Object of the Week, 17 Dec 2017 -- HCG 31 = Arp 259, an strongly interacting triplet

[Steve Gottlieb]

NGC 1741 = HCG 31 = Arp 259 = VV 524 = VV 565 = MCG -01-13-045
RA: 05h 01m 38s
DEC: -04° 15.4'
Type: Galaxy Group
Size: 1.5'
Mag: V ~ 12.5-13

French astronomer Édouard Stephan discovered NGC 1741 on 6 Jan 1878 using the 31" silver-on-glass reflector at the Marseille Observatory. He called it "extremely faint; very small; not much condensation; small faint spot, a little eccentric" and his position matches the brightest component (HCG 31A).

NGC 1741.jpg

NGC 1741 is a major merger of two distorted spirals and is clearly experiencing a very strong star-formation event. NGC 1741 forms components A and C of the galaxy group HCG 31, which lies at a distance of roughly 165 million light years. NGC 1741 is clearly interacting with HCG 31B to its right, which is also undergoing an intense burst of star-formation.

A stream of stars and gas pulled out from the interaction heads south in the direction of IC 399 = Mrk 1090. A couple of tidal dwarfs are in the process of forming in this tail. IC 399 is also a physical member of this group, but was not included by Hickson. The HST image resolves tiny, blue HII knots, strung out like beads on a necklace. The merging quartet are considered dwarfs, similar in size to the LMC, and due to their close separation of ~75,000 l.y. are expected to merge into a single large elliptical.

Note: The small face-on spiral (HCG 31D) is not a member of a group, lying far in the background at a light-travel time of 1.2 billion years, and is a challenge (V mag ~17.8) even in large scopes.

NGC 1731_SDSS.jpg

You'll probably need an 8" or 10" to spot NGC 1741 and perhaps a 12" to resolve it as a merged pair (A/C). More aperture will reveal more structural detail, of course. The B component (V = 14.7) is also visible in a 12"-16" scope (depending on your skies). IC 399 is a bit easier than the B component , although it has a similar mag (V = 14.8). Finally, I've never seen HCG 31D in my 18-inch, though as expected it was seen as a small "knot" in Jimi's 48-inch, perhaps 6" diameter.

As always,

"Give it a go and let us know!"

NGC 1731 close-up.jpg

[Norman]

Jo this is a crazy one!
Thanks for this suggestion!

Edit: ah - already had it with 12"... 4 years ago. But with rather meaningless description as reads "when i was right, i saw a quite easy big brightening which extended to a brighter star. Shape was irregular oval. I tried to isolate any structures but failed. Seeing was fairly poor"

In short: i have to revisit it :-)

second edit: pics of that night ;-)

icy dob.jpg

CS
Norman

[Uwe Glahn]

Perhaps one of the most interesting groups of the Hickson compilation for large telescopes. And Steve you are right that the merged structure is visible up from 12-inch.

12", 387x, NELM 6m5+
Component a/c visible from 104x with direct vision; with higher magnification a/c shows a mottled structure with a peak at the northern end; southeast from that peak and still in the halo of c, another structure – a becomes visible at the edge of perception; b also visible from 104x up, but much fainter than the whole a/c complex; with higher magnification b shows an 1:2 to 1:3 shape; d is not visible

24", 484x, NELM 6m5+
extraordinary formed Hickson-Group; c visible as with direct vision as the brightest object within the group; a follows c to the east as a “V” shaped detail; a much fainter than c and 1:2 east-west elongated; b steadily visible with averted vision with similar brightness than a, 1:3 elongated; d is popping in and out of view and can be hold for several seconds each, very small but laminar character; the not Hickson-catalogued object just south of a is also popping in and out of view but can not hold steadily with averted vision

27", 586x, NELM 7m0+
bright and interesting group; a and c are fused to one object; this object consists of two longish galaxies which are attached as a “V” shaped structure; c seems to be brighter but more compact; b is visible as a 4:1 elongated streak, beside the brighter core both ends shows peaks; d is popping in and out of view as a very small, stellarlike object; 0,5’ south of a another stellarlike object is visible and can hold steadily with averted vision; 2,4’ southeast a IC 399 is visible as a bright, round and concentrated companion

HCG31.jpg