I assume you're talking about the fact that distance measurements using Hubbles Law of individual galaxies can be skewed by the peculiar velocities imparted by the "local" gravitational affect of the cluster?
That's about all I know about it.
...and it's also basically correct. Since velocities are 3-vectors, it is intuitive that vast misinterpretations of the spacial locations of galaxies arise, if the measured values of the (spectroscopically determined) redshift z are interpreted as arising exclusively from a radial
velocity component associated with the Hubble expansion. It is easy to see that this (rough) simplification then leads to a distorted spacial profile of members of galaxy clusters (via Hubble's law) in such a way that instead of the true spherical cluster shape, the clusters appear as strongly elongated galaxy "jets" that always point towards the observer
. Hence the "Finger of God" nomenclature. You can easily notice this effect in my SDSS plots above.
In case of interest, there are various very good scientific papers about this point...let me know.
A more popular and short discussion with typical examples is here:
http://web.ipac.caltech.edu/staff/jarre ... /fgod.html
While the effect is easy to understand, the coding of an automatized
"finger-of-God" correction algorithm is no so easy. That's one of the tasks that I am tackling in this field from time to time...
As to your question about "lookback time". Well, since the underlying framework here is general relatiivity (GR), one has -- first of all-- to define what is meant with the notion of distance in a GR setting
. It so happens that the (measured) redshift z, is most directly related to the co-moving
distance that makes sense relativistically, (of course, provided
peculiar velocities can be neglected)!
Since GR-based distances include the "lookback" effect naturally, there is no problem, once one calculates co-moving distances in terms of z. Indeed, that's why I chose the co-moving distance. Actually, in order to calculate it, one needs to input one's favorite cosmological model (in terms of the standard parameters) . I wrote a Perl script for this purpose and used the parameters from the latest Wmap data.
No idea why you can't find the Sombrero galaxy in SDSS. It's certainly included in all other relevant catalogs.
As to the incorrect inclination of M104, it's quite unavoidable within my "mass-rendering" framework in Celestia. It's a limitation of my computer-based rendering that -- as a matter of principle-- only uses the given catalog info
. M 104 is archived with Hubble type Sa
but actually shows a highly non-canonical shape (that's why it's so famous
) . So the given angles (PositionAngle and inclination) in the scientific NGC/IC catalog are interpreted in a way by my code that appears incorrect due to inherent morphological ambiguities
. Of course one day I may well include some special treatment for a few famous galaxies into my code...
Along the same lines, I could also quote to you many galaxies in my galaxy.dsc catalog, where the spiral rotates the wrong way around. Again this is unavoidable, since unfortunately, in NO galaxy catalog there are data about the rotational sense of the spirals. Since the task was to render 10000+ galaxies, individual comparison with photographic imaging would keep me busy for the rest of my life