Object of the Week, May 1, 2016 - NGC 3239, Arp 263

Peculiar galaxy
Magnitude 11.1
RA: 10h 25m 05.5s
DEC: +17° 09′ 37″

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I first observed NGC 3239 last year and was fascinated by its irregular shape along with a picturesque clump of more distant galaxies serving as a backdrop. A triangle of medium bright stars added some foreground sparkle to the scene too – all in all, a lovely field of view in my 28 inch scope. But the conditions weren’t great and I want to come back on a darker night to see more, and last month I did.

With some patient observing and darker skies I had a great time teasing out more detail than I saw last year. My first-look impression of this tidally distorted galaxy is that it’s a lumpy mess. The brightest lumps and knots are HII regions and OB associations, and with extended observation the view is gradually embellished by two tidal tails.

The tail easiest to see is Tail B, which projects nearly due north of the main mass of the galaxy directly toward the northern most star of the triangle mentioned above. It doesn’t quite reach the star and appears broad and diffuse and curves westward. Much more difficult to see is the thin line of knots that extends the tail further west.

Even more difficult is Tail A, the faint loop that seeming originates from the brightest knot of 3239 on its southern edge. So far I’ve only seen the brightest part of the loop where it extends west from the bright knot, and think I’d need a nearly perfect night to see the entire loop with my 28 inch scope. I used 408x for my observations, but also 355x and 476x. The SQM was 21.40.

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By the way, the Tail A and Tail B labels are from a 1990 paper by Karl Krienke and Paul Hodge that includes a nice schematic of the 3239’s structure.

None of the fainter galaxies in the field of view are suspected of interacting with 3239 because they are too far away, so it seems 3239 is the result of two galaxies well into the process of merging into one. None are included in my sketch above, which focuses on only 3239.

However, tracking down the smaller and fainter galaxies is fun if you can tear yourself away from 3239. They’re all around 15th magnitude and are a nice collection of galaxies even without 3239 as a main attraction.

"GIVE IT A GO AND LET US KNOW"

Oh yes, the "Loony galaxy", VV 95. I have a sketch of a high-mag view of the central part of it with SN2012A. It was no brighter than mag 14.5 and fading when I observed it (last prior measurement; clearly fainter on that day). The round field is that of ZAOII-6 giving in excess of 500x in the 12" SCT. From left to right in it we have a foreground star, the supernova (faint), and a nonstellar knot in the galaxy. N up, W left here, so this is the "central" star and the brightest knot in your observation, Howard.

http://3.bp.blogspot.com/-RwMs9f7V1j...239+in+Leo.jpg

Hi Howard,

very nice object indeed. A really brilliant one. Nice detail in your drawing Howard. A bit surprising to me that the small knot appears that faint in your drawing yet - in DSS and the following great pic it appears more bright in comparison to the other region of the galaxy...

http://apod.nasa.gov/apod/image/1201/n3239_block.jpg


When i tried this galaxy it had been so nice and surprising to me that i made a sketch of it too.

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Shape of the bigger region was hard to catch. Observation was with 12" at 5000 ft during a very clear night of 7m0+ . Mediocre Seeing.

Just like you i just concentrated on the Arp itself - some of the companions should have been visible in 12" too i guess.

CS!
Norman

Very nice choice Howard. For sure one of the most obscure galaxies in the northern spring skies.

I can contribute an older sketch with the 16". With some knots and companions but without the tails.
16", 225x, NELM 6m0+
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A quick and dirty look through the 27" a few month ago showed spectacular knot detail. With higher power (but soft seeing) I could separate five knots. Brightest knot was the SE knot directly E of the 10mag foreground star. This knot could separated in two peaks. Second brightest was the knot directly N of the star. N of the brightest knot and at the W End of the brighter galaxy part I could see two more knots. In close proximity (3' SE) to the NGC 3239 body another galaxy was visible (PGC 30585).
Unfortunately I used to much power to see the tails. But the galaxy is still on my list for the 27", so I will start another try when conditions were better. Thanks for the reminder Howard.

Howard, What a nice surprise to see one of my favorite "odd" galaxies as object of the week. It's been a favorite of mine for many years - I like the background group of galaxies and star hopping from Gamma Leonis I usually take a quick look at Hickson 46 just 1 degree NW. There was a supernova in N3239 a few years ago; I saw it, but I forget the year. Clear skies, Miles

Glad everyone is enjoying this object! As it turns out, we had a rare clear sky last night so I took out the 8 inch for a spin and NGC 3239 was my first target. I was eager to see what an 8 inch scope could do with it and was gratified to see as much as I did:

"N3239 is a definite and slightly sparkly glow in the 8mm Ethos (118x) but was subtle overall - more like an off-center halo to the bright field star it's near than a galaxy, but I'm delighted to see any detail at all! 21.38 SQM."

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The "sparkly glow" was two of the bright HII / OB associations, and they were small and star like points that came and went with the seeing.

Nice to read about your 8" impressions Howard and that you by yourself are fascinated enough to try even with a much smaller instrument. So nice comparison to bigger apertures!

Thanks for that
CS
Norman

I last saw this one several years ago, when there was a bright supernova in it. I'll have to revisit the next time the skies clear.

Edit: It was supernova 2012A, and the scope was a 14" Dob.

My notes from last night under SQM 21.1 skies read:

At 256x, 320x and 427x visible as a bar next to a bright star with two more slightly fainter stars at one end of the major axis. On the other side of the bright star, next to the bar, I see a bright and distinct star forming region. The bar extends almost to one of the two fainter stars and then turns away from the other star. It is a tiny hook and the rest of the galaxy there is invisible. The part of the galaxy running away from the star forming regions is invisible. There are high clouds so I'll need to try again from a darker location when there are no clouds.

I've taken Howard's excellent sketch, rotated it so that north is up and labeled the various knots and features as well as the magnitudes of a few stars. The designations [HK83] are from Hodge and Kennicutt's 1983 "An Atlas of H II regions in 125 galaxies". As Howard mentioned, the tail designations (A and B) are from Krienke & Hodge's 1990 paper "The structure of the irregular galaxy, NGC 3239". The brightest HII region is also known as VV 95b (Vorontsov-Velyaminov assumed it was an interacting galaxy).

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Howard, Jimi and I looked at NGC 3239 about a week ago (May 4) during the Texas Star Party week with his 48-inch. These are my notes from that session, combined with an earlier one in February 2012.

NGC 3239 is a large, disrupted irregular, dominated by numerous HII regions. It was a fascinating sight at 488x. A mag 10 star is superimposed on the southwest side of the galaxy with the patchy, highly irregular surface brightness glow of the galaxy extending mainly north and east of the star.

A very bright, round knot of ~15" diameter is prominent on the southeast side of the galaxy. This HII complex is catalogued as VV 95b in NED and as region #6 and #10 in Hodge-Kennicutt's 1983 "An Atlas of HII Regions in 125 Galaxies". A faint star or knot is attached on the north side. A "star" recorded just off the west side of this knot turns out to be supernova SN 2012A, discovered on 7 Jan 2012, still currently around 14th magnitude (see image at http://skycenter.arizona.edu/gallery/Galaxies/ngc3239). A very faint curving tail or hook (identified as "Tail A" in Krienke & Hodge's 1990 paper "The structure of the irregular galaxy, NGC 3239") was visible extending from the very bright HII knot. It curves clockwise to the southeast in the direction of a mag 15.8 star [2' ESE of the mag 10 superimposed star].

Along the north side of the galaxy (elongated east-west) are several additional knots. About 30" due north of VV 95B is HK[83] 3/4. This close pair of knots appeared as a faint, small, irregular glow, ~6" diameter. Patchy haze is just west, but with no condensed spots. Further west, and 40" due north of the mag 10 star, is a moderately bright, small, round knot, ~10" diameter that has several HK[83] entries (28/29/31/34). To the west of this knot, the main glow of the galaxy ends near HK[83] 57/58, a faint low surface brightness knot that is elongated N-S.

Beyond the west end of the main glow is a fairly wide, very low surface brightness glow (the start of "Tail B") in the direction of a mag 12.5 star (2.4' due W of the bright star); it then sweeps broadly towards the south. A narrow extension heading southwest ends at a faint HII knot, perhaps 6" diameter, identified in NED as NGC 3239:[HK83] 75 and 80.

This is an odd one to be sure. I was in searching for galaxy clusters so logged 6 total in this area but will limit this post to the 'star' of the bunch, Ngc3239 which most folks have discussed at length. Here is observation from 21.6 site in an 18" f3.7 at 278x in fairly good seeing conditions on 4-29-2012 from a friend's place called Deep Sky Ranch well south-south-east of san jose, CA.

Ngc3239_group 1/10fov [1.8'] An EQ triangle of bright field stars dominates the field with the main glow of the galaxy to the right in my field of the upper star of the 3-star pattern. The galaxy seems to have two areas of more intense glow appearing just SE of the adjacent field star [GSC 1420:172 at mag 8.7]. One bright area is to far right of the field star or just East and this appears almost stellar. The other bright area is below and to right [NE] of the adjacent field star.

In looking at the DSS I suspect I may have confused directions in the observation. Shortly after this time I started to just state where in FOV north was and then just use right-left-below-above directions to avoid confusion of trying to state actual directions late at night when my brain cannot adjust well for field rotation.