March 2017 - Galaxy of the Month

IC 504 Group in Hydra

This interactive image of the IC 504 group was provided by the Sloan Digital Sky Survey using Aladin Sky Atlas. We also have a finder chart should help you locate these galaxies.

Over time the Galaxy of the Month selection has varied in terms of the challenges presented from objects visible in small telescopes to ones requiring the use of the larger sized ones in amateur hands. For this month’s selection, the choice is an intriguing one in terms of how difficult they will be to see.

The small group of galaxies around IC 504, which are IC 504, IC 505 and IC 506, were first discovered by Lewis Swift in 1888 using a 16” refractor in Rochester NY. This was perhaps not the greatest site even then. The galaxies were described by him as faint however the brighter pair should perhaps be visible in a modern 15-16” telescope.

The galaxies are always going to be challenging ones to observe for northern observers as they lie in the head of Hydra and therefore will never rise that high, even at their best.

These galaxies maybe physically associated and are listed in the WBL catalogue as a group of seven galaxies. The group is numbered WBL 179. I assume the other cluster members are the CGCG galaxies in the field.

IC 504 and IC 505 are classified as lenticular galaxies in some sources and IC 506 appears to be an elliptical galaxy. Deep images from the SDSS and PanSTARRS surveys however show that IC 504 appears to have spiral arms, or at least a ring of new star formation, so perhaps it is not a lenticular but a spiral galaxy. Interestingly NED also gives the classification for IC 505 as S (spiral), which suggests some confusion, although it could of course be S0. It is also suggested that IC 505 may be a binary AGN.

Despite being visually fainter than IC 504 it appears that IC 505 is also classified as the BCG galaxy (brightest cluster galaxy) for this group. As the SDSS image does not show any sign of spiral arms for IC 505 it appears that galaxy morphology classification is still as much as an art as much as a science.

The field is full of much fainter galaxies when viewed on the SDSS and of particular interest is the horseshoe shaped string between IC 504 and CGCG 32-9. I doubt that any of these will be seen visually although CGCG 32-12 (MCG +1-22-7) might be with larger telescopes. There are no visual observations of this group that I have been able to find which suggests that they are well off the beaten track. Perhaps not surprisingly there is not much information on these galaxies but the suggested distance is perhaps 60 Mpc.

For those not familiar with the PanSTARRS survey the image data is now available. Note that you cannot control the field you get and the data is not as good as the SDSS.

Owen Brazell - Galaxy Section Director