Willow Springs, 26-27 Aug 2006

by Steve Gottlieb

The skies at Willow Springs were stunningly dark, particularly to the far south where there was virtually no light extinction towards the southern horizon. The superb transparency (M33 was obvious naked-eye on Sunday morning at 3:00AM high overhead) coupled with excellent seeing made for a great opportunity to go deep with Abell galaxy clusters. So, on both nights Mark Wagner and I worked our way through a number of planetaries at high power (435x gave excellent images on both nights! ) while the summer Milky Way was still well placed and then switched over to Hickson Compact Galaxy groups (HCG) and Abell Rich Galaxy Clusters (AGC) later in the evening.

Usually I find working through more than one or two AGC's exhausting as you're pushing your scope and concentration to the limits on 15th and 16th magnitude galaxies, but under these conditions we thoroughly explored a half dozen separate rich clusters as well as a number of smaller Hickson groups that were nearby. It was easy to pump up the magnification to 300x and stretch beyond the standard galaxy catalogues (NGC, IC, UGC, CGCG, MCG, ESO) to sweep up many "anonymous" galaxies that Megastar plots (these can often be found in NED with IRAS designations). With the faint stuff, Mark and I verbally directly each other at the eyepiece to the targets and confirmed all observations in both of our 18-inch scopes.

One of the "challenge" objects I wanted to look at was a recently discovered globular, AL 3, located at 18 14 06.6 -28 38 06. This cluster was missed by the great visual observers of the 19th century and was first discovered by Andrews and Lindsay in 1967 on photographic plates taken at Armagh Observeratory in Ireland. Unfortunately, the cluster was then left in total obscurity for nearly 40 years and it wasn't identified as a globular until earlier this year! Photometry of the individual stars and a Color-Magnitude Diagram done at ESO revealed a globular cluster that is located in the Galactic bulge at a distance of ~20,000 light years from the Sun. AL 3 is probably one of the least massive gc's in the Milky Way. Surprisingly this object is easier than the tougher Palomar globulars or even another recent discovery, Djorgovski 2. It was immediately visible once I moved to the field in Sagittarius!

As this globular was recently discovered in 2005 I was very surprised to find it was relatively easy as a fairly faint, moderately large glow with a number of mag 13.5-14.5 stars superimposed, ~3' diameter. It appeared roughly round with no concentration, though with perhaps some mottling. A string of 6 stars oriented WSW to ENE is superimposed on the face of the glow along with a couple of additional stars.

Another interesting object we looked is a supermassive cluster located in the spiral arms of NGC 6946 in Cepheus. This cluster was discovered in 2000 (Astrophysical Journal, Vol 535: 748-758, "A Young Globular Cluster in the Galaxy NGC 6946") and appears to be a young (15 million year old) globular cluster embedded in a bubble- shaped star-forming region. At 323x it appeared as a 20" knot embedded within the halo of the galaxy just 3' from the core. The galaxy itself was stunning at low powers with long spiral arms that seemed to stretch on forever.

There are only a few visual supernova remnants and one of these was recently discovered in Cygnus, near Albireo. A quick look at the field of 4.7-magnitude Phi Cygni (northeast of Albireo) revealed a huge, faint filament stretching across 13' to 15' of the rich star field using an OIII filter and a 20mm Nagler.

As far as the galaxy clusters and groups here's what we explored -- AGC 3744: This cluster is very easy to locate as the central portion is less than 30' S of a 4.5-magnitude star near the bikini-bottom outline of Capricorn and is located at a distance of roughly 570 million light years. Three relatively easy members are NGC 7016, 7017 and 7018. A 4th fainter galaxy, MCG -4-49-16 is in the central core of the cluster and ESO 530-001 and -002 are just 15' away.

AGC 2572: I logged 7 members in this 600 million light distant cluster. The central core includes NGC's 7588, 7597, 7598 and 7602. The brightest member is 15th magnitude NGC 7597 which I logged as "fairly faint, moderately large, irregularly round, broad concentration within the halo rising quickly to a small bright core". Several of the fainter members are in the 16th magnitude range and are just tiny knots less than 10" in size.

HCG 94: This Hickson group is located close west of AGC 2572! No need to starhop here. The brightest member is the double galaxy NGC 7578A/B with a third much fainter member, HCG 94c close northeast.

HCG 93: This brighter Hickson group is just 30' NW of HCG 94 and contains 5 NGC galaxies -- 7547, 7549, 7550, 7553 and 7558. Only 7558 and 7553 required any effort and all 5 members fit in a 10' field!

Burbidge's Chain: With a superb southern horizon and transparency NGC 253 displayed a remarkable amount of dust structure and mottling along with HII knots. We then looked at NGC 247 which stretched across 20' of the field. Just 18' NNE is string of galaxies first described by a trio of well-known astronomers -- Geoffrey and Margaret Burbidge and Fred Hoyle in 1963 and dubbed "Burbidge's Chain". We were able to detect 4 members strung out along a north- south line. Though NGC 247 is a fairly popular target, this challenge object is still quite obscure.

AGC 4038: Another easy to locate Abell Galaxy Cluster found in the same field as 4.6-magnitude Delta Scultoris. I wrote about this cluster several years back in a Sky & Telescope article (October '99), and despite the southerly declination I was surprised that I could resolve numerous PGC galaxies from Fiddletown. The brightest members include IC 5349, 5350, 5353, 5354 and IC 5358. IC 5353 and IC 5358 are both double galaxy with very faint companions virtually superimposed! In addition we kept picking off numerous PGC galaxies -- I logged 14 total members and I believe Mark noticed a few more.

AGC 2593: This is another Pegasus rich cluster at a distance of 600 million light years. There's only one NGC galaxy here - 7649 - but a slew of tiny anonymous galaxies swarming around. I logged 8 galaxies in the same field, nearly all 10" or smaller using 323x.

AGC 2666: This last Pegasus cluster is a bit closer at 400 million light years and contains one fairly bright galaxy, NGC 7768 as well as NGC 7765, 7766 and 7767. These are all crammed into 5' of sky! Several dim anonymous galaxies are in the same field, bringing the total to at least 11 members.

AGC 76: We moved south to Pisces to view this cluster which contains several IC galaxies -- 1565, 1566, 1568 and 1569 hiding among a field containing a number of relatively bright stars. 7 members were logged in total with the brightest members, IC 1565 and IC 566 easy catches. I'm surprised this cluster was completely missed by both William and John Herschel.

HCG 5: Nearby is this small Hickson group including the double galaxy NGC 190 and two tiny companions to the north and south. (HCG 5C and 5D). I've looked at this group several previous times, but had never picked up 5D before and with a photographic magnitude of 17.3 that's not surprising!

OK, now for the planetaries --

NGC 6742 in Draco is also catalogue as Abell 50. Although it was discovered by Herschel in 1788, Abell missed this when he compiled a list of "new" planetaries based on the recently completed POSS in the late '50's. The view was striking at 323x without a filter. The planetary was slightly elongated E-W, ~30"x25" with a slightly brighter rim giving a weakly annular appearance. A very faint 15th magnitude star was just off the NNE edge and a 16th magnitude star occasionally popped out at the west edge.

Abell 51 is a toughie in Sagittarius. At 115x and OIII filter it appeared very faint, fairly small, round, ~35"-40" diameter, very low surface brightness. I couldn't quite hold this object continuously with averted at 160x using a UHC filter.

One of the stranger appearing planetaries is Sharpless 2-71 a large obscure planetary in Aquila. At 140x and an OIII filter, Sh 2-71 appeared fairly faint, fairly large, elongated at least 3:2 N-S, ~1.6'x1.0'. It appeared sharply defined with a straight border along eastern edge which runs N-S edge. The south side has a lower, irregular surface brightness and this is the faintest section though it appears to extend just as far as the north end. Without a filter the 14th magnitude central star was easy and a fainter star was close north of the central star, appearing to be a double. A trio of mag 10.5-12 stars extend beyond the planetary off the western side.

Aquila is home to a number of NGC planetaries including NGC 6741. I picked it up at 115x by blinking with an OIII filter. It appeared light blue and soft at this magnification, making the identification as a planetary evident. At 323x a small, crisp-edged disc was fairly bright, ~8" diameter. Excellent view at 565x and the planetary appeared slightly elongated ~E-W with a faint, very thin outer envelope with total size of ~10"x8".

NGC 6751 in Aquila is a pretty small object though showed lots of structure at 257x. The central star was easily seen and the rim appeared brighter and the center slightly darker. There also appeared to be a thin outer shell. A mag 14.5 star is close off the east edge and two mag 14/15 stars off the west side. At 435x, the planetary is possibly slightly elongated ~E-W and one or extremely faint stars occasionally sparkled with one possibly on the west edge. The stunning carbon star V Aquilae is in the same low power field 30' NW!

Abell 55 is one of the better Abell planetaries and also resides in Aquila. At 115x and OIII filter it appeared fairly faint, round, fairly small. Just visible without a filter once identified thought does not appear as crisp-edged. Also, 160x and a UHC filter provided a good view. Without a filter a few very faint stars were visible at the edges on the N and NW sides. The surface brightness was pretty uniform and the disc was slightly elongated SW-NE.

NGC 6772 is another relatively bright planetary in Aquila. With the good seeing and transparency, there was no problem using 323x with a UHC filter. The planetary appeared moderately bright, round, fairly large, ~60" diameter. The rim was sharply defined and slightly brighter with a subtly darker center giving a weak annular impression.

I pumped up the power on NGC 6778, a tiny planetary in Aquila. At 115x a fairly bright but small, round disc was visible with a blue- grey color. Excellent view at 435x with an irregular shape, slightly elongated ~E-W, ~25"x20". A brighter knot at the west end gave the appearance of being double or bipolar. At 565x the surface brightness was noticeably irregular and an extremely faint star was occasionally visible at the west edge and also fleeting glimpses of a centrally located star.

At 435x, NGC 6852 appeared moderately bright, fairly small, elongated 3:2 NW-SE, ~25"x18". A mag 14.5 star was close off the SE end. At this magnification the planetary was occasionally bipolar with a well- defined, very small bright knot just NW of the mag 14.5 star and a second less defined knot that forms the NW end. The two knots appeared resolved though I never had a steady view of both simultaneously. Located less that 5' ENE of a mag 7.5 star.

Abell 65 is another one of the better Abells and is found in a rich star field in eastern Sagittarius. At 115x and OIII filter, this relatively bright Abell appeared as a farily large, moderately bright glow elongated glow, extending at least 2:1 NW-SE, ~1.6'x0.7'. A mag 13 star is attached at the SE end and a mag 12 star is off the NW end.

Both nights I closed shop around 3:30 quite satiated from all the delectable views.

Steve Gottlieb