Allan,
Unfortunately, I am still noncomittal about the exact arm structure in M65, as I am too busy analyzing other galaxies...... In fact, I spent most of the last two days thinking about M66 (!!), a galaxy in which the morphology is consistent with, though perhaps not proving, a gravitational interaction with another galaxy.
You are absolutely right about the existence of a very faint anomalous outermost arm in M66 (which can be seen easily in Greg Bradley's fine image at pbase.com). This arm (or arms) makes the outer isophotes of this galaxy to be very unusual in shape!
Here is an image of the Very Faint outer arm(s) or tidal feature(s) of M66:
Also, I am reasonably certain (as I do have access to lot of multi-wavelength imaging material) that the bright portion of M66 does have considerable arm asymmetry, when we consider the difference between the two bright Spiral Arms;
the shorter of its two bright Spiral Arms looks like a stub, and, more importantly, this very short Spiral Arm
still looks short in NIR imaging (which is a low-extinction regime that traces the mass-dominant old stellar popuation) and
it still looks short in two-dimensional image maps of the CO(1-0) total intensity and CO(2-1) total intensity (both of which trace the dense & cold Molecular Hydrogen Gas that is concentrated by the density wave in a spiral arm)
Here is the (infrared) composite of J+H+K band images from the 2MASS (survey):
Isn't infrared great! As a famous lady infrared astronomer once said: "I don't care if it's plastic, as long as it's in the infrared and I can see it!"
However, one must be careful not to make too much of arm asymmetries seen in barred spirals, as moderately strong arm asymmetry is actually
normal for a barred spiral galaxy.
Nonetheless, the level of arm asymmetry (of the two bright arms) observed in M66 is
modestly outside of the normal range that is found in unperturbed non-dwarf spiral galaxies.
HOWEVER, My own (developing) interpretation of M66 is that some of the arm material is actually lifted out of the principal plane of this galaxy by gravitational perturbation! Whether this is true or not, it is a fact that a broadened and "smeared" appearance of the shorter of the two arms in a Barred Spiral Galaxy is usually associated with gravitational perturbation, as I well know from comparing very numerous images of barred spirals.
cheers, the v.bad galaxy man
P.S. In galaxies, the distinction between a ring structure and a ring structure that is composed of two spiral arms that closely overlap, is somewhat technical (in fact, the physical cause can be the same!). Professor Ron Buta studied this phenomenon in his "Catalog of Southern Ringed Galaxies"(known as the CSRG) and subsequent work.
Further, the ring morphologies observed in the galaxy population are comprehensively described and classified in his "de Vaucouleurs Atlas of Galaxies, which is an essential textbook about galaxy classification.
Madbadgalaxyman's opinionated comment of the day:
It is extremely hard (and a non-trivial task) to derive the three-dimensional structure of a galaxy from the appearance of its two-dimensional image. In spiral galaxies, the problem is made fiendishly difficult, because the naive initial assumptions of planarity and circularity do not apply to real disk galaxies seen in the real universe.
