by Albert Highe
| Scope | 17.5" f/4.5 ultralight |
|---|---|
| Eyepieces | mostly 7mm and occasionally 5mm Nagler T6's. |
As I mentioned in Part I, most of the galaxies I observed this weekend were in Abell 426 (Perseus) and Abell 194 (Cetus). I didn't appreciate it before I began, but these two galaxy clusters make a nice comparison. Both are well placed right now. More importantly, they are essentially the same distance from us, yet couldn't be more different.
The mean radial velocity of galaxies is 5396 km/s in A194, and 5470km/s in A426. Assuming a Hubble Constant of 65km/s/Mpc, each lies approximately 265-270 million light years away.
As I've mentioned in a previous report, 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. See Figure 2b, http://pw2.netcom.com/~ahighe/A426velR.jpg, where the radial velocity of galaxies in and around A426 is plotted vs. distance from the cluster center. Kent and Sargent (1983) estimate that the radius at which galaxies reach escape velocity could be as large as 7º, and present a model for determining which galaxies are cluster members based on radial velocity and distance from the center.
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. See Figure 2a, http://pw2.netcom.com/~ahighe/A194velR4.jpg, where the radial velocity of galaxies in and around A194 is plotted vs. distance from the cluster center. 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 A194 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' from the center 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. Some believe these galaxies lie along the surface of the same "bubble" as those in A194.
Until recently, the target list for A426 included objects within 3° of the cluster's center. Based on the analysis of Kent and Sargent (1983), I extended the search to galaxies out to 4°. This added another 50 targets, although some are fore- or back- ground objects. I was able to observe 40 of these new targets this weekend, bringing the total number of galaxies observed to 311. Based on radial velocity analysis, at least 31 of these galaxies likely do not belong to the cluster. At a minimum, 213 of the observed galaxies are members of the cluster.
Some galaxies make a greater impression than others. In particular, the following galaxies in A426 stood out among this weekend's observations.
IC 304/IC 305 This is a nice pair of galaxies oriented approximately N-S and separated by approximately 1.5'. They are well separated at 286X and appear to be very similar. I can hold each with averted vision 100% of the time, but they tend to disappear with direct vision.
The following are some of the "brighter" galaxies that I was able to see with direct vision and detect some structure with averted vision.
The following three are notable because I was able to hold them with averted vision at least 75% of the time. They fall into the category of "brighter" little-known galaxies.
There was one disappointment, NGC 1213. I could precisely locate its position among four very faint stars that winked in and out with averted vision. However, I couldn't convince myself I saw the galaxy. It appears to have very low surface brightness and is a rather difficult NGC object.
I got a good start on the A194 list this weekend, observing 47 objects. All of them are likely cluster members. The central field of A194 is particularly exciting. There are nine brighter galaxies within a 0.25°-diameter circle, including:
NGC 545/NGC 547 - A bright pair, visible with direct vision, 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 pair. There is some fascinating science here. 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.
NGC 535 - Visible with direct vision. Noticeably elongated. Almost 4' WSW of NGC541, along same line from NGC545/547.
UGC 1003 - Visible with direct vision. Somewhat elongated. 4.5' S of NGC541.
MCG+0-4-139 - Just visible with direct vision, but noticeably fainter and smaller than the above galaxies. 2' NNE of NGC541.
MCG+0-4-140 - Just visible with direct vision, similar to above. 4' NNE of NGC541. At the east corner of an equilateral triangle formed with a pair of 14th magnitude stars 28" apart.
NGC 543 - Visible with direct vision. Oval with stellar core. 1.5' N of MCG+0-4-140.
NGC 548 - Visible 100% with averted vision. 5' NE of NGC543.
In addition, immediately SW of the above 0.25°-diameter field is a nice trio of galaxies arranged at the vertices of a triangle with sides approximately 3'.
They appear to be similar-sized oval galaxies visible with direct vision. UGC984 is slightly dimmer than the other two.
A few more brighter galaxies are visible as you head further SW along the major axis of this cluster.
For more information about each of these clusters, including observing
lists, observation details, and finder charts, please check out:
http://pw2.netcom.com/~ahighe/a426.html
http://pw2.netcom.com/~ahighe/a194.htm
