[madbadgalaxyman (Robert)]

A good observation, Allan !

I have been trying to find something unusual about M65 for some years, and till this UV image from GALEX came out, the most unusual thing about this galaxy seemed to be its utter normality!

In essence, I do agree with you. The short spiral arm is so short that M65 might be thought of as a one-armed spiral galaxy, but this conclusion is subject to some qualifications:

To confirm the arm pattern, one really needs multi-wavelength imaging data and to have a close look at the FUV channel only, as the GALEX image in my Post actually superposes two bandpasses: Far-ultraviolet + Near-ultraviolet.
[ I might have a play with the FUV imaging data, which is usually available from the NED (extragalactic database) ]

The apparent (as is observed in the two-dimensional UV image) arm asymmetry in M65 is very very pronounced in the ultraviolet regime, though one has to take into consideration the fact that any internal extinction of the Ultraviolet light emitted by a galaxy is going to be very high due to the severe effect of interstellar dust on short wavelength light.
So, one has to be on the alert for the possibility that the arm pattern observed in a far-ultraviolet image of a galaxy may not actually be the real arm pattern; the actual arm pattern may be falsified by the presence of severe extinction from dust within a galaxy.

The Far-ultraviolet channel of the GALEX telescope/satellite is extraordinarily sensitive to the presence of very recent Star Formation and it very easily detects light from the consequent UV-luminous OB stars that are so common in the spiral arms of galaxies. Therefore, the M65 spiral arm pattern is much more clearly seen in the FUV than in the optical regime, with the qualifications that:
- old quiescent spiral arms will not be detected in the ultraviolet,
- some of the UV-bright spiral arms could very easily be hidden by dust clouds.

Even better (than ultraviolet light) for the precise delineation of Spiral Arm Patterns is the low-extinction near-infrared regime…… but I have - thus far - never seen a really sharp infrared image of M65.

The asymmetry of the two major arms could be consistent with some perturbation of M65 by an external galaxy.

One-armed spiral modes (or the presence of strong arm asymmetry between the two principal spiral arms of a galaxy) can provide supportive evidence for the idea that a galaxy has been perturbed by its neighbours, as I have argued before in IIS forums. (e.g. NGC 4027, the LMC, M96, NGC 2442, NGC 4725 , etc.)

In the case of the nearby M66, the level of arm asymmetry has been found to be large, in many & various wavelength regimes, and this fact has been used to provide some support for the idea that M66 has interacted with another galaxy…. perhaps with NGC 3628 to which it is linked by a bridge of neutral atomic hydrogen gas. (there is an outermost extremely faint arm in M66 which has no counterpart on the other side of the galaxy!)

cheers, Robert