*** The Birthday Cluster ***

a426center.jpg (33291 bytes)

Figure 1 - Some of the brighter galaxies near the center of the Perseus Cluster. NGC1275 is the brightest and lies in the SE corner of the parallelogram. Image from the STScI Digitized Sky Survey. Width is 15' and North is up.

 

Observing Challenge - 300 Galaxies in the Perseus Cluster, Abell 426

Albert Highe

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

Background

I’m fascinated by patterns of galaxies appearing within narrow fields of view. Consequently, I like to observe galaxy clusters. I'm drawn to one galaxy cluster in particular, Abell 426 in Perseus. The Perseus Cluster has special meaning to me after I "discovered" it on my birthday a few years ago. While searching for NGC 1275 with my 12.5" scope, I was surprised to see nine galaxies in a relatively narrow field of view. Four of the brighter galaxies, including NGC 1275, the brightest, lie at the corners of a parallelogram (Figure 1). I could see quite a few more galaxies while scanning the immediate area. I’ve since spent a lot of time on my "Birthday Cluster," returning to it again and again to enjoy the views and to seek out new members.

Uranometria 2000.0 shows a close up plot of this cluster on chart A4. A dashed circle with a diameter of approximately 3º indicates the location and size. However, as I hunted down galaxies in the circle, three questions arose. 

  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 Perseus Cluster. Cluster members have been studied extensively and quite a lot is known about their precise coordinates, radial velocities, magnitudes, and morphologies. A variety of sources, shown below in Table 1, provide lists of galaxies within the Perseus Cluster.  The most extensive list is by Brunzendorf and Meusinger (1999). They identified galaxies down to magnitude 19.5 within a square approximately 3º on a side. Most of these galaxies are too faint to be seen in amateur telescopes. Some of the other references include galaxies farther away from the center. However, most data are limited to galaxies brighter than magnitude 15.7. 

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 over 2400 galaxies within 3º from the center, and over 3800 galaxies within 4º 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 4º, radial velocity data are reported for approximately 9% (340 galaxies out of the more than 3800). Fortunately, the radial velocities of most of the brighter galaxies (those visible with amateur telescopes) have been measured. 

The objects on the target list meet the following criteria:

  1. Lie within 3º from NGC 1275 and

    1. have photographic magnitudes brighter than 16.5, or

    2. have photographic magnitudes fainter than 16.5, but Brunzendorf and Meusinger (1999) indicate their surface brightness is brighter than 21.0, or

    3. are as faint as magnitude 17.5 if they have more common designations, or

    4. show up well on DSS images prepared as finder charts for other targets

  2. Lie between 3º and 4º from NGC 1275 and

    1. have measured radial velocities less than 20,000 km/s, or

    2. show up well on DSS images prepared as finder charts for other targets.

These criteria pared the candidates down to 351.

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 Chincarini, Guido; Rood, Herbert J., Dynamics of the Perseus Cluster of Galaxies, Astrophysical Journal, vol. 168, p.321 (1971).
3 Kent, S. M.; Sargent, W. L. W., The dynamics of rich clusters of galaxies. II - The Perseus cluster, Astronomical Journal (ISSN 0004-6256), vol. 88, p. 697 (1983).
4 Poulain, P.; Nieto, J.-L.; Davoust, E., Isophotal shapes of early-type galaxies. II - The Perseus Cluster, Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), vol. 95, no. 1, p. 129 (1992).
5 Andreon, S.; Davoust, E.; Poulain, P., Morphological classification and structural parameters of galaxies in the Coma and Perseus clusters, Astronomy and Astrophysics Supplement Series, Vol. 126, p. 67 (1997).
6 Brunzendorf J., Meusinger H., The galaxy cluster Abell 426 (Perseus). A catalogue of 660 galaxy positions, isophotal magnitudes and morphological types, Astronomy and Astrophysics Supplement Series, Vol. 126, p. 141 (1999).

Analyzing and Verifying the Data

The Perseus Cluster is part of the vast Pisces-Perseus Supercluster. The mean radial velocity of galaxies in A426 is 5470km/sec (redshift = 0.0183).  Assuming a Hubble Constant of 65km/s/Mpc, its center lies approximately 270 million light years away.

Rich galaxy clusters like the Perseus Cluster are analogous to globular star clusters. They are dense, roughly spherical balls of galaxies in complex orbits around the center.  The concentration of galaxies in the Perseus Cluster is one of the highest known. The concentration is highest near the center and falls off rapidly farther away. There are a number of models for the concentration profile. 

Although the entire cluster recedes from us at the mean radial velocity, the velocity (or redshift) of individual galaxies can deviate significantly from the mean. Higher redshifts mean higher velocities. Based on the Hubble Constant, objects moving faster are believed to be at greater distances from us. However, since galaxies are in motion within the cluster, their velocities do not necessarily correlate with their distance from us. Depending on where they are in their orbits, their velocities relative to the center may either add or subtract from the mean velocity. The range of velocities is highest near the center of the cluster.  The range of velocities within the Perseus Cluster is one of the highest known. Near the core, corresponding roughly to the position of NGC 1275, radial velocities of members can vary as much as + 4150 km/sec.  At a radius of 3º, radial velocities cannot deviate more than + 1250 km/sec. 

Since galaxies within the Perseus 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 Sargent (1983) presented a model for determining which galaxies are members of the Perseus Cluster.  Figure 2 is similar to one of the figures shown in their paper.  However, it contains data from a larger set of galaxies (from recent NED searches) and extends to a radius of 4º (vs. 3º in the original work).  Radial velocity of each galaxy is plotted vs. its distance from the cluster center.

The solid lines represent Kent and Sargent'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. 

A426velR.jpg (48075 bytes)
Figure 2 - The radial velocity distribution profile for galaxies in Abell 426.

As already mentioned, the figure in Kent and Sargent's paper only included data out to a radius of 3º (180'). The dashed lines in Figure 2 are my extrapolation of their limit lines out to 4º. It is apparent from the figure that cluster members can be found out to 4º and beyond. Kent and Sargent estimate that the radius at which galaxies reach escape velocity could be as large as 7º.  However, the number of member galaxies becomes increasingly smaller at greater distances from the center. Consequently, I've currently limited my target list to galaxies within 4º. 

I found that the NED, SIMBAD, and "The Sky" databases have discrepancies among them. Over time, some errors have been discovered and corrected. Consulting the most recent databases was necessary to resolve some of the discrepancies. No doubt some mis-identifications still exist. However, 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

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

At least 15 galaxies are mis-plotted in "The Sky" with deviations from 30" to 60". For brighter galaxies, this amount of deviation is not likely to cause any confusion. However, for the fainter galaxies, it is likely that the galaxy would not be found or be confused with other nearby galaxies.

The Observing List

The observing list is contained in Table II – Galaxies within 4º of NGC1275.

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

Identification in an alternate catalog.

Alternate name 2 Another 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 with limiting isophote 25 mag/(arc sec)2.
Surface brightness Central surface brightness determined from the central 5 (arc sec)2, in mag/(arc sec)2
R vs. NGC1275 Distance of object from NGC1275, in degrees.
Radial velocity Reported heliocentric radial velocity in km/sec.
Cluster member Blank if considered gravitationally bound to the cluster, or if the radial velocity is unknown.
Date observed Date of most recent observation with my 17.5" f/4.5 reflector.
Magnification The magnification used for the reported observation
% time hold with averted vision The percentage of time I could hold the object with averted vision.
Remarks May include details of observation, etc.

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. References for these other designations are shown in Table IV.

Table IV - References for some less common object designations.

V Zw Catalogue of Galaxies and Clusters of Galaxies, I-VI, Zwicky F., et al., Calif. Inst. of Techn., Pasadena, 6 vols. (1961-68).
2MASX 2 Micron All Sky Survey Extended Objects
NPM Lick Northern Proper Motion Program: NPM1 Reference Galaxies, Klemola A.R., Hanson R.B., Jones B.F.,  Astron. J. 94, 501 (1987).
IRAS Joint IRAS Science Working Group. Infrared Astronomical Satellite Catalogs, 1988, The Point Source Catalog, Version 2.0, NASA RP-1190 vol. p. (1988).
B3 B3=Third Bologna Catalog of radio sources. Ficarra, A., Grueff, G., and Tomassetti, G., A New Bologna Sky Survey at 408 MHZ, Astr. Ap. Suppl. vol. 59 p. 255-347 (1985).
BM99 Brunzendorf J., Meusinger H., The galaxy cluster Abell 426 (Perseus). A catalogue of 660 galaxy positions, isophotal magnitudes and morphological types, Astronomy and Astrophysics Supplement Series, Vol. 126, p. 141 (1999).

I used another designation, BM99, to indicate those galaxies first identified in the Brunzendorf and Meusinger paper. For example, BM99-33 indicates the 33rd galaxy listed in their study. Within the last year, the Two Micron All-Sky Survey has catalogued most of these galaxies. In those cases, I've substituted the 2MASX designation as the primary identification.

Note that Table II is not an exhaustive or definitive list of observable galaxies within the Perseus Cluster. Radial velocities have not been measured for many on the list. Without knowing the radial velocity, galaxy membership in the cluster is uncertain. Also, as already mentioned, some member galaxies lie farther than 4º from the core.

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 the 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 20 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 identification.

An experienced observer with an 18" scope should be able to observe most (all?) of the galaxies listed in Table II. Some are very difficult, providing only intermittent sightings with averted vision. 

However, a large scope isn’t necessary to observe a large number of the cluster members. I also have observed quite a few of these galaxies with my 12.5" scope. On the other hand, someone with a 25" scope should be able to observe well over 400 galaxies! Anyone out there with a large scope willing to give it a try?

Are 300 objects within a 4º radius too much to tackle? How about trying for the galaxies within 1º of NGC1275 (2º diameter circle)?  Table II contains 145 galaxies within 1º of NGC1275.  So far, I have seen 137 galaxies within this smaller area. 

It is interesting to note that more than half the galaxies in Table II lie within 1º of NGC1275 , i.e. more than half lie within 1/9th the area or 1/27th the volume! This fact highlights how much denser the center of the cluster is. 

For those who are new to this cluster, I recommend starting in the center where the density of galaxies is the highest and where some of the cluster’s brightest members can be found. Approximately 25 relatively bright galaxies lie within 0.5º of the distinctive parallelogram, formed by NGC1275, 1272, 1273, and 1278. Most of these galaxies are plotted in Uranometria 2000.0. Once you have familiarized yourself with the brighter members, and can "galaxy hop" among them, begin searching for the fainter galaxies within this narrow field. Most of the fainter galaxies are not plotted in Uranometria 2000.0. To find them, either use a planetarium program that can display very faint field stars or download DSS images from the Internet. Alternatively, use the finder chart I've created. The finder chart consists of a STScI Digitized Sky Survey image of the central X field where all the galaxies on my observing list have been labeled. The finder chart is too detailed to be useful on one sheet. I recommend using it on a laptop or printing out smaller sections to use in the field. 

Only after observing all the galaxies in the central area, do I recommend searching for the outlying members. I recommend against working the list in the order shown in Table II (by RA). Even the outlying members can be conveniently observed in groups consisting of 3-7 nearest neighbors. Hopping among galaxies within these smaller groups, and then jumping to another group, made my observing sessions more efficient. Expect to spend several nights to observe all the members on the list.

Doing the background research and preparing my own observing list was unexpectedly rewarding. I started out observing galaxy clusters because it was a thrill to see interesting patterns formed by dense clusters of distant galaxies. In the end, I have a greater appreciation of the large-scale structure and workings of the universe. When I observe galaxies belonging to the Perseus Cluster, I am aware that I am looking at components of one of the largest and densest  galaxy clusters. Knowing the velocity of each galaxy allows me to visualize that universe in motion.

Selected Observing Notes

So far, I have seen 311 galaxies listed in Table II with a 17.5" scope. At least 31 of these galaxies likely do not belong to the cluster. Based on the radial velocity analysis, a minimum of 213 of the observed galaxies are members of the cluster.

Most of the galaxies on the observing list are faint. Also, initially I was more concerned about checking objects off my list rather than writing much detail about them. Consequently, most of my recorded descriptions are brief. In addition, my scale of what is "faint" or "small" changed during the course of making the observations. So, I don't think those terms are of much use. However, I feel it is important to provide some indication of the relative difficulty of seeing an object. I think two columns of Table II are more useful for that purpose - the magnification used for the observation and the percentage of time I was able to hold the object with averted vision. In most cases, objects that require higher magnification are more difficult. I generally was able to hold small, dim objects with averted vision longer at higher rather than at lower magnification. Any object that I could hold less than 50% of the time is very challenging.

MCG+7-8-4 (CGCG 541-003, PGC 12797): Plotted in Uranometria and has a listed magnitude of 15.2. However, it is only 18" away from an 11th magnitude star. After repeated attempts, I finally saw it with averted vision and could hold it 15% of the time at 286X. This is a very challenging object.

IC316: Plotted in Uranometria, and often referred to as one galaxy. However, there is a very close pair at this location, oriented north-south. I've designated them as IC316A and IC316B. IC316A is the brighter (mag 15.0), northern component. IC316B is the fainter (mag 15.8), smaller, southern component. This is a very difficult pair to split. The separation is only 11". At 267X, I can hold IC316A 100% of the time with averted vision. Half the time I can see an elongation toward the south. At 400X, I've held the smaller, fainter component about 20% of the time with averted vision.

UGC2756: Plotted in Uranometria. This is another close pair of galaxies, oriented north-south. I've designated them as UGC2756A (15.5) and UGC2756B (15.8). Although they are dimmer than IC316A, the pair are easier to split/see. They are 30" apart. I can hold A 80% of the time and B 50% of the time with averted vision at 267X.

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 "A426" 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. A426) 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. A426) is checked. The objects in Table II should appear on your screen.

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 by Albert Highe, unless otherwise noted. All rights reserved.

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