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

Observations

We have an observation by Ivan Maly with his 20-inch, observing from a dark site in the Appalachian region of Pennsylvania.