Galaxy of the Month Archive 2021
In this series of articles we draw your attention to galaxies particularly worthly of an observer's time.
NGC 3637 in Crater
April 2021 - Galaxy of the Month
For this month’s challenge we dip down into the constellation of Crater. Unfortunately, this means our targets will not rise that high as seen from the southern UK.
NGC 3636 and 3637 were both discovered by William Herschel in 1786 and then independently by Andrew Common in 1880. Although Common is perhaps best known now for his photographic work, the NGC objects he discovered were all found visually using his 36” reflector from Ealing near London. This is the same telescope that went on to become the Crossley reflector at Lick.
The two galaxies appear to form a physical but non-interacting pair. NGC 3636 is classified as an E0 and NGC 3637 as a lenticular but of a complex form with a strong bar and a ring around it. This leads to the rather complex morphological classification of (R)SB0(r). The pair are associated with a group of galaxies associated with NGC 3672 known as LGG 235, which contains only these three galaxies. The group is fairly widely spread on the sky with NGC 3672 being over a degree away from the other two. As perhaps expected from their classifications these galaxies consist of old stars with no signs of star formation going on. Perhaps surprisingly not much research has been done on this pair and most of what has been done has been on the structure of NGC 3637. The group lies at a distance of perhaps 21Mpc from us. The pair merits a couple of pages in the Annals of the Deep Sky Volume 7.
Observationally this group will be a challenge not only because of the low altitude at which it culminates but also because of the presence of the presence of the bright 7th magnitude star HD 98591 between them. Both galaxies have bright cores and it may well be this is all that can be seen of them.
I note that in the glare of the bright star there is another galaxy that does not appear in any of the other main visual catalogues, probably because on the plates they were compiled from it was overwhelmed by the glare from the star. The galaxy is only listed in surveys in the IR from the WISE satellite and also in the UV from GALEX. It will be interesting to see if it is visible in larger telescopes but I suspect the glare of the star will be too much.
My suspicion is that high power is going to be needed to work on this group so the unusual combination, for the UK anyway, of a clear transparent night with good seeing maybe required for this pair. The pair does make the Night Sky Observer's Guide (NSOG) Vol.2 which suggests they are a target for 12/14” scopes. Both galaxies make the Astronomical Leagues Herschel II list.
Owen Brazell - Galaxy Section Director
NGC 2872 in Leo
March 2021 - Galaxy of the Month
The small trio of galaxies around NGC 2872 in Leo, also known as VV 1284 and Arp 307, where it is included as part of the Double Galaxies class.
The galaxies in the group were discovered by a number of different observers with the brightest pair of galaxies (NGC 2872 and NGC 2874) being discovered by William Herschel in 1784. It took Mitchell using the 72” at Birr to find NGC 2873 in 1857 however. Whilst observing the group he also found a number of other objects that he believed to be nebulae and one of these was later catalogued as NGC 2871. Unfortunately, this later turned out to be just a star. Another case one suspects of poor seeing making stars look nebulous. The Rosse team also thought they had found another nebula in the area, which it turns was actually a knot in one of the spiral arms of NGC 2874, however it got its own NGC number as 2875.
NGC 2872 is an elliptical galaxy and possibly an AGN as it has a bright UV core and radio lobes. The group is also known as Holmberg 130. NGC 2872 is interacting with NGC 2874. NGC 2873 is probably only optically associated as it appears to be at a different redshift, otherwise not much seems to be known about it. The Galaxy Zoo project classifies NGC 2873 as a red spiral, one of the new types of galaxy classification to come out of that project. Intriguingly NGC 2874 also seem to be classified as heading that way although the GALEX images show a lot of star formation happening in its arms which is slightly odd if that is the case. Very deep images of NGC 2874 show signs of tidal streamers, possibly from an interaction or merger with a dwarf galaxy. The distance to the group is probably of the order of 110 million light-years (33Mpc). Arp lists the group under his classification of Double Galaxies.
Hubble does seem to have observed the group a long time ago and no colour images seem to have been made however there is a black and white image.
Interestingly the DSS image of the group shows NGC 2872 as quite a small elliptical. However deeper images from newer surveys (SDSS, PanSTARRS) show it has a bright core and a very much larger extended halo and here it is classified as an E2. Perhaps surprisingly as an Arp galaxy not much research appears to be available on this system.
The group is very tight so observing this will require high power and steady seeing to split the galaxies. NGC 2872 and NGC 2874 should be quite easy but finding NGC 2873 may be a challenge for larger telescopes. Steven Gottlieb found it very faint even with a 24” and requiring averted vision to hold. Interestingly the group does not make the Night Sky Observer's Guide (NSOG), which is perhaps surprising as it has an Arp designation, it is perhaps time to remember that the core NSOG series is now pretty old and telescope sizes in particular have moved on. It does make Luginbuhl and Skiff (L&S) however where the suggestion is that from high altitude the two brighter galaxies are visible in 15cm but a 30cm shows them better. Interestingly they suggest that NGC 2873 should be visible in 30cm which does not tie in with Steve G’s observation above. I note that with a 25cm from the UK the group was not regarded as particularly impressive.
Owen Brazell - Galaxy Section Director
NGC 2389 Group in Gemini
February 2021 - Galaxy of the Month
Although rather late in the season this small group of galaxies in Gemini is still high in the sky at a reasonable hour.
The group, consisting of the galaxies NGC 2389, NGC 2388 and NGC 2385, was all discovered by William Herschel. He first found NGC 2389 itself in 1788 and then the other two galaxies in 1793, perhaps by this time he had moved to front view mode and gained an extra magnitude or so in what he could see.
There are a number of non-existent NGC objects in the area in NGC 2386, NGC 2390 and NGC 2391. These are all stars that were discovered by the Rosse team at Birr and thought to be nebulae but which later turned out to be stars. One suspects that they were fooled on a night of poor seeing/transparency when the stars looked nebulous, an issue that still plagues visual observers in the UK.
The group is also catalogued as WBL 142 which contains just these three galaxies. The group would appear to lie about 200 million light-years from us. NGC 2388 itself is an interesting type of galaxy known as a Luminous Infra-red Galaxy (LIRG). These are normally galaxies where there is a lot of star formation going on that is hidden by dust, which is then heated by the starlight to glow in the Infra-Red. They are also usually very red in colour. All the galaxies in the group are classified as spirals but the type of NGC 2388 is unknown. All the galaxies in the group appear disturbed but there are no obvious signs of interaction, such as tidal tails etc..
NGC 2389 is an almost face on spiral with a relatively bright nucleus and signs of a bar which fits with its SAB(rs)c classification in the rather involved de Vaucouleurs system. For some reason NED also lists this galaxy as NGC 2388E, although this may have come from the RNGC by Sulentic and Tifft. NGC 2389 is a well-developed spiral with lots of young blue stars whilst both of the others are somewhat reddish. The UV GALEX image backs this up with NGC 2389 itself showing lots of star formation and the other two being practically invisible. NGC 2385 appears close to edge on with a well-developed dark dust lane. The suggestion is that NGC 2389 and 2388 form a pair with NGC 2385 a more distant part of the group.
The Night Sky Observer's Guide (NSOG) Vol 1 recommends the group as a target for 40-45cm telescopes but also suggests that the small faint edge on galaxy UGC 3879 nearby should also be visible. I suspect that from UK skies probably 45-50cm may be needed to see all the galaxies in the group. All four should be in the same FOV with a medium power widefield eyepiece. I suspect however that UGC 3879 is going to be a challenge for all but the best nights.
Also in the same medium power field is the faint spiral NGC 2393, a Stephan discovery, so there are a lot of objects to go for in this field. There is also a nice pair of galaxies nearby associated with NGC 2275 which has been covered before in the GOM series.
Owen Brazell - Galaxy Section Director
NGC 2805 in Ursa Major
January 2021 - Galaxy of the Month
The small group of galaxies around NGC 2805 has an interesting discovery history. Three of them, NGC 2805, NGC 2814 and NGC 2820 were discovered by William Herschel in 1791. The last galaxy in the group is listed as either NGC 2820A or IC 2458. However, it appears that Bigourdan, who discovered IC 2458 in 1899, really meant to apply that designation to a knot in NGC 2820 so the galaxy is not the IC object. This small galaxy is also known as Markarian 108. John Herschel also managed to add to the confusion in the area as he recorded another nebula that became NGC 2816. NGC 2816 is however just another observation of NGC 2820 so that number should be retired.
The group also became known as Holmberg 124 after Erik Holmberg’s catalogue of double and multiple galaxies in 1937 that he found from early photographic plates, an effort that was corrupted by poor images and led to a number of false identifications. The group is also catalogued as LGG 173 which adds NGC 2880 to the group to make a 5 galaxy system, which is slightly odd as NGC 2880 is almost 2 degrees away from the others.
The group is classified as a poor galaxy group and consists of mostly late type spirals. The distance to the core of the group is around 90 million light-years or so. GALEX images in the UV show a lot of active star formation going on which suggest that the group has interacted in the recent past to stir up the star formation. The group does not show up so well in the IR WISE images. NGC 2814 in particular shows a number of knots, a bit like M82, and almost looks like two galaxies in collision. NGC 2820A also appears very disturbed.
NGC 2805 is a face on spiral with well-defined arms in the central part but somewhat asymmetric arms further out. On the Sloan Digital Sky Survey (SDSS) image there is a diffuse patch that I initially thought was another dwarf galaxy in the system but is more likely to be an artifact from the bright star in the field as it does not appear on other survey images. There is a nice image at ManTrapSkies.com. NGC 2805 was also host to the recent supernova 2019hsw. Interestingly although the galaxies around NGC 2820 show signs of interacting with each other from their radio emission and there is a tidal bridge between all of them, there is no tie up with NGC 2805. It also appears that there may be a tidal dwarf galaxy created in the streams from NGC 2820 to the NE of it.
The group is fairly compact and will fit in the field of a medium power (say 260x) hyperwide eyepiece. None of the galaxies in the group make the Night Sky Observer's Guide (NSOG) which is perhaps rather surprising. They do however make Luginbuhl and Skiff (L&S) where they suggest that a 30cm telescope is required to see much. NGC 2805 is a fairly low SB galaxy so perhaps only the small diffuse nucleus will be seen. I suspect the use of high power may help on the smaller galaxies to see if any detail can be seen. The NGC 2820 triplet does make the Interstellarum Field Guide, although not NGC 2805. NGC 2805 does make the H400 II list but the others in the group don’t.
Owen Brazell - Galaxy Section Director