Enhanced foreground mitigation in thermal SZ Compton- y maps via polarization and deprojection

Abstract Residual foreground contamination in thermal Sunyaev-Zeldovich (SZ) Compton- y parameter maps ( y -maps) arises mainly from Galactic emissions — thermal dust and synchrotron radiation — on large angular scales, and from cosmic infrared background (CIB) anisotropies on small scales. Unlike the thermal SZ effect, Galactic foregrounds are strongly polarized. Exploiting this distinction, we introduce a hybrid Needlet Internal Linear Combination (Hybrid NILC) method that combines Planck total-intensity and polarization frequency maps in the component-separation pipeline, thereby improving the suppression of residual Galactic emission while preserving the unpolarized SZ signal by leveraging the intrinsic TE and TB correlations of thermal dust and synchrotron. Using Planck PR4 data, we find that the Hybrid NILC y -map exhibits about 40 % lower cross-correlation with the IRAS dust tracer than the standard temperature-only Planck y -map, indicating reduced residual Galactic contamination. Simulations further indicate that, for future high-sensitivity surveys such as LiteBIRD , the Hybrid NILC will become increasingly effective at suppressing Galactic residuals. We further address small-scale extragalactic contamination by selectively deprojecting specific moments of the CIB using a Constrained Hybrid NILC variant, achieving an improved balance between CIB suppression and noise penalty compared to previous implementations in the literature. These novel approaches — particularly the joint use of temperature and polarization in component separation — offer a powerful framework for disentangling polarized and unpolarized signals.