Abell Galaxy Cluster 2218

[Martin Meredith]

A couple of nights ago having completed some upgrades to the Quattro (flocking the interior of the light shield, adding a fan, light-shielding the primary, adding a Catseye hotspot to the primary to make collimation even simpler) I decided to try it out on something rather distant and faint: the Abell galaxy cluster AGC 2218 in Draco. 

When I first started getting interested in Abell galaxy clusters not so long ago I reckoned that perhaps a few dozen of the 2700+ clusters observable in the Northern hemisphere might be in reach.  Later, when I plotted the locations of the AGCs on all all-sky map recently I limited it to those whose 10th brightest member (m10) was at least magnitude 16 (some 232 in total), so AGC 2218 didn't even appear. But seeing this very distant cluster on http://www.faintfuzzies.com raised the challenge: is it possible to enjoy a meaningful look at a 2 billion light-years distant cluster during a brief-ish sensor-assisted observing session?

Conditions were reasonable once the moon had set: 78% hum, 12C, SQM 20.25 (NELM = 5.9). All shots are tracked in alt-az. 

First, as a warm up I revisited AGC 2199 in Hercules before it sank too low. Here's a mean-stack of 8x45s, more or less centred on NGC6166. This cluster contains 88 galaxies in a field of around 1.5 deg diameter, so we're just seeing the central 50% or so here. This cluster is fairly close as AGCs go, at 'only' 417 million light years, and correspondingly bright, with an m10 of 13.9 , 11.8 visual.

 

 

Now for the main target, Abell 2218. This is substantially fainter, richer and visually more compact: it has an m10 of 17.7 and contains over 200 galaxies in a 9' field (the screenshot represents approx 26' x 20'). Fortunately the field of AGC 2218 is quite easy to find at only a short star-hop S of NGC6214 and can be spotted centred a little to the south-west of the brightest galaxy UCGA 413 (shaped like an integral sign). The cluster has the appearance of a bunch of Scottish midges as seen through a tent wall but is a considerably more welcome sight.   

 

 

I spent a fair amount of time examining this field, comparing it to map sources and the DSS plates, but above all wondering what life (*) is like on those distant shores and how it has changed since these photons started their journey.

 

I will now revise my estimate of how many AGCs are within reach of modest scopes (particularly for those with even more sensitive cameras)  . There are a hundred or so more distant Abell groups than this one too. At 5+ billion lys Abell 851 in Uma and Abell 370 in Cetus are the most distant that I've come across. Targets for another night perhaps…

 

Thanks for reading

 

Martin

 

(*) inevitable, surely 

 

[Martin Meredith]

I had a quick go last night at a slightly deeper Abell cluster, AGC 2125, which is at a distance of around 2.8-2.9 billion light years (redshift 0.2465). This cluster literally straddles the border between UMi and Draco and is near PGC2686703 but I star-hopped from SAO16799, a couple of fields away.

Unfortunately, the clouds were coming and going and this is the best I could do by stacking and rejecting in between. SQM 19.8 4C 86% humidity.

Checking on Aladin I'm picking up at least half a dozen galaxies in the inner region but it is difficult to tell how many more are there -- even the DSS plates don't show much. I can only find magnitudes for two members. This is one to revisit under better conditions.

Zoomed (2.5x) and rotated with N up.

Whether this counts as a 'meaningful view' is open to interpretation but I enjoyed tracking it down and sitting out the clouds in a perverse sort of way...

Next target is AGC 1722 in UMi at redshift of 0.328 

cheers

Martin

[Martin Meredith]

On Friday I was aiming to take a further step out to 3.4-3.7 BLYs (redshift 0.328) in search of Abell 1722 in UMi, the 5th most distant Abell cluster. Conditions were moderately good (SQM 20.1, 67% humidity) to start, but clouds were coming and going so again it was a case of stacking and rejecting (the hot pixel trails tell the story). I hopped across from NGC 5144 which is about 1 FOV (around 0.5 deg) away. The cluster was around 29 deg above the horizon.

This is a sum stack of 16 x 45s before the clouds became too much of a pain. 

Here's a 4x zoomed and rotated image of that region with a tentative marking of possible cluster members. The brightest member is mag 18.2 [red] and there are 86 members in total, although only a few are visible here.

Here's is a great image which I used to identify members.

Final instalment to come...

Martin

[Martin Meredith]

Is the most distant Abell cluster, AGC 370 in Cetus, detectable in near-live mode? This group is altogether more challenging, sitting at around 4.7-5 billion LYs (redshift 0.375). On Saturday I had the opportunity to find out. Conditions were not so great, with gusty wind and some local light pollution in the southerly direction (SQM reading of 19.5 = NELM 5.1; humidity 87%).

At around 9pm point the cluster was at around 36 deg. It is located very close to PGC 175370, about 1 deg away from NGC 1015.

Unfortunately, wind gusts strong enough to blow the light shield off stopped play before I could do very much but I did manage to observe while LodestarLive built up a stack of 8 x 45s. This is the result.

Here's a 4x zoomed and rotated version of the central region. A DSS image of the same portion is shown for comparison. See here for the Hubble version  .

By comparison with the DSS image there are two clear candidate members visible as marked. The upper object is ACO 370 13 while the lower slightly curved object has mag 20.4  according to Aladin so it is likely that I'm picking up the combined light of several close members since I doubt I can reach that mag with this scope in short exposures. There's a suspicion of other members too in this shot.

 

Martin

 

 

 

[Martin Meredith]

Just to wrap up for now...

My aim has been to determine how many of the Abell clusters are within reach of an 8" f/4 scope with the Lodestar-C camera in near-live mode. In part I wanted to have a realistic limit on the number to plot on an all-sky planning map, but really I just wanted an enjoyable challenge, and it has certainly been that.

The Abell clusters are interesting 'objects' as they cover a large distance range and hence also vary in brightness and apparent field width, so are suitable for all sorts of scopes. For the closest clusters a wide-field scope/camera combination is needed to pick up all the members. At around 0.4 billion LYs clusters such as Abell 2199 not only display multiple members but also show some of their morphology. For me the views of Abell 2218 at 2 BLYs were also better-than-expected. Seeing a multitude of specks at such a distance is rewarding. Going a little deeper (2.8-3.7 BLYs) for Abell 2125 and 1722 produced impoverished views and for the most distant cluster at nearly 5 BLYs we're definitely in faint fuzzy territory!  But it seems fair to say that no Abell cluster is off-limits for moderate scopes with sensitive cameras in short exposures.

Having obtained these initial images it will be really interesting in the future to see the effect of varying observing conditions (or equipment).  I aimed for moonless skies but otherwise conditions were not perfect (e.g., SQM never exceeded 20.25). How much impact would travelling to a SQM 21+ site have? More resolution would be good too but looking at the DSS image of even the deepest cluster I think more can be teased out at my current focal length. And of course I'd expect the more sensitive Lodestar-M2 to pick up fainter members. I wonder what kit would be needed to image the gravitationally-lensed arcs in some of these clusters (notably AGC 370)?

cheers

Martin

[laser_jock99]

Good stuff- I really must hook my X2 to the 12" F2.9 scope in it's dark sky site one night!  

[Martin Meredith]

Thanks LaserJock. Please do post your results if and when you get the chance. That beast should have no trouble picking out a few more details 

Martin