ar X iv : a st ro - p h / 02 08 17 8 v 2 5 O ct 2 00 2 GAIA and the Extragalactic Distance Scale

The local expansion field (v220 < 1200 kms ) and the cosmic expansion field out to 30 000 kms are characterized by H0 = 58 [km s −1 Mpc]. While the random error of this determination is small (±2 units), it may still be affected by systematic errors as large as ±10%. The local expansion is outlined by Cepheids and by Cepheid-calibrated TF distances of a complete sample of field galaxies and by nearby groups and clusters; the cosmic expansion is defined by Cepheid-calibrated SNe Ia. The main source of systematic errors are therefore the shape and the zero point of the P-L relation of Cepheids and its possible dependence on metallicity. GAIA will essentially eliminate these systematic error sources. Another source of systematic error is due to the homogenization of SNe Ia as to decline rate ∆m15 and color (B − V ). GAIA will discover about half of the 2200 SNe Ia which will occur during a four-year lifetime within 10 000 kms. For many of them groundbased follow-up will provide useful photometric parameters (and spectra), which will allow to fix the dependence of the SNe Ia luminosity on ∆m15 and (B−V ) with high accuracy. At the same time they will yield exquisite distances to a corresponding number of field galaxies. — GAIA will also revolutionize the very local distance scale by determining fundamental distances of the companion galaxies of the Milky Way and even of some spirals inand possibly outside the Local Group from their rotation curves seen in radial velocities and proper motions. Moreover, GAIA will obtain trigonometric parallaxes of RR Lyrae stars, of red giants defining the TRGB, of stars on the ZAMS, of White Dwarf defining their cooling sequence, and of globular clusters, and determine the metallicity dependence of these distance indicators. It will thus establish a self-controlling network of distance indicators within the Local Group and beyond.