Securing the TRGB Distance Indicator: A Pre-Requisite for a JWST Measurement of H_0

The tip of the red giant branch (TRGB) is arguably one of the most precise and efficient distance indicators for nearby galaxies. TRGB distances are a key rung in the distance ladder, contributing to a precision measurement of the Hubble constant (H_0), independent of Cepheid variable stars. These TRGB distances are measured with HST F814W (i.e., I-band) observations where the luminosity of the TRGB has only a modest and well-characterized dependence on metallicity.

In the NIR, the TRGB is up to 2 mag brighter offering profound observational gains. Simply put, JWST is capable of measuring TRGB distances out to ~50 Mpc over a 125x greater volume than HST, and reaching galaxies with diverse properties across all morphological types. The TRGB with JWST will become the primary distance indicator in the nearby universe.

However, the luminosity of the TRGB in the NIR is expected to have a greater dependence on metallicity and possibly stellar age. Thus, for JWST to be used for TRGB distance work, it must be empirically calibrated and robustly tested. We propose observing 4 nearby galaxies that have existing HST ACS and WFC3 IR observations from a JWST preparatory program. We will calibrate the TRGB in the F090W, F150W, F277W, and F356W NIRCam filters and the F090W and F150W NIRISS filters in parallel. We will quantify the precision of TRGB distances obtained with simultaneously imaged F090W+F277W filters, which offers a ~50% increase in efficiency of JWST observations for distance work. A calibration in this first cycle of JWST will enable TRGB distances with JWST throughout the mission lifetime and charter a path to reduced uncertainties in the local measurement of H_0.