Object of the Week, May 15, 2017 - NGC 3631, Arp 27

[Howard B]

Spiral galaxy
Ursa Major
RA 11h 18m 13.4s
DEC +53d 26m 37s
Magnitude 10.4
Size: 4.5 arc minutes

I’ve observed NGC 36531 only once with my 28 inch scope, but I think it’s an object that will show its spiral arms to scopes much smaller. My notes from May 2015 read:

“This is a beautiful face on galaxy! Just south of the Big Dipper’s bowl, it’s big and relatively bright. One spiral arm is pretty straight and is the brightest part of the galaxy save the core. 253x, 21.24 SQM.”

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Halton Arp’s notes:

“Another Spiral Galaxy with One Heavy Arm. The heavy arm is the nearly straight one above the core, cutting across the general sweep of arms, heading just south of east. Also note the faint one extending out on the north side, and the faint halo on the east side.”

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Located about 54 million light years distant, 3631’s prominent heavy arm will probably be the first detail you’ll see in the spiral disk – it’s pretty bright for a spiral arm. Under a really good sky I’ll bet it’s visible with a 16 inch scope – but no doubt someone here has seen it in a smaller scope. Also look for a bright knot near the heavy arm (left side of my sketch) which appeared pretty star-like to me.

"GIVE IT A GO AND LET US KNOW"

[Steve Gottlieb]

Hopefully you'll get some smaller scope observations, Howard, but here are my notes through Jimi's 48" in April 2011 (on my birthday!):

48" (4/4/11): beautiful face-on spiral with two long, winding arms and branching extensions! This very bright galaxy is sharply concentrated with an intensely bright small core that increases to an extremely bright stellar nucleus. A prominent patchy arm is attached on the west side of the core and rotates counterclockwise to the north, then sharply bends back to the east, extending to the east end of the 4' diameter galaxy. What appears to be an offshoot arm turns counterclockwise on the east side and continues all the way to the south end of the galaxy. The second arm is attached on the southeast end of the core and sweeps around the south side towards the west and then continues to the northwest end of the galaxy. An offshoot or another patchy arm continues east near the edge of the northern halo. The two main arms are nearly connected by a slightly brighter region on the south side.

[Howard B]

That must have been a wonderful happy birthday view of 3631 Steve - I can just imagine all the detail.

[Mark McCarthy]

Not small aperture, but I observed this April last year from 19.54 SQML sky in my 20-inch: "Face-on spiral. Large, spiral form apparent with DV. Halo mottled and cloudy. Compact core, stellar nucleus. Halo is dim but large and rounded. Two not quite stellar brightenings in the eastern part of the halo."

I did not highlight a heavy arm, but the spiral form was very plain. Worth a revisit for sure.

[Uwe Glahn]

You win the bet Howard - the spiral structure and above all the heavy arm is visible with 16-inch. Of course the whole spiral structure was hard to detect and more visible because of the dark lanes between the arms. But the brighter knots, the heavy arms and the dark structures made it possible to "see" the spiral of this nice galaxy.

16", 180x-257x, NELM 6m5+, Seeing III
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[Don Pensack]

From my notes with the 12.5":
med.size, face-on spiral w/stellar core, Sc?Sd? nice, round shape w/diffuse arms, core is sml % of visible Gal., lower SB than average for size, mottled overall, dark lanes 1 side of core.
SQM 21.6, 183x