Arp empty ring galaxies

[Martin Meredith]

I find the Arp Catalogue of Peculiar Galaxies fascinating (BTW the whole doc can be compiled into a pdf and downloaded from the NED site at Caltech: http://ned.ipac.caltech.edu/level5/Arp/Arp_contents.html) and have spent some of the past week or so seeking out examples of some of the categories. One such is the set of "empty ring galaxies". These are suggested to be the result of galaxy collision where the result is for the bulk of the galactic material to be forced out to the periphery post-encounter. Visually they appear like smoke rings, with the colliding galaxy still in the vicinity. They seem to be pretty faint and I'm very grateful for sum-stacking to be able to see much at all.

Here are three I captured on different nights. In each case since the apparent size is so small I've shown a zoomed and inverted (gimp) image alongside an image taken from the 200-inch (!) Palomar scope plates.

First, Arp 146 in Cetus. Spight et al (1990) report a mag of 15.8 for the whole system and 17.14 for the companion; visual mags are probably 2 brighter. The Palomar plates show a complete ring of varying thickness, with a sperm-like ingressor galaxy. My 12x30s sum stacked image is an incomplete ring and lacks the tail of the other galaxy. Tracking was a little off (the hot pixels tell the story) so I think I could do better with this object. A longer FL would of course help a lot!

Next, Arp 147 (IC298) also in Cetus. This is a similar picture except the collision involved an elliptical. The resulting ring apparently contains 9 black holes. This is mag 14.6 and is at distance of around 430 MLy. Again, my image fails to complete the ring, lacking at the point where it is thinnest. 

Finally, Arp 141 in Camelopardalis. This is an irregular ring. The system is mag 12.5. The inverted image in this case seems to show less of the ring than the original.

OK, well not quite finally, here's another Arp (273) in Andromeda which while not a ring shows signs of interaction with the s-shaped galaxy. 

The benefits of stacking, particularly sum over mean, are evident here in these zooms. Left is a single 30s sub, then 8 x 30s mean stacking, then 8 x 30s sum stacking, with the Palomar plate at the right. Sum stacking as we've seen before brings out more detail.

Cheers

Martin