Arp 104, also known as Keenan's system, is entry 104 in Halton Arp's Atlas of Peculiar Galaxies catalog for spiral galaxy NGC 5218 and globular galaxy NGC 5216. The two galaxies are joined by a bridge of galactic material spanning 22 000 light years.

In 1790 William Herschel discovered the galaxies, and in 1926 they were studied by Edwin Hubble. In 1935 Philip C. Keenan first published a paper about the bridge connecting the galaxies, which was rediscovered in 1958 at the Lick and Palomar observatories. [Source: Wikipedia]

Galaxies NGC 5216 (bottom) and NGC 5218 (top) really do look like they are connected by a string. That string is a cosmic trail of gas, dust, and stars about 22,000 light-years long. The interacting galaxy pair is some 17 million light-years away in the constellation Ursa Major. The debris trail that joins them, along with NGC 5216's comma-shaped extension and the distorted arms of NGC 5218, are a consequence of mutual gravitational tides. The tides disrupt the galaxies as they repeatedly swing close to one another. Drawn out over billions of years, the encounters will likely result in their merger into a single galaxy of stars. Such spectacular galactic mergers are now understood to be a normal part of the evolution of galaxies, including our own Milky Way. [Source: APOD]

The image was captured with the venerable Meade 12"LX200 Classic and the Atik 383L+ mono CCD at F7.16. Astronomik's LRGB filters were used. All subs were taken at 1x1 bin, -10C, with 10 minutes each for RGB and 7 1/2 minutes each for L.

Image:

Lum : 57 subs (7.10hr) on Jun 8th, 9th, 10th, 11th and 22nd.

Red : 10 subs (1.67hr) on Jun 23rd, 24th and 25th.

Grn : 10 subs (1.67hr) on Jun 23rd and 25th.

Blu : 10 subs (1.67hr) on Jun 23rd and 25th.

Processing was done with PixInsight, following (for the most part) kayronjm's tutorial of Feb. 24th. Only L was used to develop the Luminance image. R, G and B were not collected for the color mix. They may be added later (see Edit below).

North is up, and this is a very slight crop. Interestingly, I had to edit the above APOD information. APOD appears to have their galaxies switched, calling 5218 5216 and vice versa. Best as I can tell, the Astrobin designation of the galaxies is correct.

The background is particularly noisy -- or, there are reflections in the optical train that I cannot get out. I was unable to eliminate all of the noise and artifacts while trying to keep as much of the tidal bridge. New flats were done towards the end of this effort. They did not resolve the issue.

EDIT Jun 26th: The "Original" image is from L only. Added RGB on Jun 26th for image B. Not sure it contributed a lot to the mix.