Galaxy of the Month Archive 2020
In this series of articles we draw your attention to galaxies particularly worthly of an observer's time.
NGC 2889 in Hydra
March 2020 - Galaxy of the Month
For March’s GOM we stay in in the constellation of Hydra and look at the pair of galaxies NGC 2889 and NGC 2884. William Herschel discovered NGC 2889 in 1786 but it was left to Heinrich d’Arrest to discover NGC 2884 in 1865.
Both of these galaxies appear to be spirals but whereas NGC 2889 is seen almost face on NGC 2884 is viewed at quite an angle. The two galaxies are listed as a pair but are not currently interacting, however distance measurements would suggest that the two galaxies may be an optical pair rather than a physical pair as NGC 2889 is listed as having a distance of about 61 Mpc whereas NGC 2884 is listed with a distance of maybe 50 Mpc. They could still be a pair however if there is a sufficiently large local component to their velocities as many distance velocity calculations are dependent on the Hubble flow being the dominant component of the velocity.
NGC 2889 was home to SN 2007rb.
Perhaps unsurprisingly there is not much research been done on either of these galaxies and what there is mostly concentrates on generic properties of the bar in NGC 2889. Deep images also suggest that NGC 2884 is a barred spiral, however NED gives it a classification as an S0/a (a lenticular galaxy) so there appears to be some confusion over its classification.
The two galaxies are close enough together that they will fit in the same medium (150x) to high (270x) power field of view. However given that Herschel did not see NGC 2884 it may be much fainter than NGC 2889 so a high power may be better to pick it out. Night Sky Observer's Guide (NSOG) suggests that NGC 2889 will be visible faintly in 8-10 inch scopes whilst 16-18 will show more detail and bring out NGC 2884.
Nearby is the asterism that was given the NGC number 2879 that was also found by d’Arrest on the same night he found NGC 2884. Unfortunately it is not a true fuzzy but merely a grouping of four stars. There are a number of other very faint galaxies in this field that are likely to be beyond all but the largest telescopes visually but are probably within the range of EAA systems.
Owen Brazell - Galaxy Section Director
NGC 2992 in Hydra
February 2020 - Galaxy of the Month
The pair of galaxies that make up the major components of the group associated with NGC 2992 and NGC 2993 were first discovered by William Herschel in 1785. They were later catalogued by Arp as Arp 245 and are also in the extended VV of interacting galaxies catalogue as VV 1311. Arp added the pair to his catalogue in the group of galaxies with the appearance of fission. The group lies perhaps 100 million light-years from us. Also close by is the galaxy RFGC 1621 which may also be associated with the group, although it is not part of the interacting pair.
The main pair are seen perhaps 100 million years after the closest point of the encounter (perigalacticon) and the interaction has already drawn out tidal tails from both galaxies. Indeed there might already be a dwarf galaxy forming in the northern tidal tail of NGC 2992.
NGC 2992 is also a Seyfert AGN, although a fairly mild one with a classification of 1.9. It is showing some signs of revived activity, perhaps due to infall of material on to its central black hole from merger products. Radio imaging shows bubbles of material coming out at right angles from the nucleus of NGC 2992 showing a classical biconical system. There may also be small scale spiral structure in the centre which could be fuelling the accretion disk around the central black hole.
Both NGC 2992 and NGC 2993 are connected by a faint tidal bridge as well as having their own tidal tails. RFGC 1621 appears to be a dwarf galaxy from its blue colours. Although observed with GALEX the pair do not seem to show the colours associated with new star formation. There is however a magnificent image of this pair at Adam Block's caelumobservatory website which does show new star formation in NGC 2993 as well as in the tidal tails. Both of these galaxies are catalogued as spirals, although of course they are now distorted from their encounter.
Although not in the H400 lists NGC 2992 did make it into both Hartung’s Astronomical Objects for Southern Telescopes and Walter Scott Houston’s Deep Sky Wonders has also described the pair, although Houston does not say what size telescope was used. Night Sky Observer's Guide (NSOG) Volume 2 suggests that 12-14 inch telescopes should show the pair clearly and 20-22 inch telescopes may show the tidal tails, although I expect this is probably from a high altitude site. I am not sure if the small flat galaxy is visible given its magnitude of around 17. Given the closeness of the pair I think using a medium to high power eyepiece would be the choice for observing these. Unfortunately from the UK they never rise above the 1 airmass line so try and observe them on the meridian.
Owen Brazell - Galaxy Section Director
We have an observation by Ivan Maly with his 20-inch, observing from a dark site in the Appalachian region of Pennsylvania.
NGC 2563 in Cancer
January 2020 - Galaxy of the Month
The small group of galaxies around NGC 2563 was once through to be part of a larger group known as the Cancer I cluster however this has been shown just to be a collection of disparate groups of galaxies and is not a true cluster. The cluster is now better known as the NGC 2563 group.
The brightest galaxies in the group, NGC 2562 and NGC 2563, were discovered by William Herschel in 1787. NGC 2560 and 2569 were discovered by d’Arrest in 1862 using the 11” refractor at Copenhagen and NGC 2570 by Copeland in 1873 using Lord Rosse’s 72” at Birr.
The group is also catalogued as WBL 178, a group of 11 galaxies which also includes the galaxies NGC 2556 but not NGC 2569 and NGC 2570. The group is also included in the LGG catalogue as LGG 158, a group of 14 galaxies which includes the NGC galaxies 2558, 2562, 2557, 2563, 2556, 2560 and 2569, showing how different selection criteria can include different objects.
A much deeper study has suggested that there may be as many as 64 members of this group, although many are small and faint. NGC 2563 is definitely the brightest cluster galaxy (BCG) for this group and most, if not all, of the galaxies in the core of the group are lenticulars which suggests this is quite an old group. It appears that galaxies on the outside edge may well be still joining the group as they are still gas rich spirals.
The group is also bright in X-Rays and observations suggest that most of the gas has been stripped from the galaxies by RAM pressure stripping and there are signs of interactions within the group, although mostly from observations in the radio region in the 21cm band. The X-Ray emission is probably coming from the hot gas between the galaxies. The group appears to be about 228 million light-years from us.
Observationally the core of the group is quite compact and will fit in the field of view of a medium power eyepiece. If we assume that all the galaxies in the LGG list are actually part of the group then the whole group spans about 45’ on the sky.
There are observations of this group in the book 'Galaxies and How to Observe Them' by Steinicke and Jakiel and also in Night Sky Observer's Guide (NSOG) Volume 1 where they suggest the two brighter galaxies are targets for 12-14” telescopes. There is no individual coverage of the other galaxies in the group there although there is a sketch showing NGC 2560, NGC 2562 and NGC 2563. Luginbuhl and Skiff (L&S) suggest that a 25cm telescope maybe needed to show NGC 2562 and NGC 2563. There are many other fainter galaxies in the core of the group that large telescopes and EAA systems may pick up.
Of particular interest is the galaxy UGC 4332, which appears to be undergoing some form of merger from the distortions visible in it. Observations of the fainter members of the group maybe hampered by the 6th magnitude star just south of it so the use of high power is recommended to keep it out of the field of view.
Owen Brazell - Galaxy Section Director