Arps in Ursa Major

[Martin Meredith]

Ursa Major is home to more than 30 members of the Arp catalogue. Here are 7 displaying very different kinds of peculiarities that I looked at a few weeks back.

First up is Arp 1 (NGC 2857). This mag 12.2 galaxy is classified as a low surface-brightness spiral, although I've seen fainter. In the same field is Arp 285, consisting of mag 13.0 NGC 2854 (the NE component in this shot) and NGC 2856. This latter galaxy has a narrow jet that I was not aware of when I observed it (otherwise I would have spent more time on it). It is clear on the DSS image and just barely visible at a push on the inverted shot (rotated to match: it actually points towards Arp 1). I believe this is the remnant of a bridge formed from galaxy interactions. 

 

Next is Arp 18 (NGC 4088), also in Ursa Major. This galaxy, certainly meriting the appellation 'peculiar', is a "spiral with detached segments", clearly seen here. This is a mag 10.5 galaxy with a compact nucleus, surprisingly bright arms. As a bonus we also have spiral NGC 4085 in the shot. Somewhere between the two galaxies is a mag 20.7 high redshift quasar that I've failed to capture. Another one to revisit.

Here's Arp 27 (NGC 3631), a "spiral with one heavy arm" whose location I've indicated in gray. Arp also talks about an 'absorption tube crossing from inside to outside of the S arm' (the S arm is on the right in this image). I'm not sure what he means by an absorption tube but there is a very narrow curved structure that seem to sprout out of the arm at about 2 oclock. Also marked is a reasonably bright yet distant quasar, somewhere in the 8.5-10 billion year range.

Downsizing now, in the centre of this shot next to the bright mag 8.6 star is Arp 151, not surprisingly a member of the category "galaxy with jets" -- the jet pointing upwards towards 11 o'clock. The end of the jet (SE here) is the stellar-like active galactic nucleus Markarian 40.

Arp 224 (NGC 3921) looks like a pearl ring but is classed as "Amorphous spiral arms". I left this stacking for a while longer to get a better look at the complete ring and also the faint extensions on the opposite side of the loop. 

The area immediately to the left of Arp 224 is Abell Galaxy Cluster 1400, which is a smattering of mag 18+ galaxies at around 1.2 billion light years. A fair few are visible here. Arp 224 is at about a quarter of the distance, as is the near edge-on spiral NGC 3916, perhaps the cause of the loop structure?

Finally, here's Arp 264 (NGC 3104). Here the monica "Irregular clumps" fits. This is a very strange object and quite faint, needing a good stretch. Actually, this one is not quite in Ursa Major but just over the border in Leo Minor.

Thanks for looking

Martin