Variations in Star Formation History and the Red Giant Branch Tip

We examine the reliability of the tip of the red giant branch (TRGB) as a distance indicator for stellar populations with different star formation histories (SFHs) when photometric errors and completeness corrections at the TRGB are small. In general, the TRGB-distance method is insensitive to the shape of the SFH except when it produces a stellar population with a significant component undergoing the red giant branch phase transition. The I-band absolute magnitude of the TRGB for the middle and late stages of this transition (~1.3-1.7 Gyr) is several tenths of a magnitude fainter than the canonical value of M_I ≈ -4.0. If more than ~30% of all stars formed over the lifetime of the universe are formed at these ages, then the distance could be overestimated by ~10%-25%. Similarly, the TRGB-distance method is insensitive to the metallicity distribution of stars formed except when the average metallicity is greater than ⟨[Fe/H]⟩ = -0.3. If more than ~70% of all stars formed have [Fe/H] > -0.3, the distance could be overestimated by ~10%-45%. We find that two observable quantities, the height of the discontinuity in the luminosity function at the TRGB and the median (V - I)₀ at M_I = -3.5, can be used to test whether the aforementioned age and metallicity conditions are met.