Determining the location of the tip of the red giant branch in old stellar populations: M33, Andromeda I and II

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The absolute bolometric luminosity of the point of core helium ignition in old, metal-poor, red giant stars is of roughly constant magnitude, varying only very slightly with mass or metallicity. It can thus be used as a standard candle. Here, we review the main difficulties in measuring this location in any real data set and we develop an empirical approach to optimize it for tip of the red giant branch (TRGB) analysis. We go on to present a new algorithm for the identification of the TRGB in nearby metal-poor stellar systems. Our method uses a least-squares fit of a data adaptive slope to the luminosity function in 1-mag windows. This finds the region of the luminosity function that shows the most significant decline in star counts as we go to brighter magnitudes; the base of this decline is attributed as the location of the tip. This technique then allows for the determination of realistic uncertainties which reflect the quality of the luminosity function used, but which are typically ∼0.02 mag rms + ∼0.03 mag systematic, a significant improvement upon previous methods that have used the tip as a standard candle. Finally, we apply our technique to the Local Group spiral galaxy M33 and the dwarf galaxies Andromeda I and II, and derive distance modulii of 24.50 ± 0.06 mag (794 ± 23 kpc), 24.33 ± 0.07 mag (735 ± 23 kpc) and 24.05 ± 0.06 mag (645 ± 19 kpc) respectively. The result for M33 is in excellent agreement with the Cepheid distances to this galaxy, and makes the possibility of a significant amount of reddening in this object unlikely.