Keenan`s System

OBSERVING REPORT

NGC 5216 & 5218 - Keenan`s System in Ursa Major

Date: August 8, 2010
Time: 05:30am UT (August 9)   10:30pm local time (UT-7hs) 
Telescope: 24" (61cm.) f/4
Observer: Leo Cavagnaro

Click on the image to enlarge

Finder chart

How to find it,
About 2° 55' to the southwest of the star 10 Draconis (visual magnitude 4.6).

Stars in map up to visual magnitude 6.0

The Keenan`s system, a pair of interacting galaxies about 17.3 million light years distant, also known as Arp 104, was first remarked upon by P. C. Keenan who noticed the remarkable filament between the peculiar type spiral NGC 5216 and the globular galaxy NGC 5218. These galaxies are situated in the well known constellation Ursa Major, in the northern sky. A way to find the zone where this pair of galaxies lies is to use the 3.6 magnitude star Thuban (α Draconis). The galactic pair is situated about 4° west-southwest of it. 

The Keenan`s System. North is up

At 90x through this big telescope both galaxies are easily seen immersed in a field where the brightest stars are in the east side (left) of the field of view here. Another nearby galaxy, NGC 5205, is visible in the field of view (lower right on DSS image).
NGC 5218, the northern member of the Arp 104 pair and catalogued as an SBb/P by the NGC/IC Project web page, is well known to have a disturbed structure. It looks, in relation to its companion, brighter, elongated, and smooth in brightness.
Observing the galaxy with averted vision some very small dots seem to be immersed in the galaxy.
On the other hand, NGC 5216, discovered by Friedrich Wilhelm Herschel (1738 - 1822) in 1790, an E type galaxy, looks round and smaller than NGC 5218, with a small and bright core. 

NGC 5216

Higher magnification is necessary to see more structure in these galaxies.
According to Cullen et. al. (2007), a prograde close passage between the disky NGC 5218 galaxy and the elliptical NGC 5216 occurred about 300 Myr in the past. The smooth stellar tidal bridge is a product of that interaction. A DSS image of this pair shows that tidal bridge connecting both galaxies. However, observations at 90x and even at 271x don`t show this structure at all.
At 271x, NGC 5218 appears elongated and with some small zones somewhat brighter.

NGC 5126 looks round in shape with a small round core. Averted vision is useful for a better observation of the galaxy. Its fainter outer parts are better detected at this power.

NGC 5218

Sketch of the Keenan`s System made by the author. North is up.