Abell galaxy clusters

[Martin Meredith]

I don't think we yet have a dedicated Abell galaxy clusters thread, so please feel free to contribute observations here. 

In total, there are 5250 such clusters, including some 1174 that are the 'Southern Abells' with declinations lower than -16, so more than enough for a single observing season 😉

These clusters were catalogued by Abell, Corwin and Olowin (and are nowadays often referred to with the designation ACO) and have a wide spectrum of visual appearances. They are mainly very faint -- about 90% have nearly all of their member galaxies fainter than mag 16, but that for me is part of the challenge and excitement of observing them with EEVA techniques. There is something very special about a very rich galaxy group which occupies a compact region of the view and is teeming with faint objects. Add to that the uncertainty of the group's appearance, and indeed whether it will appear at all, and I think a good evening's viewing can be had with these groups.

Most ACOs are, if not exactly uncharted territory, barely make it on to the deepest charts, and I imagine very rarely observed at all. I used to pass quickly by when scanning the charts for interesting objects, but now when I see a completely empty circle enclosing a supposed ACO group, I often stop to take a look (though only if I have at least 10-15 minutes to spare).

I'm going to start this thread with an observation from last night. It was meant to be clear but the promised clear skies were actually plagued with scudding low clouds throughout the brief session (I also had a plane fly right through one sub). In the end there were more frames that were completely clouded than clear (my darks library benefited from the initial waiting around...). In a moonlit night under such conditions I would normally have called it a day, but I perservered as I was interested in testing some aspects of calibration in Jocular. Surprisingly, even though the overall captures took longer than usual, it was (just about) feasible to do some observing under such conditions, and I think speaks to the value of the EEVA approach. 

I observed 4 ACOs in all but I'm just posting one, and I think at first glance it will appear like the winner of the under-whelming object of the year. This is ACO 2397 in Aquarius, a few arcminutes away from NGC 7164. This is the result of nearly 9 minutes observation with the clouded out subs ruthlessly culled from the stack. In this annotated view the cluster is represented by the solid ring, but I wonder whether the galaxies marked at the right also belong to the cluster?

 

 

How many galaxies are present in the cluster? The listed member count is 146 but this number needs to be interpreted with care as it corresponds to the count of cluster members with magnitudes up to 2 magnitudes fainter that of the third brightest member (so in this case roughly in the range 18-20). Hence the actual member count is higher (potentially much higher). 

Abell et al also report the magnitude of the tenth brightest galaxy. In this case it is 17.9. The group has a redshift of 0.224, a richness of 3 (scale 0-5, where 5 is the richest) and a distance class of 6 (scale 0-7, where 7 is the most distant). I calculate that the distance is somewhere between 2 and 2.7 billion light years.

An image of this cluster (N down) appears as plate 27 of article [1], where some of these galaxies can be ID'd (so are not just patches of noise!), and table 13 of that article provides galaxy photometry for the cluster. This shows that the brightest galaxy is a dizzingly intense m17.68 (marked on my shot) and there appear to be over 530 members down to around mag 23.

Individual subs show nothing at all, and it takes quite a few subs before the first members can be confidently identified. But that is part of the pleasure of observing faint ACOs.

[1] http://articles.adsabs.harvard.edu/pdf/1983ApJS...52..183B

Of course, there are much brighter ACOs out there such as 539 in Orion, 2151 in Hercules and 426 in Perseus, which are much closer and spectacular in a different way.

cheers

Martin
 

 

Edited September 23, 2020 by Martin Meredith

[Martin Meredith]

I've been checking some of my earlier ACO captures and found this one from a year ago. ACO 2666 in Pegasus is at the other distance extreme -- distance class I, with 34 members within 2 mags of the 3rd brightest. It provides a nice illustration of the wide variety within this catalogue. This group is at about 390 million LYs.

The brightest galaxy is the (inevitable) elliptical (mag 13.3 NGC 7768), but it is surrounded by some gorgeous edge-ons (from top to bottom, mag 16.8 type Sc, mag 15.5 type Sbc, mag 16.3 NGC 7766, ending at the bright NGC 7767, mag 14.5 type S0-a.  The edges of the field contain some further edge-ons, all at a similar distance. Perhaps the most interesting galaxy here is the near face-on Sc type at about 1 o'clock. This alone might be worth a closer look. The 'ring' appears quite knotty.

The bright star to the L of the central elliptical is GR Peg which varies from mag 10.4-11 about 5 times a year. 

[glafnazur]

Great stuff Martin, I look forward to seeing more of these. 👍

[Bill S]

Abell galaxy clusters - plenty to go at. I looked at a couple the other night and I must have another look at them but they were rather 'spot the galaxies'. (Abell 347 in Andromeda and 349 in Triangulum.) I've been looking at them again and realise that the galaxies are rather small. Like Martin I decided to look what I'd got from the past. This one, Abell 2151 in Hercules, seems to have quite lot to it. Otherwise known as the Hercules Galaxy Cluster. It's got up to 200 galaxies in it and is around 500 million light years away.

 

I could not resist running a spot of plate-solving and annotation to see what's what.

 

 

Or if that's not enough using HyperLeda (and ASTAP) we get:

 

 

Good luck unravelling that.

Alvin Huey's guide is a great resource for these galaxy clusters:

http://www.faintfuzzies.com/Files/AbellGalaxyClusters v1.pdf

Cheers

 

[Mike JW]

Abell 2937. I find this area very confusing.

Using the link in VizieR the centre of the cluster is put where the tiny red square is located (in the middle, just above the bright star.)

Using Aladin the cluster centre is shown where the cursor is and given as two objects superimposed on each other with the info at the cursor given as a galaxy cluster

 

Abell 2666 - much easier to sort and a very interesting fov.

Thanks for posting.

Mike

[Mike JW]

Bill - using your scope at f2.5 is a real winner - great wide view. I have tried reaching focus on my Dob with a 0.5 reducer on the camera but even with an adapter I had no luck at getting an image. Mike

[Bill S]

Mike - Because of the spacing to the reducer I'm not getting a true 0.5x reduction. I reckon the focal ratio is about 3.3.

You say you could not get an image. What sort of adapter were you using? A low profile 2 inch to 1.25 inch? Or are you referring to the C/CS thread to 1.25 inch filter thread adapter so you can attach the focal reducer?

With a focal reducer you usually need to focus inwards.

A big advantage of cameras such as the Lodestar and Ultrastar is their 1.25 inch diameter body will go well into the focuser. I have a parfocalising ring on my Lodestar so I don't push it in too far (and it allows me to set the camera in at the same point each time I set up).

Cheers

Bill

 

 

[Martin Meredith]

Fantastic image Bill. You could spend a good hour exploring the different galaxy types in that cluster. 

I didn't spot the reducer until Mike pointed it out. That's a great idea and seems to have no noticeable distortions. You must have it well collimated!

Thanks for the additional info Mike. Abell 2937 is indeed confusing position-wise. The article I link to shows two great clumps of galaxies. I've rotated one of their north-down plates to match mine (which is on the left in case not obvious!) centred where my solid circle is. It is hard to know how far out the cluster extends -- and I suppose at these distances incorporating the other ellipses I've marked might mean this has a too-large extent.  On the other hand, many Abell clusters are part of much larger structures. See https://cdsarc.u-strasbg.fr/viz-bin/Cat?J/MNRAS/445/4073 for a catalogue of these (Bill, you'd need a 0.1 reducer to fit some of these on the Lodestar 🙂

[Mike JW]

Martin - thanks for the info.

Bill - the reducer adapter is http://www.scopestuff.com/ss_npea.htm. What spacer do you use because that might be a smart move on the 15. Converting the f4.5 to f2.25 is a bit sever even for the quality optics of my mirror but something around f3 or f3.5 might just work.

Mike

[Bill S]

Mike - the adaptor you give link to looks good to me. That should not stop you pushing the Ultrastar into the focuser.

One thing to note with the c to 1.25 inch adaptor (needed to allow you to add the reducer) is that you want one with no shoulder on it. A shoulder will stop you pushing the camera into the focuser tube.

So a plain one like this:

 

Not one with a shoulder like this:

 

 

The adaptor I use to attach the reducer to my Lodestar is ~14 mm.

I do have an even shorter one but I do not normally use that.

I hope this is useful.

Cheers

Bill

 

 

[Mike JW]

Thanks Bill. Mike

[Martin Meredith]

Here's the brighter (by far) of two Abell clusters I observed a couple of nights ago. Replaying this now in Jocular as an animation, I can see light clouds passing through every sub, but the averaging effect meant I could get an image without realising (until now) that this was happening -- the power of EEA! The second cluster is less than a degree away but is distance class 6 and while I was able to spot about a dozen tiny galaxies, the cloud had thickened up by this time. In fact, there are 4 Abell clusters within1.5 degrees of each other with distance classes of 3, 5, 6 and 6, so a good hours could be spent here on a clearer night without really moving the scope. 

Abell 71 presents an interesting composition because it contains mag 4.3 eps And (164 light years away) and a triplet of constrasting NGC galaxies around 260 million light years distant, so  well over a million times further out than eps And. I always find these conjunctions mind-boggling. The mag 13.8 elliptical is NGC 183 and above it are a pair of edge-on-ish spirals, mag 15.6 type Sab NGC 184 and the well-matched mag 15.7 So-a type NGC 181.

Although it seems somehow unlikely, this  triplet may well be part of the Abell cluster itself based on distance estimates. The two lines I've marked represent more or less the diameter of Abell 71 (in the long direction). What I find appealing is that this is an unusual quasi-linear grouping of the clusters brightest members. It takes a bit of scrutiny, but this is a chain of galaxies, mainly with magnitudes around 17.0, some fainter, a couple brighter.  The unusual structure is clearer on the chart (after mental rotation). It is hard to see where such a group has a centre.

I read in a very interesting 1985 article [1] entitled A POSSIBLE 300 MEGAPARSEC FILAMENT OF CLUSTERS OF GALAXIES IN PERSEUS-PEGASUS that Abell 71 might serve as a link between the Pisces-Cetus supercluster and Perseus-Pisces supercluster. Its distance and location is right for this. I imagine there has been more recent work to confirm or refute the idea [edit: yes, it is even mentioned in Wikipedia! https://en.wikipedia.org/wiki/Perseus–Pegasus_Filament -- a filament a billion light years long].

I have no idea whether the 'strung-out' ness of these galaxies is related to it being part of such a filament, or accidental. In either case, it is for me part of the charm of EEA to spot something potential interesting and then later find that it is indeed a fascinating part of our universe.

cheers

Martin

[1] http://articles.adsabs.harvard.edu/pdf/1985ApJ...299....5B

Edited October 7, 2020 by Martin Meredith

[Mike JW]

Hi Martin,

When looking at NGC 183 visually little did I realise what lurked near by. I saw the chain straight away and great to ponder if the galaxies lie within a filament connecting the super-clusters. I have looked at the article you mention - page 9 - lower diagram would suggest this is not the case from the RA/DEC data? but looking at other articles then it could well be. As you say the distance is in the right region.

Apparently the Perseus -Pegasus filament you mention lies next to the Pisces - Cetus filament (1 billion x 150 million lyrs) and includes the Virgo super-cluster.

Of course the reality gets very complex - this article looks at the Perseus end -https://iopscience.iop.org/article/10.1086/301196/fulltext/ - and see image below showing the main cluster filament and also various minor filaments. The dots are galaxy plots.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

http://egg.astro.cornell.edu/alfalfa/ugradteam/uat16talks/uat16_introAPPSS.pdf - a very interesting power point which includes references to  the filament.

Mike

[callisto]

The thought of how far these galaxies are blows my mind 😮..............Love it.

 

Mark

[AKB]

Before the freezing fog really hit last night, I had an eclectic romp around the eastern sky using C9.25 / Hyperstar / Atik 460EX with about 1.3º FOV (and 1.75" pixels.)

In this category, I observed Abell 426 (Perseus A), one of the best known (and, no doubt, imaged) galaxy groups.  Also, taken together, one of the most massive observable objects.  There's really too much in here to talk about, but from an observational point of view, colour would have have been a plus!  Just to help out a bit, I've included an annotated image. 

Of particular interest, though, is NGC 1275.  It's a strong radio source, and actually two galaxies converging at a rate of 3000 km/s.  But there is some way to go before they merge, since they're apparently ~200,000 ly apart.

Despite the date on the image, it was actually captured at 20:20 on 22-Nov-2020.

 

[Mike JW]

It has been a while since we had a Abell posting. Recently I visited Abell 568 in Gemini. It would appear to be a bit short of galaxies.

This shot is at the centre of 568.                                                                                This  is the zoomed in shot, showing the triangle of galaxies - around the 1 billion lyrs away.

                                                                                                                                   There are other  possible candidates but NED does not label them

[icpn]

If you have not already found it this is a good guide http://www.faintfuzzies.com/Files/AbellGalaxyClusters v1.pdf

 

 

[Martin Meredith]

I couldn't resist a look at Abell 2065 in Corona Borealis last night. This has the reputation of being a challenge for visual observers with the brightest galaxy at around about mag 16, but for EEVA the full beauty of this dense cluster is more easily attained. With a listed diameter of about 25 arcmins, this fits nicely on to the Lodestar. Apparently this cluster contains ~400 galaxies and lies at around 1100 million light years distance.

Apologies for the coma -- I spotted that my secondary needed the merest tweak and I seem to have ended up with it right out of kilter, secondary adjustment being the beast that it is, at least on my Newt. The galaxies are pretty forgiving though.

Martin

[Mike JW]

I have just had a look at the objects in CRB having been inspired  by your Abell 2065. Plenty to track down.

Mike

[Mike JW]

Last night my chance came to take a look at CRB with the 15. I actually looked at 7 Abell clusters.

Abell 2065 - galaxies everywhere. Shot below to complement Martin's view.

and here is a zoomed in view of the central part. Two of the objects indicated by the red lines are about mag 20. I suspect all the fuzz spots are galaxies.

Forget the label that says ASI 174, it was the Ultrastar (not sure why that label has appeared)

Mike

[Martin Meredith]

Here's Abell 2510 in Pisces. This is not one of those impactful galaxy clusters -- the galaxies are very faint and understated, and dotted around the field. This is a distance class 6 cluster and I'm often intrigued when there is nothing marked on the charts to see if I can pick up any galaxies.

I've marked some of the galaxies for which I have data and which are just about visible in the image, but I find it difficult to believe the data. The galaxies are to the  lower left of the label in each case. From top to bottom we have

5186886 4104 MLyrs, mag 20.3

5186892 2373 MLyrs, mag 19.5

5186887 4200 MLyrs, mag 20.8

3576831 1603 MLyrs, mag 19.4

5184351 5726 MLyrs, mag 21.8

3334273 2738 MLyrs, mag 18.7

These are all B filter magnitudes.

I'm pretty sure there's no way I can detect a mag 21.8 (or even a mag 20.8) with my kit, even with 8m of exposure! The distances marked show an incredible spread too. It would be interesting to see more shots of this and to try to get alternative distance/mag estimates...

[Mike JW]

I tend to agree about questioning if you are picking up mag 20 galaxies. My understanding is once you go beyond mag 18 the data gets less reliable. I have just cross referenced the fuzz spots to the Aladin/NED info and they coincide with galaxies and the magnitudes given (more or less), the 21.8 is given as 21.4!

Mike

[Martin Meredith]

Thanks for checking on NED. The use of different filters for the reported magnitudes can often explain differences like these, but not in this case. Now, your behemoth might be able to pick up mag 21....

I'll repeat the exercise on another cluster (hoping for a couple of clear nights tomorrow and Friday).

Martin

[Mike JW]

Hi Martin,

Below is my effort from last night. I went for 8 minutes to match your time. Picking up the same fuzz spots as your shot and I have added in some quasers as well, including a mag 21.8 - questionable!!! At the top of the fov are lots of fuzz spots which I assume are actually galaxies.

 

Mike

[Martin Meredith]

Maybe this part of the sky contains some kind of magnitude booster... Great to see some z > 3 quasars. Coincidentally I'd just added some z ~ 4.5 quasars in Pegasus to my target list for this evening.

Martin