*** Abell 1367 ***

A1367_15_15.jpg (39194 bytes)

Figure 1 - Some of the brighter galaxies approximately 10' NW of the  center of A1367.  The brightest galaxy in this image is NGC3842. Image from the STScI Digitized Sky Survey. Width is 15' and North is up.

Observing Challenge - 100+ Galaxies in the Leo Cluster, Abell 1367

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.

 

Background

My observing project of the Perseus Galaxy Cluster, Abell 426, was very rewarding. I found I enjoyed searching for and reading the academic sources, sorting through the data to identify cluster vs. non-cluster galaxies, preparing my own observing list, and hunting down galaxies that are seldom, or have never been, seen by human eye. Consequently, I decided to perform similar studies of other rich Abell galaxy clusters. Abell 1367 was next on my list.

Uranometria 2000.0 shows a close up plot of A1367 on chart A11. A dashed circle with a radius less than 1º indicates the location and size. As with the A426 study, 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 Leo Cluster. A variety of sources, including those shown below in Table 1, provide lists of and information about galaxies within A1367.  An interesting recent article is Donnelly et al. (1998). They propose that A1367 consists of two merging clusters. Unfortunately, the papers I've found do not contain data as complete or as accessible as for the Perseus Cluster. 

To generate the most up-to-date and complete list, I ran several "near name" searches on the NASA/IPAC Extragalactic Database (NED) website. Searches generated 542 galaxies within 1º from the center, and 1682 galaxies within 2º from the center. The majority of these galaxies are very faint, and only recently have been catalogued by the Two Micron All Sky Survey.  Most have no visual magnitude or radial velocity data. For example, within 2º, radial velocity data are reported for approximately 13% (217 galaxies out of 1682). Fortunately, the radial velocities of most of the brighter galaxies (those visible with amateur telescopes) have been measured. 

There was some difficulty conducting the above searches. The question is "where is the center of A1367?" Various sources give different locations. In addition, one source gives two locations corresponding to the calculated centers for each of two merging clusters. Below are Epoch 2000.0 coordinates provided by a few sources.

Source RA (h:m:s) DEC (d:m:s)
SIMBAD 11 44 44.6 +19 41 59
NED 11 44 29.5 +19 50 21
Godwin and Peach (1982) 11 44 17.5 +19 50 31
Donnelly et al (1998) (SE) 11 44 50

(NW) 11 44 22

+19 41 44

+19 52 27

 

The SIMBAD and NED centers are separated by 9'. Interestingly, the SIMBAD center is a little over 1' away from Donnelly et al's SE cluster center and the NED center is within 3' of Donnelly et al's NW cluster center. Likewise, Godwin and Peach's center is approximately 2' of Donnelly et al's NW cluster center. When preparing the Finder Chart image and the data for Table II, the searches were centered about the NED coordinates.

 

The objects on the target list meet the following criteria:

  1. Lie within 1.25º from the center of A1367, and

  2. Have photographic magnitudes brighter than about 17, or

  3. Show up well on DSS images prepared as finder charts for other targets.

These criteria pared the candidates down to 205.

 

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 Dickens, R.J. and Moss, C., Redshifts of Galaxies in the Cluster Abell 1367, Mon. Not. R. astr. Soc., vol 174, 1976, p. 47-58.
3 Gregory, S.A. and Thompson, L.A., The Coma/A1367 Supercluster and its Environs, The Astrophysical Journal, vol 222, June 1978, p. 784-799.
4

Godwin, J.G.; Peach, J.V.; Photometry of the cluster of galaxies A1367, Mon. Not. R. astr. Soc., vol 200, 1982, p. 733-746.

5 Donnelly, R.H.; Markevitch, M.; Forman, W.; Jones, C.; David, L.P.; Churazov, E.; Gilfanov, M., Temperature Structure in Abell 1367, Astrophysical Journal, Vol 500, June 1998, p. 138-146. 
6 Iglesias-Paramo J., Boselli A., Gavazzi G., Cortese L., Vilchez J.M., The R'-band luminosity function of Abell 1367: a comparison with Coma, Astron. Astrophys. vol. 397, January 2003, p. 421-430.
7 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).

 

Analyzing and Verifying the Data

The mean radial velocity of galaxies in A1367 is 6595km/s (redshift = 0.022). Assuming a Hubble Constant of 65km/s/Mpc, its center lies approximately 330 million light years away.  It is essentially the same distance away as Abell 1656, the Coma Cluster. A1367 and A1656 are the two largest clusters in the vast Coma Supercluster of galaxies.

Since A1367 and A1656 are the same distance from us, it is interesting to compare them. As mentioned elsewhere, rich galaxy clusters like Abell 1656 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 away from the center. The Coma Cluster is the largest known galaxy cluster. 

A1367 is considerably smaller than A1656. In addition, Donnelly et al (1998) indicate that A1367 appears to be a merger of two clusters.  Unfortunately, I found no reference that analyzes galaxy membership as a function of radial velocity and distance from the center. However, since A1367 shares membership with A1656 in the same supercluster, I decided to apply the results of A1656 by Kent and Gunn (1982) to A1367. 

In Figure 2 the radial velocity of each galaxy is plotted vs. its distance from the cluster center. Figure 2a is for A1367. Figure 2b is for A1656. The solid lines in Figure 2b represent Kent and Gunn's (1982) velocity profile for cluster member galaxies vs. distance from the center. Galaxies with radial velocities outside these limit lines are considered field galaxies. I don't have the knowledge and experience to apply Kent and Gunn's approach to A1367. Instead, I fitted a function to their limit lines for A1656 and scaled it to A1367.  Published values for the mean radial velocity and velocity dispersion for A1367 were the only input parameters. The dashed lines in Figure 2a are the result. Surprisingly, they appear to define the limits of A1367 rather well. There appears to be a break in the data around 70' from the center. Consequently, galaxies on the target list lie within a radius of 1.25º.

A1367velR.jpg (43177 bytes) A1656velR.jpg (67953 bytes)
Figure 2a - The radial velocity distribution profile for galaxies in and around Abell 1367.

Figure 2b - The radial velocity distribution profile for galaxies in and around Abell 1656.

Of the 205 target galaxies within 1.25º, 111 galaxies (54%) have published radial velocities. How many of the galaxies without radial velocities are cluster members? I believe most of them are. The majority of the known radial velocities (97 out of 111) lie within the dashed limit lines in Figure 2a. If this ratio is assumed to hold for the remaining galaxies, another 12 are likely not bound to the cluster. On the other hand, this number could be greater. It is possible that measurements of radial velocities favor brighter, closer galaxies.

Note that it is difficult to tell whether any galaxies beyond 1.25º belong to the cluster. Even galaxies with radial velocities the same as the cluster average may interact with nearby field galaxies, causing them to eject from the cluster. 

To aid locating each galaxy, I created a database of galaxies belonging to the Leo 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.

Approximately 25 galaxies are mis-plotted in "The Sky" with deviations up to 3'. These are unusually large deviations. Even with bright galaxies, one might be confused by such large errors.

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 1.25º of the center of Abell 1367.

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

Table III - Description of headings in Table II

Headings

Description
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.
Comments  

You’ll notice several designations in Table II, including objects from NGC, IC, UGC, MCG, CGCG, and PGC catalogs. However, some less common designations are also used. Full names for these other designations are shown in Table IV.

Table IV - Some less common object designations.

KUG Kiso Ultraviolet Galaxy Catalogue
MAPS-NGP Minnesota Automated Plate Scanner - North Galactic Pole
AGC Arecibo General Catalog
2MASX 2 Micron All Sky Survey Extended Objects
ABELL Abell Clusters of Galaxies
ARK Arakelian Emission Line Objects

Observing Methodology

Preparation is very important. The most useful charts are those I prepared from STScI Digitized Sky Survey images of the central 1º X 1º field and selected outlying areas. I labeled the target galaxies in Photoshop. For use in the field, I print out charts of regions approximately 20' X 30'. Feel free to download the finder chart I've created for the central 1º X 1º field. 

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. For my 17.5" scope, I generally use a 7.5mm Takahashi LE or 7mm Nagler T6 eyepiece, providing magnifications of 267X and 286X, respectively. For viewing the smallest and faintest galaxies, and for splitting the very close galaxy pairs, the 5mm Takahashi LE, providing 400X, works best. Detecting the faintest members also requires very dark skies with good to excellent transparency.

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've observed about 131 objects on the list. Nine may not be cluster members.

Creating a database for "The Sky"

I find it useful to have a separate symbol for the galaxies in Table II. That way, I can independently display those objects vs. other galaxies or objects. To create a special symbol, click on "Preferences" under the "View" tab. Click on "Add" in the "Preferences" window and type in a name. I use "A1367" to designate my objects. Then define the characteristics for the "font," "line," "fill," and "symbol."  Then click "save" and then "OK" to exit "Preferences".

Create a text file from Table II or use the file I've provided.

Then click "Import" from the "Data" tab. Use the "Browse" button to find the text file from the previous step. "Data Class" should be set to "Objects/Points." Select the name you gave your objects (e.g. A1367) under "Object Type." Then click on "Define Fields." You will need to highlight the appropriate column(s) for each of the data fields. You must define all of the "Required" fields. Everything else is optional. If you created your own text file, you must be careful to have the identical character spacing between tabs for each line of text.

When you are done, click "OK" and then "Compile." Go back to the "View" tab and click on "Filters."  Make sure the box for your object set (e.g. A1367) is checked. The objects in Table II should appear on your screen.

I understand SkyMap Pro is much easier to use.

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: June 20, 2004