Optimizing the tip of the red giant branch distance estimator

Abstract

Until very recently, our knowledge of the local peculiar velocity field has been severely hampered by the lack of reliable distance measurements. HST has dramatically changed this situation, allowing astronomers to obtain accurate distances to more than 150 nearby galaxies. This number could easily reach 400 if enough observing time would be dedicated to snapshot observation of the objects in the catalog of Karachentsev et al (2004). Such a dense grid of objects correctly placed in their 3D position would provide key information on the amplitude of peculiar motions, the radial domain of bound groups, the clustering and morphological segregation properties of galaxies, and the incidence of extreme dwarfs galaxies. The key instrument to measure distances with HST is the Tip of the Red Giant Branch technique. The full exploitation of this powerful distance estimator requires a deeper understanding of the possible sources of errors and biases, such as the absolute calibration of the I-band magnitude of the tip and its dependency on age and metallicity of the underlying population, the possible contamination by AGB stars, the breakdown of the methodology in sparsely populated colour-magnitude diagrams and when the tip is close to the photometric limit.