The Tip of the Red Giant Branch as a Precision Distance Indicator

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Session 67 -- Cosmology and Distance Indicators
Oral presentation, Thursday, January 13, 2:15-3:45, Crystal Forum Room (Crystal City Marriott)

[67.06] The Tip of the Red Giant Branch as a Precision Distance Indicator

B.F.Madore (NED/IPAC/CALTECH/JPL), W.L.Freedman (OCIW)

We present an analysis of synthetic I versus (V--I) color-magnitude diagrams of Population II systems to investigate the use of the observed discontinuity in the I- band luminosity function as a precision primary distance indicator. In the simulations we quantify the effects of (1) signal-to-noise, (2) crowding, (3) population size, and (4) non-giant-branch-star contamination on the methodology adopted for detecting the discontinuity, measuring its luminosity, and estimating its uncertainty. A variety of systematic effects are observed; and these are discussed in the context of observables, such as the signal-to-noise ratio and/or surface brightness. With reasonable exposure times (of a few thousand seconds) using moderate, to large sized telescopes ($>$2.5m aperture) it is concluded, that from the ground the tip of the red-giant-branch method can be successfully used to determine distances good to $\pm$10\% for galaxies out to 3 kpc ($\mu \sim 27.5$ mag), and from space a factor of four further in distance ($\mu \sim 30.5$ mag) can be reached using HST . The method applies equally well wherever an old population of red giant stars is detected, whether that population resides in the halo of a spiral galaxy, the extended outer disk of a dwarf irregular, or in the main body of an elliptical galaxy.

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