Refining a New Probe of the Hubble Tension: Testing Homogeneity Assumptions in the J-region Asymptotic Giant Branch Method

The J-region Asymptotic Giant Branch (JAGB) method leverages the near-constant mean magnitudes of carbon-rich AGB stars to calibrate Type Ia supernovae out to ~40 Mpc, making it a powerful independent rung on the extragalactic distance ladder to measure the Hubble constant (H0) and probe the origins of the Hubble Tension. A critical assumption of JAGB consistency has recently come under scrutiny, with luminosity function asymmetries producing systematic offsets in H0 of up to 7 km/s/Mpc. To directly test this assumption, we conducted a spectroscopic program with the Kast double spectrograph at Lick Observatory targeting photometrically-selected J-region stars. We present the initial catalog of spectra that will be used to constrain the JAGB systematic error budget.