March 2016 - Galaxy of the Month
NGC 2699 Galaxy Group in Hydra
This interactive image of the NGC 2699 was provided by the Digitised Sky Survey using Aladin Sky Atlas. You can download a finder chart for the area, with another zooming in on the group of galaxies. There's a SkyTools version as well.
This month’s choice is the group of galaxies surrounding NGC 2699 in Hydra. The Lyon catalogue (LGG) of compact galaxy clusters lists it as number 164 which consists of the galaxies NGC 2695, 2699, 2706 and 2708. Note that NGC 2706 is about 30’ away to the north from the rest of the group.
There are a bunch of other galaxies in this field in NGC 2697, 2698 and 2709 which although in the same general area do not seem to be associated with the group as defined by the LGG selection, either that or the radial velocities have not been measured well enough.
To add to the confusion, the UZC-GC catalogue lists the group as number 81 and it suggests the group contains the 7 galaxies NGC 2690, NGC 2695, NGC 2697, 2698, NGC 2699, NGC 2706 and NGC 2709. This is a catalogue based on automated near neighbour search techniques taking into account the recession velocities.
The brightest two galaxies in the group NGC 2695 and 2708 were discovered by William Herschel in 1785. The rest by a ragtag list of the main NGC observers with varying size telescopes, which suggests the faintness of some of the other members.
Deep images suggest that NGC 2708 has been undergoing some form of interaction from the tails that appear to be coming off it. This may also be why it is catalogued as an emission line galaxy, although it does not appear to have been an AGN so this maybe from some enhanced star formation.
The majority of the group appear to be spirals with just 2699 being a confirmed elliptical. The recession velocity would put the group at a distance of around 24 Mpc.
It is probable that NGC 2708 is the same object that John Herschel found and got added into the NGC as NGC 2727. This is another example of where different charting programs plot objects depending on whether they believe in the historical corrections or not. For more information on the dubious objects in the field see Harold Corwin’s notes on the NGC. There is also some information on these in Wolfgang’s book.
Given the range of brightness of objects in this field I think it will be a challenge for observers. The brightest two should be visible in 22-30cm telescopes but I suspect the fainter galaxies at around 13th magnitude are going to require scopes of the order of 40cm or above to find, certainly from the kind of typical skies we have in the UK. Finding the fainter galaxies will not be helped by the fact there is a 7th magnitude star in the field.
Owen Brazell - Galaxy Section Director