The Infrared Tully-Fisher Relation in the Ursa-Major Cluster

We present new magnitudes derived from 1.65 micron images for 23 galaxies in the Ursa Major cluster. Magnitudes now exist for all but one spiral meeting our criteria for cluster membership and having HI velocity width greater than 187 km/s and inclination greater than 45 deg. These spirals fit a Tully-Fisher relation with dispersion in intrinsic magnitudes (after known observational uncertainties and the effect of cluster depth are removed) of 0.36 and a slope of 10.2 +/- 0.6. The magnitude dispersion is smaller than found in the Virgo cluster but still significantly larger than claimed by some authors. We find a hint that the Tully-Fisher relation may turn over at the bright end. Adding the central surface brightness of the disk as a third parameter flattens the slope of the Tully- Fisher relation and may give a distance estimate with slightly less dispersion, but the significance of the decrease must be tested on an independent sample.