Thermal Sunyaev-Zel'dovich cross-correlations with unWISE galaxies: disentangling radio contamination, dust properties, and electron pressure

Abstract

Cross-correlations between the thermal Sunyaev-Zel'dovich (tSZ) effect and galaxy surveys provide a sensitive probe of hot gas in low-mass halos, but on the angular scales of greatest astrophysical interest they are also highly vulnerable to residual foreground contamination. We analyze the cross-correlation of two low-redshift unWISE galaxy samples with Planck PR4 and ACT DR6 microwave temperature sky maps directly in harmonic space, fitting simultaneously for correlated tSZ, cosmic infrared background (CIB), and radio emission. Using the nine Planck bands to perform model selection, we find that a three-component model consisting of tSZ, radio emission, and a CIB amplitude term is strongly preferred over a model that omits radio contamination, with a significance of $9.5σ$ for the unWISE Low-z sample and $11σ$ for the unWISE Mid-z sample respectively. For the unWISE Low-z sample ($\bar{z}=0.14$) the preferred \textit{Planck} fit gives an effective CIB emissivity index $β_0=1.79\pm0.29$, an effective CIB dust temperature $T_0=22.0\pm5.1 K$, and a radio spectral index $β_r=-2.18\pm0.24$; for the unWISE Mid-z sample ($\bar{z}=0.23$) we find $β_0=1.41\pm0.13$, $T_0=28.0\pm4.2 K$, and $β_r=-2.62\pm0.26$. We apply this radio-inclusive recipe to the ACT+Planck 90, 150, and 220 GHz maps to obtain the best small-scale measurements. This removes the apparent negative galaxy-tSZ cross-correlation seen in the fiducial ACT DR6 NILC reconstruction and in no-radio fits. The cleaned tSZ cross unWISE spectra remain positive to $\ell \simeq 6000$ and are broadly consistent with the Planck-only reconstruction on overlapping scales. We show that these radio-cleaned spectra are well described by a conventional halo model calibrated to the galaxy population and a two-parameter generalized NFW electron pressure profile.