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Walking on the Moon

Arp 295: A bridge between two galaxies

Martin Meredith

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Arp 295 is an interacting pair of galaxies just over the border from Pisces in Aquarius. The southernmost object is an almost perfectly edge-on spiral galaxy at magnitude 14.5 which makes it into the Flat Galaxy Catalogue (FGC 2525). To its upper left is another spiral (PGC 72155) at a similar magnitude (14.6). Both galaxies are around 320 million light years distant. To the right is the mag 14.7 elliptical IC 1505, which is at a similar distance. 

Arp 295.png

The peculiarity of the interacting pair comes from the bridge of material joining the two spirals (and also extending away on the other side of FGC 2525), discovered by Wilson and Zwicky. Arp classified the group as 'Double galaxy with long filaments', and it is perhaps the best example of the type. Given the angular separation of just under 5' between the two, and their distance, the bridge of gas and stars is about a quarter of a million light years long -- about twice as long as the diameter of the Milky Way. It is tilted with respect to the flat galaxy, and the filament on the other side is at a slightly different angle still. All these details were important in the famous 1972 Toomre and Toomre article on Galactic Bridges and Tails which used computer simulations to show how such features can arise from close encounters between pairs of galaxies. In the case of Arp 295 the encounter is thought to be quite recent and destined to repeat itself until the galaxies eventually merge.

This is a longer-than-normal stack (on the other hand, 8 minutes is a paltry amount to spend on this fascinating system ;-). Although the bridge started to appear after 4 subs of 30s, the filament is not of uniform thickness and I had to stack for a while for the weaker sections to show through. There is also a plume to the upper left of the companion galaxy PGC 72155 which is starting to appear in the image, but more exposure is needed to pull it out. Captured with the usual kit: 8" f4 Newt, Lodestar X2 mono, StarlightLive v3, no filters, live dark subtraction. Moonless, windless, moderate seeing. The objects were around 40 degrees above the horizon. Original image inverted and automatic white balanced in GIMP. North is nearly straight up...

There are lots of other galaxies in the image. From around midnight to 4 on the rim of the circle are 4 mag 16 galaxies. Less apparent and closer in to FGC 2525 are 3 galaxies (or galaxy fragments perhaps). This APOD image shows them well. The closest one, just to the right is a ring galaxy (on my photo it looks stellar).

If anyone likes a challenge there is another complete but very thin bridge in Arp 104 in Ursa Major.


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Great capture and write up. I have tried observing this ARP from my location but the bridge has proved elusive. Needs darker skies. Another reason I enjoy your captures.


Edited by Astrojedi
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Fascinating objects Martin, lots to explore in that image. The inversion works really well to bring out  the faint stuff. I have never heard of the Flat Galaxy Catalogue, we need a catalogue of catalogues! I haven't been out with my EAA kit for a long time, all I can manage at the moment are short visual bursts, but hoping to start my EAA season very soon. 

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Thanks Hiten and Rob. Indeed, there are lots of catalogues. If you browse Vizier you'll find most of them. There are in fact two flat galaxy collections I know about, the Flat Galaxy Catalogue and the Revised FGC (RFGC) ;-) There are a bunch of obscure but potentially interesting listings I like too, such as the 'galaxies behind the milky way' catalogue. Fortunately (?), new catalogues are being added every week, usually based on some academic study, and some of them turn out to be interesting for amateur observers.


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