June 2017 - Galaxy of the Month
NGC 6764 in Cygnus
From the UK, June and July are practically dead months for deep sky observing as it never gets astronomically dark so choosing a Galaxy of the Month target is pretty hard and, some may say, pointless.
For this month, I have chosen the galaxy NGC 6764 in Cygnus. Cygnus is perhaps not an obvious constellation to look for galaxies in but it does share a border with Draco and it is close to here that NGC 6764 lies.
First discovered in 1885 by Lewis Swift using a 16” refractor NGC 6764 is a barred spiral, not dissimilar to NGC 7479 in that we can see a strong bar but weak spiral structure.
The galaxy itself is a hybrid active galaxy and its spectrum shows it as a LINER, although it has also been classified as a type 2 Seyfert. By hybrid it means that the optical emission spectra cannot be explained by a single ionizing agent so in this case as well as a central black hole there must be other sources of radiation. It is suggested in this case that it is a circumnuclear ring ionized by hot young stars from a recent star formation pulse.
The galaxy spectra has also been classified as a Wolf-Rayet type so it is perhaps a composite starburst/AGN galaxy. NGC 6764 does also have radio jets from the central black hole. There are also signs of radio lobes associated with the galaxy.
Interestingly WISE images in the infrared show very strong signal in the centre of the galaxy which could be dust, indeed the PANSTARRS image below does show dust in the central region along with the expected blue star formation regions in the bar.
In terms of distance NGC 6764 is of the order of 111 million light years from the earth. There is some suggestion it may be physically associated with NGC 6759 given that the redshifts are similar.
Visually I would expect that at magnitude 11.9 the nucleus should be easy to find and larger telescopes should show the bar as well. The spiral arms may be visible under dark skies with the larger telescopes in amateur hands. There is also a small galaxy LEDA 214715 which, at mag 15, should be visible in large amateur telescopes very close to NGC 6764.
Perhaps unsurprisingly the galaxy does not appear in any of the standard references, although it does appear in the Webb Society Deep-Sky Observer's Handbook (WSDSOH) Volume 4 with an observation with the 82” at McDonald Observatory.
One of the few observations I have seen from the UK suggests that it was an averted vision object with a 35cm telescope. Steve Gottlieb reports that he can see the bar and a halo along with detail in the bar in a 24”, along with the companion.
Owen Brazell - Galaxy Section Director