The Renaissance of the Planetary Nebula Luminosity Function (PNLF)

Over the past 35 years, the PNLF has primarily been used as a distance indicator. Narrow-band imaging with 4-8m class telescopes has enabled distance measurements for over 50 galaxies, but its application was largely limited in range to the Virgo and Fornax clusters. Recently, though, the MUSE integral field spectrograph, along with techniques such as the narrow-band differential filter method, allowed PN surveys to reach much greater depths and with greater efficiency than traditional imaging. The increased sensitivity and improved photometric accuracy have more than doubled the effective range of the PNLF method to ~40 Mpc. At these distances, the PNLF can not only calibrate the luminosities of SN Ia across a range comparable to Cepheid variables, but also measure isolated galaxy distances in the relatively unperturbed Hubble flow, thus providing a direct measurement of H0. The PNLF also serves as an important independent check for systematics across distance methods (e.g., Cepheids, TRGB, SBF). It is also applicable to both early- and late-type galaxies. We will review the initial success of the MUSE-PNLF project and outline the steps needed to confirm that the PNLF achieves a precision comparable to that of the Cepheid, TRGB, and SBF methods. We propose a multi-year project to observe up to 50 carefully selected galaxies with VLT/MUSE to significantly enhance the reliability of the distance scale and our confidence in resolving the Hubble Tension.