Ken,
The anomalous blue extension on one side is very interesting.
I suppose that it could just be an outer spiral arm, seen almost in our line-of-sight. Very tricky to find out what is really going on here, as everything is foreshortened.
When we look at spiral galaxies in a face-on orientation, a single outer spiral arm is not that unusual......
a "one-armed mode", in the opaque jargon of professional astronomy, refers to a galaxy which is one-armed at a specific range of galactocentric radii....it is not uncommon to find that even famous galaxies that we
believe that we know well, actually have only a single spiral arm at some radius.
(Nearly all barred spirals have a trace of one-armed structure; with one weak arm and one strong arm. Perturbations by bars and/or by external galaxies can cause a one-armed look.)
Just by looking at your very fine "two-dimensional" image (obviously, no
physical depth is available in a galaxy image), one cannot tell whether the anomalous "arm" is close to the galaxy or far from it. But the velocities of the stars or gas (via spectroscopy) can sometimes give us the true "3D" picture of how a galaxy is actually structured in real space.
Given the outer asymmetry of the image of this galaxy, it does seem possible that this galaxy has undergone some kind of mild perturbation. Mind you, all galaxies are actually perturbed, to some extent!
A candidate perturbing object is UGC 7176 to the south of NGC 4157.
See the attached DSS image taken from server1.wikisky.org :
This little dwarf galaxy is probably significantly bent and warped, that is, it is probably non-planar. Furthermore, its redshift is close enough to that of NGC 4157 to indicate that it may well be in physical proximity. These two facts would support its identity as the perturbing object.
Another possible perturber is the very unusual spiral
NGC 4088, which is probably a member of the same group of galaxies.
I have seen a lot of these types of galaxy pairs before, and I think there is a good chance that there has been an interaction.
Another common cause of asymmetry and non-planarity in a galaxy image that we see edge-on is the existence of a "warp", which implies that a spiral or S0 galaxy is not coplanar....in other words, there is a genuine bend, twist, or even a bifurcation into two different planes.
As I do have a personal "rogues gallery" of ~500 different galaxies with warps.......to me, the hypothesis of a warp or bend in the disk of N4157 seems less likely. (However, I think that there is some literature evidence for the existence of a warp)
If I ever get "A Round Tuit", I will track down some HI data, to see if the gas layer is disturbed in N4157.
cheers,
madbadgalaxyman
