*** Abell 1656 ***

A1656.jpg (56634 bytes)

Figure 1 - A dense collection of galaxies near the  center of A1656.  The two brightest galaxies in this image are NGC4889 and NGC4874. Image from the STScI Digitized Sky Survey. Width is 15' and North is up.

Observing Challenge - 400+ Galaxies in the Coma Cluster, Abell 1656

Albert Highe

For those who want to jump straight to the observing list, click here.

Download the finder chart I've created for the central 1º X 1º field. 


Uranometria 2000.0 shows a close up plot of A1656 on chart A8. A dashed circle with a radius of approximately 2º indicates the location and size. As with the other studies of galaxy clusters, I tried to answer three questions. 

  1. Do all the observable galaxies within the circle belong to the cluster? 
  2. Are there observable cluster members that are not plotted in Uranometria? 
  3. Do cluster members lie outside the circle?

Sourcing the Data

To answer those three questions, I searched the academic literature for information on the Coma Cluster. Cluster members have been studied extensively and quite a lot is known about their precise coordinates, radial velocities, magnitudes, and morphologies. The two papers I found most helpful are references 2 and 3 listed in Table I. 

Reference 2 is the most extensive survey I found, containing 6724 objects down to approximately magnitude 21 within a 2.63º square area centered on the Coma Cluster. Unfortunately, few of the objects are identified, cluster members are not distinguished from non members, and cluster members lie outside the surveyed area. 

Reference 3 presents an excellent analysis of which galaxies are likely gravitationally bound to the cluster based on distance from the cluster center and radial velocity. It uses data from Tifft and Gregory (1976). Within a radius of 3º, the sample is complete to mag 15.7, and within a radius of 6º, the sample is complete to mag 15.0. Unfortunately, the data isn't sufficient to create an observing list since many fainter galaxies are visible in moderately large amateur scopes.

Reference 6 is a more recent survey listing 450 galaxies down to mag 16 within a 9.8º square area centered on the Coma Cluster. This article contains an outstanding summary of much of the desired information for each galaxy: identification from multiple catalogs, coordinates, radial velocity, magnitude, size, orientation, etc. Unfortunately, it only goes to mag 16 and includes a large number of galaxies from the surrounding Coma Supercluster.

No source provided the data I wanted - within a radius of 5º, a list of galaxies down to approximately mag 17.5 whose identities, coordinates, and radial velocities were know. I generated multiple lists using a variety of parameters in "Near Name" searches on the NED web site. The volume of data is large - over 1200 objects. 

Table 1 - Selected references

1 Tirion, W.; Rappaport, B.; Remaklus, W., Uranometria 2000.0 Deep Sky Atlas, Vol 1, Second Edition, Willmann-Bell Inc. (2001).
2 Godwin, J. G.; Metcalfe, N.; Peach, J. V., The Coma Cluster - I. A Catalogue of Magnitudes, Colours, Ellipticities and Position Angles for 6724 galaxies in the field of the Coma cluster, Royal Astrononomy Society Montly Notices, vol. 202, p. 113 (1983).
3 Kent, S. M.; Gunn, J. E., The Dynamics of Rich Clusters of Galaxies. I - The Coma Cluster, Astronomical Journal, vol. 87, pp. 945-971 (1982).
4 Tifft, W. G.; Gregory, S. A., Direct observations of the large-scale distribution of galaxies, Astrophysical Journal, vol. 205, pp. 696-708, (1976).
5 Gregory, S. A.; Tifft, W. G., Gross Optical Properties of the Coma Cluster, Astrophysical Journal, vol. 206, pp. 934-938, (1976).
6 Doi, M.; Fukugita, M.; Okamura, S.; Tarusawa K., Automated Surface Photometry for the Coma Cluster Galaxies: The Catalog, Astrophys. J. Suppl. Ser. 97, 77 (1995).
7 Gregory, S.A. and Thompson, L.A., The Coma/A1367 Supercluster and its Environs, The Astrophysical Journal, vol 222, June 1978, p. 784-799.


Analyzing and Verifying the Data

The Coma Cluster is part of the vast Coma Supercluster. The mean radial velocity of galaxies in A1656 is 6925 km/s (redshift = 0.02310). Assuming a Hubble Constant of 65km/s/Mpc, its center lies approximately 350 million light years away, approximately the same distance as the Leo Cluster, A1367. 

Rich galaxy clusters like the Coma Cluster are analogous to globular star clusters. They are dense, roughly spherical balls of galaxies in complex orbits around the center.  The concentration is highest near the center and falls off rapidly farther away. The Coma Cluster is the largest known galaxy cluster. 

Since galaxies within the Coma Cluster can have such a wide range of velocities, it is more difficult to determine if galaxies within the field are in the foreground or background. Kent and Gunn (1982) presented a model for determining which galaxies are members of the Coma Cluster.  Figure 2 is similar to one of the figures shown in their paper.  However, it contains data from a larger set of galaxies (updated to include all galaxies for which radial velocities have been reported) and extends to a radius of 5º (vs. 6º in the original work).  Radial velocity of each galaxy is plotted vs. its distance from the cluster center.

The solid lines represent Kent and Gunn's velocity profile for cluster member galaxies vs. distance from the center. Galaxies with radial velocities outside these limit lines are considered field galaxies. However, there is some uncertainty in the model. Some galaxies outside, but near the limit lines, may be gravitationally bound to the cluster. 

A1656velR.jpg (67953 bytes)
Figure 2 - The radial velocity distribution profile for galaxies in Abell 1656.

Kent and Gunn were reluctant to associate galaxies beyond 5º (300') with the cluster and stopped their limit lines at 230'. Galaxies with radial velocities the same as the cluster average may interact with nearby field galaxies, causing them to eject from the cluster. Consequently, the target observing list (Table II) contains only those galaxies within 4º  from the center of A1656.

In summary, the objects on the target observing list meet the following criteria:

  1. Photographic magnitudes are brighter than 17.5

  2. Within 1º  of the center of A1656, radial velocities are less than 30,000 km/s

  3. Greater than 1º  from the center of A1656, radial velocities fall within the limits defined in Reference 3

  4. Brighter galaxies with more common designations that have radial velocities outside the limits are also included.

  5. Galaxies with radial velocities outside the limits are noted.

  6. Galaxies lying further than 4º  from the center of A1656 are not included.

These criteria pared the 1200+ candidates down to 729.

To aid locating each galaxy, I created a database of galaxies belonging to the Coma Cluster shown in Table II and loaded it into "The Sky", running on my laptop computer. A planetarium program containing the custom database is a useful observing tool for three reasons. 

  1. I can readily identify which galaxies are cluster members. 
  2. I can see the entire set in order to plan a night’s observing strategy. 
  3. With all the galaxies plotted at the correct coordinates, I know I will be searching in the right place.

Unlike with other galaxy clusters I've studied, the identifications and positions of galaxies in "The Sky" had few errors. The only limitation of using the database in that software is that some visible galaxies are not included and membership in the cluster is not known.

To insure that a galaxy (by whatever name) exists at each of the listed coordinates, I downloaded an image for each object from the STScI Digitized Sky Survey

The Observing List

The observing list is contained in Table II – Galaxies within 4º of the center of the Coma Cluster, Abell 1656.

The headings of the columns in Table II are described in Table III below.

Table III - Description of headings in Table II.


Name Generally the most common catalog name.
Alternate name 1

Identification in an alternate catalog.

Alternate name 2 Identification in an alternate catalog.
RA Right Ascension for Equinox 2000 in hours, minutes, and seconds.
DEC Declination for Equinox 2000 in degrees, arc minutes, and seconds.
Magnitude For most of the data, this is the photographic apparent magnitude.
R (arcmin) Distance of galaxy from center of cluster in arcmin.
Radial velocity Reported heliocentric radial velocity in km/sec.
Cluster member? Remarks indicating likelihood of being a cluster member.
Date observed Date of most recent observation with my 17.5" f/4.5 reflector.
Magnification The magnification used for the reported observation
% averted vision The percentage of time I could hold the object with averted vision.

Observing Methodology

Although a large number of galaxies are clustered together in the central 1º, using a low power, wide field eyepiece will show very few at one time. Most of them are very small and faint. Higher magnification improves contrast and shows more detail. For observing faint, low contrast galaxies, I find using an exit pupil of approximately 1 - 1.5mm provides the optimum view. When using the 17.5" scope, I generally use magnifications of 267X or 286X (7.5mm Takahashi LE or 7mm Nagler T6 eyepieces, respectively). For viewing the smallest and faintest galaxies, and for splitting very close galaxy pairs, I generally use 400X (5mm Takahashi LE or 5mm Nagler T6).  Of course, seeing must be very good to distinguish small galaxies from foreground stars. Detecting the faintest members also requires very dark skies with good to excellent transparency.

Preparation is very important for efficient, productive observing sessions. As mentioned above, for planning, I start with a custom database loaded into "The Sky". For unambiguous identification, I use photographic finder charts. A photographic image contains stars that can not be displayed by existing software. A galaxy’s precise location can be found relative to a pattern of nearby, even faint, foreground stars. In many cases, only after concentrating on that particular location did fainter galaxies "pop" into view with averted vision. If I had been looking as little as 1' away, I would likely have missed, or misidentified, many of them.

Because the concentration of galaxies is so much higher in the central region, I prepare different observing aids for the center vs. outlying areas. For the central 1º X 1º field, I downloaded an image from the STScI Digitized Sky Survey (DSS) and labeled galaxies using Photoshop. The resulting finder chart is too detailed to be useful when printed on a single 8-1/2" x 11" sheet. Consequently, I divided the file into six regions and printed out finder charts approximately 20' X 30'.  Alternatively, the single large file can be loaded onto a laptop for use in the field. Please feel free to download and use the labeled 1º X 1º finder chart for your search.

For galaxies outside of the central 1º X 1º area, I used a different approach. I downloaded a 15' X 15' DSS image for most of the individual galaxies, labeled it using Photoshop, and printed it.  However, intermediate-sized finder charts are helpful to organize the search by focusing on galaxies near to each other (rather than following a list ordered by right ascension). Using "The Sky", I printed out several intermediate-sized finder charts that define regions approximately 1-2º across and contain an average of 10 galaxies each. In a binder, I place individual DSS images behind each corresponding intermediate-sized finder chart. 

With a finder scope, I can quickly star hop to a small region defined by one of the intermediate-sized finder charts. Thereafter, it is relatively easy to use the intermediate-sized finder chart to hop from one galaxy field to the next at 286X while viewing through the eyepiece. The 15' X 15' DSS images are then readily available for galaxy identification.

Most of the galaxies on the observing list are faint. Most of my recorded descriptions are brief. In order to provide some indication of the relative difficulty of seeing an object, I record the magnification used for the observation and the percentage of time I was able to hold the object with averted vision. Any object that I could hold less than 50% of the time is very challenging.

Selected Observing Notes

Currently I have observed 382 galaxies in Table II. Based on radial velocity data, at least 41 are likely not gravitationally bound to the cluster. Based on results so far, I expect to increase the number observed to well over 400. 

Creating a database for "The Sky"


Contacting me

If you have questions, or want to report any errors, please contact me at: ahighe @ ix.netcom.com

I also would appreciate hearing about your observations, especially if you use a scope of a different size. Thanks.

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Copyright © 2003-2004 by Albert Highe, unless otherwise noted. All rights reserved.

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Changes last made on: July 26, 2004