Object of the Week, June 1, 2014 - AGC 2065, the Corona Borealis Galaxy Cluster

[Howard B]

Galaxy Cluster
Corona Borealis
RA 15:22:50 (approximate center of the group)
DEC +27:43:00

Size:
Magnitude: 15.5 to as faint as you can go…

AGC 2065, the Corona Borealis Galaxy Cluster, is well known for being about 1 billion light years distant and consisting of myriads of closely spaced and faint galaxies. A few of the individual galaxies are magnitude 15.5 or so, but most are fainter so the cluster presents challenges no matter what size scope an observer points here.

Edwin Hubble discovered this cluster in the 1930’s and one of its galaxy members, PGC 54876, was used by Milton Humason in 1936 to help determine the famous distance/velocity relationship that shows the universe is expanding. PGC 54876 is one of the brighter members of AGC 2065, so if you can see any of the individual galaxies in this group, this should one of them.

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George Abell came along in the 1950’s and included the Corona Borealis Galaxy Cluster in his catalog of Rich Cluster of Galaxies, with AGC 2065 being entry 13.

More recently, AGC 2065 has been shown to be gravitationally bound to several other galaxies clusters that will coalesce into a single group in a few billion years.

I first saw AGC 2065, or at least a few of its members with my old 20 inch f5 scope in the 1990’s, but it wasn’t until last summer that I got around to pointing my 28 inch scope in its direction. Already well down in the west I still got a spectacular view of bunches of faint galaxies:

“Abell 2065 is the very definition of “lumpy darkness”. When I first found the field of view I could see a mottled texture more than individual galaxies, but 408x and determined averted vision showed me the individual galaxies shown in my sketch. But even so, the size and orientation is uncertain because there are so many galaxies here that are so faint. 21.61 SQM” (21.85 the following night).

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The first portion of the cluster to catch my attention was the three galaxies grouped together near the center of the cluster, PGC’s 54880, 54881 and 54883. Being far enough away from the two bright-enough-to-be-irritating field stars helps, as does the tight and distinctive triangle they make. I found this a good place to start my galaxy hops through the rest of the cluster.

There’s something of an unofficial and friendly competition to see who can detect the most galaxies visually in AGC 2065. Regardless of the winning score so far, all I know is that I have a long way to go to catch up!

"GIVE IT A GO AND LET US KNOW"

[Uwe Glahn]

Cool OOTW Howard,

first contact years ago was, when a friend of mine tested the transparency with the visibility of individual galaxies in his 16". When the sky was very good, he pick up 4-5 galaxies without problems.

Last year I took some hours to see what is possible with my 27" under fairly good but not perfect skies. I remember a hard observation to work out all possible galaxies. Many of them were extremely faint and I lost the orientation a few times. Unfortunately I had to overdraw the sketch or better the brightening of the members a lot to distinguish the different mags. In fact - all galaxies even with 27" were pretty tough but it was a lot of fun to discovery the Corona Cluster.

27", 586x, NELM 6m5+, Seeing good
http://www.deepsky-visuell.de/Zeichnungen/Abell2065.jpg
website with inverted version (http://www.deepsky-visuell.de/Zeichnungen/Abell2065.htm)

[FaintFuzzies]

Great object Howard!

I coped the below from another thread.


Abell 2065 is one of the most distant galaxy clusters visible in large amateur telescopes. This is a very tough group, but fun to work on.

20 galaxies were seen mostly with the 7 and 5mm TMB Supermonocentrics. I used a 7.5mm Tak LE to reorient myself from time to time. I used a 22" reflector at Lassen Peak parking lot. NELM was 7.5 or 7.6 at best.

Three galaxies are obviously brighter than the others.

I’ve labeled the 22 detected galaxies. There are two galaxies that I could not determine the destination of. They are marked as “?”

PGC 54883 and PGC 54881 forms a nice double galaxy separated by a mere 20”. Both are considerably faint and round.
The other bright galaxy, MAC 1522+2747C, is a considerably faint 3:1 elongated even surface brightness patch. PA = 60 and about 15” long. This is located way up on the north edge of the cluster. Actually this was the first galaxy I detected while centering the cluster on my eyepiece.

Starting with the western bright star, the one at mag 10.8.
There are five galaxies in its immediate environs that I spotted. All are very small and very to extremely faint.
PGC 54869
MAC 1522+2742A
PGC 54867
MCG+5-36-17
MAC 1522+2741D

Working my way to the mag 12.9 star to the SW. I spotted only one additional galaxy, MAC 1522+2739B as an extremely faint round glow.

Back to the original bright mag 10.8 star, and working my way to the east, I see a lot of galaxies.
MCG+5-36-20 – is a very faint round patch. 1.4’ east of the star.
PGC 54883 and PGC 54881 is a pair of considerably faint and round. Almost merged and separated by 20”. A nearly equilateral triangle of galaxies due south of the pair of PGC’s are detected as three extremely faint round smudges. They are MCG+5-36-21, PGC 54878, MAC 1522+2741G

MCG+5-36-20 lies between the pair of PGC’s and the 10.8 mag star as a very faint, very small round patch.

MCG+5-36-19 is a very faint round patch lying about 1.5’ north of MCG+5-36-20

MCG+5-36-22 lies very close to a ~16 mag star, as an extremely faint fuzzy star. I almost missed it.
MAC 1522+2739C – extremely faint, very small round patch
MCG+5-36-24 – extremely faint, very small round patch
MCG+5-36-23 – very faint, very small round patch
MAC 1522+2742D – extremely faint, very small round patch

Two questionable fuzzy spots, labelled as "?", were detected, but not labeled as galaxies in Megastar. I’ll need to research it more on the internet. They are located…
1 – just 17” SW of a mag 15 star, which lies 2.2’ WSW of the northern mag 10.5 star.
2 – forms a nice double with a mag 16 star. The pair is located 2.2’ SSE of the mag 10.5 star

Edit: This was from my original write-up. I've done a quick NED check and both are indeed galaxies.


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[Jimi Lowrey]

Howard your OOTW inspired me to complete a project I started to see how many galaxies I could see in ACO 2065. I had already made finder charts for this project last year.

Tonight with not the best seeing I was able to check of my finder chart 58 galaxies on the south side of the cluster. I am sure I will be able with better seeing to see many more. I will keep you posted on my progress.

[Atlas]

I have observed Abell 2065 last year under good skies (NELM ~ 6.5) in southwestern Germany. At 350x I could make out ca. 25 galaxies, three of them with direct vision, the others indirectly and intermittently. I have marked them with circles in the attached DSS printout.

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This year I have also tried Abell 2005 and Abell 2079. They lie roughly at the same distance as Abell 2065 and could be physically connected. Under mediocre conditions I could see only 2 galaxies in Abell 2005. Abell 2079 was even harder with the bright star beta CrB in the vicinity.

Greetings
Johannes

[akarsh]

Wow! Jimi, you've already done everything that you said you wanted to try during TSP (the UCD in M60, counting galaxies in the Corona cluster).

Anyway, this has become one of my favourite objects, ever since the moment I saw it in Jimi's 48". There were galaxies popping in and out all over the field! It was very easy to resolve galaxies in Jimi Lowrey's 48", but this wonderful view motivated me to try it in the 18".

I concur with Howard, with the notion of “lumpy darkness”. The first time I attempted this cluster was at TSP 2012 with my 18" (my eyepiece collection and understanding of visual observing has improved quite a bit since then). There, I noticed nothing at first, but then, I noticed that the background was lit with a dim, mottly light. It was a hair-raising experience. I could just about resolve the central mass back then.

Picking up motivation from the 48" view of the cluster, on the last night of TSP, Santhosh and I spent two hours on this object. Unfortunately, it was about 60 degrees high when we started, so it was a bit more than 30 degrees high when we were done -- bad, but still we were able to study it.

With careful and intense averted vision, I managed to pick up about 5 galaxies. I wish I had a sketch, but I don't. I'm quite certain I'll return to this object many more times and try to give it a more thorough look.

Clear skies

Regards
Akarsh

[akarsh]

Jimi and I had an excellent view of this cluster through my (finally recoated) 18" from the excellent skies of the Warner Mountains, California. We did not have an SQM, but it was clear to both of us that this had to be an exceptional sky. Despite the summer milky way and Jupiter being high up, the surroundings were dark. Here is my report on identifying 10 galaxies through my 18":

We observed this cluster after culmination, at about 60 degree altitude (airmass ~1.1). Under these conditions, Jimi and I had no trouble seeing the cluster -- it might be better described as "Lumpy Brightness" than as lumpy darkness! There were galaxies popping in-and-out of view as is typical in larger aperture scopes, but the 4 brightest galaxies could be held intermittently for longer durations with averted vision (est. 70~80% holding if one tried to). Using a correctly-oriented POSS I image from MegaStar as reference, Jimi counted 10 galaxies and I too identified 10 galaxies. I initially thought I had 12, but two of the objects I thought were galaxies turned out to be stars. Translating from my rough sketch of the position of various galaxy cores, here is a list of galaxies identified. Most V magnitudes come from averaging the 'g' and 'r' magnitudes from Sloan photometry.

The central brightest 4 galaxies:

PGC 54888, PGC 54883: The cD galaxies of the cluster. It was easy to see them as one blob, but it took some extra effort to resolve them into two cores. (Sloan puts these at V ~ 15.9, 15.7)
PGC 54876 (V ~ 15.5)
PGC 54884 (V ~ 16.2)

The rest:

PGC 54869 and SDSS J152218.16+274259.0: I did not resolve this pair, but the combined light from the pair was seen as an elongated glow.
PGC 54867 (V ~ 16.3)
PGC 54870 (V ~ 16.0)
PGC 54875 (V ~ 16.0)
PGC 1815633 (V ~ 16.9). The visual mag of this makes me wonder if I was seeing the combined light from the tight grouping.
PGC 54891 (V ~ 15.7): Possibly seeing the combined light from the neighboring edge-on and this galaxy.

Double star:
PGC 1816284: Turns out this is a tight double-star, per the SDSS imagery. SIMBAD has it marked as a galaxy, and it also has a LEDA designation!

Clear Skies!
Akarsh