Extending the M33 Extinction Map with Radio and Far-Infrared Gas Observations

As the third-largest galaxy in the Local Group, M33 requires high-precision, wide-area extinction maps to recover the intrinsic luminosity and enable accurate studies, yet the available maps remain limited in coverage. In this work, we extend the stellar-based extinction map within the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) footprint to the outer disk by calibrating empirical AV-H i and AV-total hydrogen relations in the PHATTER overlap region. Within the PHATTER footprint, we retain the stellar-based map at 6″ resolution, and outside the area covered by the stellar-based extinction map we produce an extended map at ≈12″ resolution. The resulting map preserves a continuous morphology across the PHATTER boundary and reveals clear spiral arm and interarm contrast and patchy dust substructures across a broader area. The median and maximum extinction values, median(AV)≈0.91 mag and AV,max≈2.62 mag, are consistent with the high-extinction range reported by previous M33 studies, while naturally reflecting differences in tracers and spatial coverage. The extinction map provides a practical, publicly available dataset for galaxy-wide extinction correction and for further studies in M33.