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

Abstract: 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-$C_\ell$ combined estimator, we compute the angular power spectrum up to $\ell=191$ from birefringence and clustering maps at $N_{\rm side}=64$. 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 $$A<sup>{\mathcal{D}_\ell}=(2.22\pm2.09)\times10<sup>{-4}\,\text{deg}$$, 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 $g_{\phi\gamma}=10<sup>{-15}\,\text{GeV}<sup>{-1}$ for masses around $10<sup>{-32}\,\text{eV}$ 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.