Spiral galaxy distance indicators based on near-infrared photometry

We compare two methods of distance determination to spiral galaxies using optical/near-infrared (NIR) observations, the (I–K) versus MK 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 to similar galaxy samples published previously. The CM relation can be used to determine distances to field spiral galaxies with MK > −25.5, to at least MK ≈ −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 done 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 to 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 on the order of 25%. 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%. However, one of the main advantages of the use of the NIR CM relation is that we only need photometric data to obtain distance estimates; use of the TFR requires additional kinematic data, although it can be used to significantly greater distances.