Feasibility of JWST observations of primary distance indicators in the MW, LMC, and SMC

Determine whether the James Webb Space Telescope can conduct useful observations of primary distance indicators—specifically Cepheid variables, the tip of the red giant branch (TRGB), carbon-rich asymptotic giant branch (JAGB) stars, and Mira variables—in the Milky Way, Large Magellanic Cloud, and Small Magellanic Cloud at the requisite precision to serve as geometric anchors for the JWST-based distance ladder, given that these targets are very bright and JWST currently lacks the rapid spatial scanning and gyro-guided fast slew capabilities that enabled HST observations of these systems.

Background

Accurate calibration of the local distance ladder relies on geometric anchors observed with the same facility used to measure primary distance indicators in Type Ia supernova host galaxies. HST has successfully used four anchors (NGC 4258, Milky Way, LMC, SMC), enabling
% precision in H0 when combined with Cepheid measurements across many SN Ia hosts.

JWST’s current distance-ladder applications have been limited to NGC 4258 as the sole anchor. Extending JWST calibrations to include the MW, LMC, and SMC would improve precision and redundancy but may be challenging because JWST lacks HST’s specialized observing modes (rapid spatial scanning and gyro-guided fast slews) that were essential for bright targets in these nearby systems. Establishing JWST’s capability in these anchors is therefore a key unresolved question affecting the future precision and robustness of JWST-only H0 determinations.