Spiral galaxy distance indicators based on near-infrared photometry

We compare two methods of distance determination to spiral galaxies using o ptical/near-infrared (NIR) observations, the (I-K) versus M-K colour-absolu te magnitude (CM) relation and the I- and K-band Tully-Fisher relation (TFR ). Dust-free colours and NIR absolute magnitudes greatly enhance the usefulnes s of the NIR CM relation as a distance indicator for moderately to highly i nclined spiral galaxies in the field (inclinations between similar to 80 de grees and 90 degrees); by avoiding contamination by dust the scatter in the CM relation is significantly reduced, compared with similar galaxy samples published previously. The CM relation can be used to determine distances t o field spiral galaxies with M-K > -25.5, to at least M-K approximate to -2 0. Our results, supplemented with previously published observations for which we can - to some degree - control the effects of extinction, are consistent with a universal nature of the CM relation for field spiral galaxies. High-resolution observations made with the Hubble Space Telescope can provi de a powerful tool to calibrate the relation and extend the useful distance range by more than a factor of 2 compared with ground-based observations. The intrinsic scatter in the NIR CM relation in the absolute K-band magnitu des is similar to 0.5 mag, yielding a lower limit to the accuracy of distan ce determinations of the order of 25 per cent. Although we find an unusually low scatter in the TFR (probably a statistica l accident), a typical scatter in the TFR would yield distances to our samp le galaxies with uncertainties of only similar to 15 per cent. However, one of the main advantages of the use of the NIR CM relation is that we need o nly photometric data to obtain distance estimates; use of the TFR requires additional kinematic data, although it can be used to significantly greater distances.