TESTS OF THE TULLY-FISHER RELATION .1. SCATTER IN INFRARED MAGNITUDE VERSUS 21 CM WIDTH

We examine the precision of the Tully-Fisher relation (TFR) using a sa mple of galaxies in the Coma region of the sky, and find that it is go od to 5% or better in measuring relative distances. Total magnitudes a nd disk axis ratios are derived from H and I band surface photometry, and Arecibo 21 cm profiles define the rotation speeds of the galaxies. Using 25 galaxies for which the disk inclination and 21 cm width are well defined, we find an rms deviation of 0.10 mag from a linear TFR w ith dI/d(log W(c)) = -5.6. Each galaxy is assumed to be at a distance proportional to its redshift, and an extinction correction of 1.4(1 - b/a) mag is applied to the total I magnitude. The measured scatter is <0.15 mag using milder extinction laws from the literature. The I band TFR scatter is consistent with measurement error, and the 95% CL limi ts on the intrinsic scatter are 0-0.10 mag. The rms scatter using H ba nd magnitudes is 0.20 mag (N=17). The low width galaxies have scatter in H significantly in excess of known measurement error, but the highe r width half of the galaxies have scatter consistent with measurement error. The H band TFR slope may be as steep as the I band slope. As th e first applications of this tight correlation, we note the following: (1) the data for the particular spirals commonly used to define the T FR distance to the Coma cluster are inconsistent with being at a commo n distance and are in fact in free Hubble expansion, with an upper lim it of 300 km s-1 on the rms peculiar line-of-sight velocity of these g as-rich spirals; and (2) the gravitational potential in the disks of t hese galaxies has typical ellipticity <5%. The published data for thre e nearby spiral galaxies with Cepheid distance determinations are inco nsistent with our Coma TFR, suggesting that these local calibrators ar e either ill-measured or peculiar relative to the Coma Supercluster sp irals, or that the TFR has a varying form in different locales.