Spiral galaxy distance indicators based on near-infrared photometry

We compare two methods of distance determination to spiral galaxies using optical/nearinfrared (NIR) observations, the (I - K) versus M_K colour-absolute magnitude (CM) relation and the I- and K-band Tully-Fisher relation (TFR). Dust-free colours and NIR absolute magnitudes greatly enhance the usefulness of the NIR CM relation as a distance indicator for moderately to highly inclined spiral galaxies in the field (inclinations between ∼ 80° and 90°); 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 to field spiral galaxies with M_K > -25.5, to at least M_K ≈ -20. 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 provide 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 magnitudes is ∼ 0.5 mag, yielding a lower limit to the accuracy of distance determinations of the order of 25 per cent. Although we find an unusually low scatter in the TFR (probably a statistical accident), a typical scatter in the TFR would yield distances to our sample galaxies with uncertainties of only ∼ 15 per cent. However, one of the main advantages of the use of the NIR CM relation is that we need only photometric data to obtain distance estimates; use of the TFR requires additional kinematic data, although it can be used to significantly greater distances.