his class of galaxy groups.

  This group is going to be challenging for observers as none of the galaxies are bright enough to have an NGC or IC designation and the low altitude as seen from the UK, although it does crawl above the 30 degree line, will not help either.

  The brightest galaxy of the group is MCG 1-25-9 at 13.8 B so it should be visible in 30-40cm scopes from a dark sky. The rest of the group is somewhat fainter. The group is so tight that you should try using the highest power your telescope and the conditions will allow as this is also going to be the best when trying to tease out the details of the fainter members of the group.

  Unfortunately, despite being a Hickson group it does not appear in the Night Sky Observer's Guide Vol. 2. There is some information on the group on Reiner Vogel’s Hickson page.

  Owen Brazell - Galaxy Section Director

• NGC 1740 in Orion

  January 2023 - Galaxy of the Month

  For this month’s challenge I am going very deep into Orion. I am going to throw this out there and apologise that the challenge may be too extreme. The challenge is the faint galaxy pair NGC 1740 and NGC 1753.

  NGC 1740 was discovered in 1830 by John Herschel using an 18.3 speculum metal telescope. NGC 1753 had to wait another 50 years or so for Lewis Swift to find it using a 16” refractor in 1889, I suspect at this date from the Lowe observatory on Echo Mountain. Unfortunately it seems that confusion arose in the area and it appears that there is some conflict between the NGC numbers 1740 and 1742 due to an error in John Herschel’s positions. NGC 1742 is almost certainly just a star. It was found by Ball using Lord Rosse’s 72” at Birr.

  NGC 1740 is classified as a lenticular galaxy S0 at a distance of perhaps 59 Mpc. NGC 1753 is classified as a spiral SBa (pec) with a redshift derived distance of 57 Mpc so they may be a physical pair. Perhaps unsurpingly for such non-descript galaxies there is not much written on them. Both galaxies do appear surprisingly bright in the GALEX images which suggests that there is some star formation going on, which would be surprising for a lenticular galaxy as most of the star forming gas is normally stripped out of these.

  Holmberg thought NGC 1740 was a double nebula with NGC 1742 and classified it as Holmberg 84 in his catalogue of double and multiple galaxies. Unfortunately, as noted above NGC 1742 does not exist. It is possible that Barrachi using the 40” Great Melbourne telescope found these two galaxies independently.

  I have to say both of these are going to be very challenging objects and probably a bit faint really for any spectacular views. The altitude is also not going to help as seen from the UK but they do rise above 30 degrees. The galaxies are relatively close together and will be in the field of view of a modern widefield eye at a power of 265x. I suspect that using a high power to increase the contrast is probably going to be the only way to pick up NGC 1753. Using a medium power eyepiece may also bring the galaxy NGC 1729 into the field. This is also a faint object though.

  Steve Gottlieb suggests that even with his old 17.5” both these galaxies were no more than faint smudges. Perhaps unsurprisingly they are not in the Night Sky Observer's Guide (NSOG) Vol. 1.

  Owen Brazell - Galaxy Section Director