Galaxy of the Month Archive 2019
In this series of articles we draw your attention to galaxies particularly worthly of an observer's time.
IC 2184 in Camelopardalis
March 2019 - Galaxy of the Month
This peculiar V shaped galaxy pair in Camelopardalis was first discovered in 1900 by Bigourdan using the 12.4” refractor in Paris.
Originally thought to be a single galaxy it was added into Vorontsov-Velyaminov’s catalogue of interacting galaxies as VV 644. Due to its bright emission in the UV/Blue bands it was also classified as Markarian 8.
There seemed to be a lot of confusion as to the nature of this object with some observers claiming there were four galaxies present. This mystery was cleared up when Hubble imaged the pair. The Hubble image shows distinct tidal tails as well as the massive star forming regions formed when the gas from the galaxies crashed into one another.
The presence of these star forming regions and the existence of many Wolf-Rayet stars has also led to IC 2184 being classified as a Wolf-Rayet galaxy. Wolf-Rayet (WR) galaxies are a subset of emission-line and HII galaxies, whose integrated spectra show broad emission features attributed to the presence of WR stars.
Images taken in H-Alpha show the existence of a number of bright knots, which are the starburst sites. These knots would appear to be immersed in a diffuse envelope. The rapid rate of star formation is indicated by the fact that one of the knots may have up to 850 WR stars in it. Given this the knots cannot be that old and their age has been estimated of to be of the order of 4-6 million years. The existence of these knots may be the reason that earlier observers thought there were more than two galaxies here.
From the Hubble image it would appear that we are pretty much seeing the galaxies edge on. The galaxies are perhaps 165 million light-years away and at this distance would be perhaps 45 and 40 thousand light-years across. The whole system is perhaps 65 thousand light-years across, so quite small galaxies. The pair will merge into a single galaxy and given their distance will have probably done so by now.
Visually this galaxy pair will be a challenge, not so much for their faintness as they are around 13th magnitude, but because of their size. They are quite small so you are going to need high power to separate the pair.
Perhaps not surprisingly IC 2184 does not make the standard references. Steve Gottlieb has quite an old observation in his IC notes which suggests that it was fairly faint with a 24” reflector and even at 375x he could not split the pair. I guess it has not made it onto the list for Jimi Lowery’s 48”. Uwe Glahn does have a nice drawing of the pair however with his 27” in the Interstellarum Field Guide.
Owen Brazell - Galaxy Section Director
NGC 2585 in Hydra
February 2019 - Galaxy of the Month
In 1886, whilst observing with the 26.3” refractor at the University of Virginia’s Leander McCormick Observatory, Frank Muller came across a group of what he believed were four new nebulae.
Unfortunately the observers at the Leander McCormick observatory were notorious for their poor reported positions for newly discovered nebulae and in this case the positions that he gave Dreyer were at least an arc-minute out.
Based on the descriptions Muller gave however, Dreyer was able to associate three of the nebulae with a small group of galaxies which later became catalogued as NGC 2583, NGC 2584 and NGC 2585. The fourth object that Muller discovered was later determined to be a triple star, although the number NGC 2586 has sometimes been erroneously assigned to another nearby galaxy MCG -1-22-12.
Two of the galaxies, NGC 2584 and NGC 2585 appear to form a physical pair and show signs of tidal interactions. They were given the number 1225 in the extended VV (Vorontsov-Velyaminov) catalogue of interacting galaxies. As the extended catalogue was never published in a journal (only online) the VV numbers beyond 835 are not included in the main extragalactic databases such as NED.
There is conflicting information about whether NGC 2583 forms part of the group but its distance and redshift suggest it probably doesn’t.
The two NGC galaxies are going to be challenging to see visually as they are at around 13-14th magnitude. If they are not challenging enough then there are two other galaxies that appear in the same field as NGC 2584/5 that have catalogue numbers from the MCG (Morphological Catalogue of Galaxies) catalogue at around 15th magnitude as MCG -1-22-6 and MCG -1-22-7.
There are suggestions that they may also form part of a physical group with NGC 2584/5. Their redshifts are quite similar to the main pair, although the redshift of MCG-1-22-6 is perhaps a little further from that of NGC 2583. The redshift of NGC 2585 is around 6868 km/s whilst that of MCG-1-22-6 is 6590. Similar to the issues with NGC 2583 however, if the distances/redshifts associated with them are correct then it is perhaps unlikely unless the group is very widely spread in space. At a stretch MCG-1-22-12 (the galaxy erroneously assigned NGC 2586) could also be part of the group as its redshift is similar to that of NGC 2585. The group is not assigned any number in either the WBL or LGG catalogues which suggests that they at least did not see it as a physical cluster.
The group is very small and will be challenging to find as, despite the fact it lies in the northern part of Hydra as it does not rise much above the 30 degree (one airmass) altitude line as seen from the southern UK.
It is tight enough that all the galaxies in the main group should appear in the field of a medium power (say 250x) eyepiece. MCG-1-22-12 is slightly further away but even then it should fit in the same field as the others with a modern hyperwide eyepiece at say 200x. It may be worth using this kind of power to help darken the sky background and try and bring the galaxies out.
NGC 2585 is a barred spiral galaxy and NGC 2584 is an Sc spiral with lots of ongoing star formation and as they both appear to be face-on spirals probably only the core will show. NGC 2583 is an elliptical galaxy. As always when observing galaxies try and find a dark site with little light pollution.
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