How Robust is the Cosmic Distance with Tip of Red Giant Branch against Stellar Population Variations?

TL;DR

This study assesses the robustness of the Tip of the Red Giant Branch (TRGB) as a standard candle, finding it remains reliable despite variations in stellar population parameters, supporting its use in precision cosmology.

Contribution

The paper quantifies how metallicity, alpha-element enhancement, age, and helium abundance affect TRGB luminosity, confirming its robustness as a distance indicator.

Findings

Metallicity and alpha-element enhancement significantly dim the TRGB luminosity.

Age and helium abundance have minor effects on TRGB brightness.

Net variation in TRGB magnitude remains below 0.028 mag under typical conditions.

Abstract

The tip of the red giant branch (TRGB) provides a key standard candle for extragalactic distance measurements and for refining the Hubble constant. We test its robustness by quantifying how metallicity, $\alpha$-element enhancement, age, and initial helium abundance modulate the TRGB luminosity, using synthetic composite color--magnitude diagrams in the $I$ and $F814W$ bands. We find that metallicity and $\alpha$-element enhancement are the primary drivers of TRGB variation, while age introduces only a modest effect and helium abundance is negligible. At fixed age and helium content, increasing the mean metallicity by 0.5 dex or the $\alpha$-element enhancement by 0.3 dex produces the well-known systematic dimming of 0.046 and 0.050 mag, respectively, in $M_I^{\rm TRGB}$, and of 0.093 and 0.044 mag, respectively, in $M_{F814W}^{\rm TRGB}$. By comparison, changes in age of 3~Gyr and in initial helium abundance of 0.10 yield minor luminosity shifts, with average changes of 0.031 and 0.009~mag, respectively, in $M_I^{\rm TRGB}$, and of 0.035 and 0.027 mag, respectively, in $M_{F814W}^{\rm TRGB}$, substantially smaller than those caused by variations in metallicity or $\alpha$-element enhancement. For mixed stellar populations under typical stellar-halo metallicity conditions, the net variation in $M_I^{\rm TRGB}$ arising from each combination of the $\alpha$-element enhancement, age, and initial helium abundance remains below 0.028~mag, well within reported systematic uncertainties. Together, these results reaffirm the TRGB as a highly robust distance indicator and support its continued use as an independent anchor for precision cosmology in the era of the Hubble-tension debate.