6 Conclusions

The main conclusions of this paper can be summarized as follows:

1.

For the galaxies NGC 2403 and NGC 7331, MOND rotation curves agree acceptably with the observed curves when these galaxies are taken at the Cepheid-based distance.

2.

For NGC 3198 at the Cepheid-based distance of 13.8 Mpc, the MOND curve shows small (<10 km s^-1) but significant systematic deviations from the observed curve.

3.

If the distance to NGC 3198 is taken to be 12.5 Mpc, or 10% less than the Cepheid-based distance, the MOND curve is an acceptable representation of the observed curve. This lower distance is probably within the uncertainties of the Cepheid method.

4.

These conclusions are unaltered by utilizing recent near-infrared photometry of NGC 3198 which does show evidence for a small central bulge and bar component.

5.

For NGC 2841, the rotation curve predicted by MOND when the galaxy is taken to be at the upper limit on the Cepheid-based distance (15.6 Mpc) remains inconsistent with the observed curve, with systematic deviations of more than 30 km s^-1.

6.

The smallest distance for which the MOND curve is compatible with the observed curve (given the uncertainties involved in the tilted ring technique for modelling warps), is 17 Mpc or 20% larger than the Cepheid-based distance. The preferred MOND distance is 23 Mpc.

7.

The TF distance to NGC 2841, based upon the Cepheid-re-calibrated TF relation is 24 Mpc. If the distance to this galaxy is really 14.1 Mpc, then it would deviate from the mean I-band TF relation by more than 1 mag.

8.

NGC 2841 has been the host of a type Ia supernova, 1999by. If this galaxy is at the Cepheid-based distance, this would be one of the least intrinsically luminous supernovae ever observed. Calibrating the peak luminosity by the Phillips relation, the SN-based distance is 23.5 Mpc.

It is clear that NGC 2841 remains a critical case for MOND. The discrepancy between Cepheid-based distance and both the TF and SNIa based distances to NGC 2841 suggests that there may be a problem with the derived Cepheid-based distance.

In general, it is evident that accurate distance determinations to nearby galaxies are extremely relevant to the question of the viability of MOND. MOND, as a modification of Newtonian dynamics attached to an acceleration scale, is far more fragile than the dark matter hypothesis in this regard. It would be useful to obtain more Cepheid-based distances to the sample of galaxies with well-observed rotation curves. Particularly useful would be a Cepheid distance estimate to the Ursa Major cluster as many of these galaxies have well-measured rotation curves and near-infrared photometry.

Acknowledgements

We are grateful to M. Milgrom for a critical reading of the manuscript and we thank the referee A. Bosma, for giving useful comments which have improved the paper.