Stairway to axions: the cross-correlation of birefringence and galaxies from NPIPE and Quaia

We present the first measurement of the cross-correlation between anisotropic birefringence and galaxy number counts, utilizing polarization data from Planck NPIPE and the Quaia quasar catalog. By employing a QML/pseudo-Cℓ combined estimator, we compute the angular power spectrum up to ℓ=191 from birefringence and clustering maps at Nside=64. Our analysis indicates that the observed spectrum is well consistent with the null-hypothesis, with a probability to exceed of 37% and an estimated scale-invariant amplitude of Aℓ=(2.22±2.09)×10−4deg, at the 68% confidence level. Finally, we derive constraints on the axion-parameters within an early dark energy model of birefringence. Our findings reveal an unprecedented upper bound on the axion-photon coupling down to gϕγ=10−15GeV−1 for masses around 10−32eV and high initial misalignment angles. This result opens a previously unexplored window in parameter space, providing the first constraint in this ultra-light mass regime.