*** Abell 194 ***

A194.jpg (18119 bytes)

Figure 1 - Galaxies near the center of A194.  The bright pair in the upper left corner are NGC 545 and NGC 547. Image from the STScI Digitized Sky Survey. Width is 15' and North is up.

Observing Challenge - 100 Galaxies in Abell 194

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

Uranometria 2000.0 shows a close up plot of A194 on chart A16. A dashed circle with a diameter less than 1º indicates the location and size. As with other 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 Abell 194. A variety of sources, including those shown below in Table 1, provide lists of and information about galaxies within A194.  A particularly excellent article is Chapman et al (1988). They investigated and analyzed galaxies within 5º of the center of A194. 

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 4700 galaxies within 2º from the center, and over 10,000 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 or the Automated Plate Measurement United Kingdom Schmidt (APMUKS(BJ)).  Most have no visual magnitude or radial velocity data. For example, within 4º, radial velocity data are reported for approximately 24% (2372 galaxies out of the more than 10,000). Fortunately, the radial velocities of most of the brighter galaxies (those visible with amateur telescopes) have been measured. 

I had a quandary while conducting the above searches. The question is "where is the center of A194?" Various sources give different locations. I've noticed this before with other Abell Clusters. For example, here are Epoch 2000.0 coordinates provided by a few sources.

Source RA (h:m:s) DEC (d:m:s)
SIMBAD 01 26 01 -01 22 02
NED 01 25 33 -01 30 25
Chapman et al (1988) 01 25 57 -01 20 25
NGC 541 01 25 44 -01 22 46

 

The proposed centers of A194 span almost 12'. I found no convincing argument to favor a particular one. There is even some confusion within Chapman et al (1988) who first states the indicated coordinates for the center. However, later in the same paper the authors say that NGC 541 is at the center. My choice of center is inconsistent as well. The NED coordinates are the center for the Finder Chart image. When preparing the data for Table II, the searches were centered about a convenient compromise of the above possibilities  - the coordinates of NGC 541.

 

The objects on the target list meet the following criteria:

  1. Lie within 2º from the center of A194 (taken to be NGC 541), and

  2. Have measured radial velocities less than 10,000 km/s, and

  3. Have photographic magnitudes brighter than about 17, or

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

These criteria pared the candidates down to 156. Only 117 have radial velocities between 4000 and 6600 km/s.

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 Zwicky, F.; Humason, M. L., Spectra and Other Characteristics of Interconnected Galaxies and of Galaxies in Groups and in Clusters. III, Astrophysical Journal, vol. 139, January, 1964, p. 269.
3

Chincarini, G.; Rood, H. J., The structure of the galaxy cluster A 194, Astrophysical Journal, vol. 214, June 1977, p. 351-358.

4 Chapman, G. N. F.; Geller, M. J.; Huchra, J. P. , Linear clusters of galaxies - A194, Astronomical Journal, vol. 95, April 1988, p. 999-1022. 
5 van Breugel, W.; Filippenko, A.V.; Heckman, T.; Miley, G., Minkowski's Object: A Starburst Triggered by a Radio Jet, Astrophysical Journal, vol 293, June 1985, p. 83-93.
6 Arp, Halton; Atlas of Peculiar Galaxies, Astrophysical Journal Supplement, vol. 14, November 1966, p 1.

 

Analyzing and Verifying the Data

The mean radial velocity of galaxies in A194 is 5396 km/s.  Assuming a Hubble Constant of 65km/s/Mpc, its center lies approximately 265 million light years away. It is essentially the same distance away as Abell 426, the Perseus Cluster.

Since A194 and A426 are the same distance from us, it is interesting to compare them. As mentioned elsewhere, rich galaxy clusters like Abell 426 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 concentration of galaxies in the Perseus Cluster is one of the highest known. 

In contrast, A194 does not fit a spherical model. There is no dense central concentration of galaxies and the range of radial velocities of its members is much narrower. In fact, A194 is rather peculiar. It is the prototypical "linear" cluster. There is preferential alignment of its members along a particular axis.  It is analogous to a rather rich open star cluster. Some of the differences between A194 and A426 are evident in Figure 2 where the radial velocity of each galaxy is plotted vs. its distance from the cluster center. Figure 2a is for A194. Figure 2b is for A426. 

Chapman et al (1983) concluded that any galaxy within 2º of the center and with a radial velocity between 4000 and 6600 km/s is likely a member of the cluster.  Figure 2a clearly shows the band of galaxies between 4000 and 6600 km/s. Since member galaxies lie within such a narrow range, foreground and background galaxies, with different radial velocities, stand out well. The concentration of galaxies above 9000 km/s around 200' corresponds to the galaxy cluster Abell 189. Note that the concentration of galaxies between 4000 and 6600 km/s also increases around 200'. This corresponds to the nearby NGC533 Group. This cluster has a similar velocity distribution and lies just over 3º away from A194. 

A194velR4.jpg (43265 bytes) A426velR.jpg (48075 bytes)
Figure 2a - The radial velocity distribution profile for galaxies in and around Abell 194.

Figure 2b - The radial velocity distribution profile for galaxies in and around Abell 426.

To aid locating each galaxy, I created a database of galaxies belonging to the A194 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.

Some galaxies are mis-plotted in "The Sky" with deviations up to 4.5'. 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 2º of the center of Abell 194.

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.

I Zw Catalogue of Galaxies and Clusters of Galaxies, I-VI, Zwicky F., et al., Calif. Inst. of Techn., Pasadena, 6 vols. (1961-68).
NPM1G Lick Northern Proper Motion Program: NPM1 Reference Galaxies, Klemola A.R., Hanson R.B., Jones B.F.,  Astron. J. 94, 501 (1987).
AGC Arecibo General Catalog
2MASX 2 Micron All Sky Survey Extended Objects
UM University of Michigan Emission Line Objects
LEDA Lyon Extragalactic Database
APMUKS(BJ) Automated Plate Measurement United Kingdom Schmidt (B_J_)
SDSS Sloan Digital Sky Survey
MRK Markaryan objects
FGC Flat Galaxy Catalogue

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 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 smaller DSS images, labeled them using Photoshop, and printed them. These small finder charts were typically 15' X 15' for individual galaxies, but could be as large as 20' X 30' if some galaxies were relatively close together.  In addition, 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 1º-2º across and contain an average of 10 galaxies each. In a binder, I place the small 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' (and similar-sized) 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've observed 49 objects on the list. All except two are likely cluster members.

NGC 545/NGC 547 - The brightest galaxies within A194, easily visible with direct vision. They form a close pair only 30" apart, aligned NW-SE. NGC 545 appears to be slightly oval, whereas NGC 547 is circular. They appear to be immersed in a common halo. This pair is also known as ARP 308.

NGC 541 – Also known as ARP133. Visible with direct vision and nearly as bright as the above pair 4’ ENE. In some photographs, a trail of material can be seen connecting NGC 541 with the above pair. Minkowski’s Object lies 1’ ENE of NGC 541 along a radio jet apparently emanating from it and directed towards NGC545/547. Uranometria apparently labels NGC541 as Minkowski’s Object, which is not correct. In the NED, Minkowski’s Object has the designation APMUKS(BJ) B012314.23-013754.6. I thought I might have glimpsed it, but couldn’t be sure. Minkowski’s Object will take better observing conditions and/or a larger instrument.

Minkowski's Object is more than just a challenging object to observe. I found the science fascinating. The paper by van Breugel et al. (1985) has a nice discussion. Minkowski's Object appears to be a rather peculiar dwarf galaxy. Note the tendrils in Figure 3. Stellar activity in dwarf galaxies is typically rather low. The massive, hot, blue stars have long died and dwarfs have settled down to a quiet existence. You see a nice distribution of stars on the main sequence. Minkowski's Object is quite different. It is very blue, indicating the existence of a large number of young, hot, blue stars. It also shows emission spectra typical of "starburst galaxies". Starburst galaxies are those that show unusually high star-forming activity. M82 and NGC253 are a couple of well-known examples. Minkowski's Object is also at an interesting location - near the tip of a jet of ionized plasma emanating from NGC541. No doubt this jet comes from a massive black hole in the center of NGC541. I believe it was Minkowski who proposed that the increased stellar activity in the peculiar dwarf was initiated by the shock front of the jet - hence the name. 

MinkowskiNGC541F.jpg (33579 bytes)
Figure 3 - Closer view of Minkowski's Object. Image from Halton Arp's "Atlas of Peculiar Galaxies". Image is approximately 3' wide and North is up.

 

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: May 16, 2004