View Full Version : Abell 2147 and Arp 324

2010-Mar-08, 04:36 AM
Arp 324 is a chain of galaxies in a western corner of Hercules right on the border with Serpens Caput. It is located in the corner I call the realm of Abell galaxy clusters. As there's a line of 3 them with Abell 2147 at the south end. 2152 and 2151 are the other two, 2151 is the famous Hercules Galaxy Cluster. In fact Arp 324 is almost at the center of 2147 cluster which is about 475 million light years distant. Arp put Arp 324 in his Group character: Chain of galaxies category. How many he saw in the chain I don't know. Some sources say 6 some 9. Arp's comment on the group: "Diffuse elongation of E's along line joining them." I think he is referring to the two main ellipticals which seem to exist in one diffuse, very elongated halo. Whether this is real or a line of sight illusion I don't know. I found nothing on this question. In any case such elongated halos are a pretty good sign that these halos are tidal in nature. Not easy to pull stars from such massive elliptical galaxies so I assume it was these two interacting that caused these tidal features. The northern galaxy is classed as cD:E+ galaxies which is a rare type of extremely large elliptical galaxy usually found to be anchoring a galaxy cluster, which is exactly what it is doing. It is known as UGC 10143. The smaller southern one is UGC 10143 Notes 2 or MCG +03-41-051 and is classed simply as an E3 galaxy.

I really doubt this is a true galaxy chain but more a line of sight apparent chain of random galaxies in the cluster. Some of which, like the two big ellipticals and III Zw 075 below are interacting pairs.

Between the two main elliptical galaxy is a "cute" ring spiral galaxy with two fine arms coming off the ring. It is UGC 10143 NOTES01 or MCG +03-41-052. It is a puzzle to me. Everyone considers it part of the chain. Here's a typical comment on it: "Compact red object with a faint envelope. This is a component of a chain (connected by a luminous band) of which the compact galaxy III Zw 075 is also a member." By red they mean infrared which I don't pick up. So it is more blue white in my image. The problem is its red shift is considerably greater than that of the others putting it about 600 million light-years from us, 125 million further than the Abell Cluster that contains Arp 324. Sometimes a galaxy can pick up high speed due to the gravitational interaction with its cluster neighbors. That may explain it or it may not be a member of the group. I'm leaning toward the latter. It is quite obviously classed as SAB(r)ab.

III Zw 075 is the pair of elliptical galaxies below the ring spiral that are only 6" apart. I don't know why the note above implied it was a single galaxy. They appear to be a pair of interacting compact dwarf spherical galaxies in the Abell cluster. They seem to exist in a common envelope of stars though this may be an illusion. I find no redshift data on this pair.

At the bottom of the "chain" is a flat edge on spiral. It is UGC 10143 NOTES03 or KUG 1559+160. It is nearly impossible to find a catalog that includes all of these. Several include most however. Like the ring spiral its red shift is a bit out of line with the Abell cluster. In this case it is about 75 million light-years closer by its red shift than Abell 2147. Since most cluster galaxies are elliptical I'm leaning toward this one not being a member as well.

Another spiral like galaxy "in the chain" is 2MASX J16021582+1557252. It is between the upper large elliptical and the ring spiral. It's northern arm seems far bigger and brighter than the southern. It might be two superimposed galaxies though NED and SDSS only list the one at in this immediate area. It, like the other spirals has a red shift that doesn't fit the two main ellipticals for Abell 2147. It is about 100 million miles beyond them based on its red shift.

This covers the basic 6 galaxies in the "chain". While other sources list more I'm unable to identify them with any certainty. Some are listed by their MAC entry. The only listing of this catalog I have is accurate only to about 1 minute of arc in Declination and 15" in RA. Another PGC 56783 is non existent in all of my sources. As best as I can determine at least two of these are referring to the little interacting trio of galaxies above the two major ellipticals and a bit west. From south to north they are SDSS J160214.00+160111.2, SDSS J160213.92+160127.2 and SDSS J160214.72+160127.5. Redshift data shows the first two are members of the Abell 2147 cluster. I have no data on the third member, it appears to be involved with the halo so probably is also a true member of the cluster.

Other nearby likely members of the cluster are SDSS J160211.40+160021.5 down and right of the trio above (the two leftmost galaxies point to it), SDSS J160215.30+160220.1 fainter and above the triplet and beside a dim blue star, SDSS J160217.58+160010.6 a bright elliptical at the top of the halo about the northern main dC elliptical and SDSS J160217.96+160027.9 the faint round galaxy just above the last galaxy. Continuing north along the same line as the last two but further north than the triplet is the rather bright elliptical member of the cluster and maybe Arp's chain, SDSS J160219.55+160209.6. Most of the other ellipticals in the image appear to be members of the cluster as well. Though a few are far beyond the cluster yet are as big and bright as cluster members.

If you've been following my posts I've been noticing the SDSS often missed very blue galaxies. So the extremely blue galaxy west of Arp 324 caught my eye. It seems an exception because it was picked up by the SDSS as SDSS J160154.20+160016.7 and has a red shift that puts it in the Abell cluster. So not all of these super blue galaxies are not picked up.

Down near the bottom left of center is a very weird galaxy with a red shift of 510 million light-years and thus likely a cluster member. It is right beside a rather bright blue star to its east. It is classed as S0 but seems to have a very odd blue ring around it. NED's primary listing for it is GIN 464. Blow up the image to see the ring detail.

Trying to do a typical annotated version of this image is virtually impossible. I'd have labels on top of labels making a mess and taking me far more time than I have available. So I've just pointed out a few objects, not members of Arp 324 or Abell 2147 that caught my eye. Objects like SDSS J160143.48+155706.3, a quasar at 12 billion light years shining at magnitude 21.3. A question mark indicates a galaxy not in NED that I think should be as far dimmer galaxies are included.

For those who have been asking the limiting magnitude of this image isn't as deep as I'd have liked, humidity cut down transparency. It is about magnitude 22 for tiny compact galaxies, 22.3 for stars and about 21.8 for fuzzy galaxies. All these are measured in green light.

Arp's image, with the 48" Schmidt it appears:

SDSS image

14" LX200R @ f/10, L=6x10', RGB=2x10'x3, STL-11000XM, Paramount ME

Attached besides the main image is an annotated image and a 1.5x enlargement.

Full image with less compression:


2010-Mar-08, 06:46 PM
Beautiful alphabet soup of galaxies, Great info on this chain of galaxies also. Clear Skies.
Congrats on the image contribution of ngc4390 (Sn) on UniverseToday.

2010-Mar-09, 01:04 AM
Beautiful shot. I continue to be amazed at the detail and low noise you get with relatively short exposures.

2010-Mar-09, 02:26 AM
When you use 18 micron pixels you do pick up a lot of photons. Color is usually taken using 24 micron pixels when imaging faint galaxy halos. When I go to 1x1 binning like I did for the C179 cluster in M32 then I have to at least double the time in the L channel. Clouds cut me off at 7 rather than 8 so there's a bit more shot noise in that than I'd like. Seeing like that happens once every 5 years so no way to get more data so went with what I had. I am guessing my extremely dark skies also help. I know when I image with a moon which would be similar to being near a city I can't dig nearly as deep without the image getting really grainy. I really need to get far more time in such conditions but rarely do.

Also I've learned a few ways to deal with shot noise that probably would cause the major imagers to cringe but it works for me.


2010-Mar-09, 11:47 PM

When you've got all those pixels to work with, binning is an easy choice!

I'd be interested in the shot noise approach, and no one should cringe about anything that isn't changing the signal.


2010-Mar-10, 05:01 AM
It's not so much the number of pixels as the focal length and seeing that doesn't support it. Since a compressor gives me no additional field of view binning eliminates an optical element and source of reflections.

What I do about shot noise in the background depends on the image. No two are the same. I'd need a book to cover it. Due to binning real detail has little visible shot noise, even the faint plumes, it is the background that has the problem. Adding color makes it worse. So first thing I do is select the stars in the RGB image making sure I got them all. I manually select non stellar objects, again being sure I have it all. Then I invert to select the background and I totally desaturate it. Without color it won't show at all in the LRGB combine no matter how noisy. A lot of the noise in a combined background is due to uneven color from the RGB background, this removes it.

L can be somewhat the same. With masks and color select I select all real detail then invert to select the background only. This can get tricky if there's IFN in the image, you don't want to remove that or faint detached plumes. Lots of bluring of the mask edges is required of course. I usually end up with a dozen layers to make it easier if I screw up and need to go back. Rather than blurring the featureless background I use color select to find the darkest "holes" and raise them compressing their range but leaving some variation, just making it far smaller and thus less noticeable. Exactly how all this is done depends so much on the image I can't begin to cover it. Probably, if I redid an image I'd do it differently as I never seem to remember what exactly I did with an image. I tried making actions for basic functions but soon found they didn't exist so it was not possible.

After creating the LRGB the background will be totally neutral grey though with some slight intensity differences. I use the final background mask and give it a bit of a tint. What that is depends on the image. Sometimes neutral works but more often it gets a very slight bluish tint. Doesn't take much, go gentle here. That seems to hide any remaining background problems.


2010-Mar-10, 05:05 PM
Thanks Rick, your hand-crafted noise reduction approach is quite effective.

2010-Mar-13, 01:12 PM
I concur with Andy, Great info and tips on your methods, Thanks for sharring and Clear Skies.