Cross-correlation of Planck cosmic microwave background lensing with DESI galaxy groups

We measure the cross-correlation between galaxy groups constructed from DESI Legacy Imaging Survey DR8 and Planck cosmic microwave background (CMB) lensing, over overlapping sky area of 16 876 deg(2). The detections are significant and consistent with the expected signal of the large-scale structure of Universe, over group samples of various redshift, mass, and richness N-g, and over various scale cuts. The overall signal-to-noise ratio is 40 for a conservative sample with N-g >= 5, and increases to 50 for the sample with N-g >= 2. Adopting the Planck 2018 cosmology, we constrain the density bias of groups with N-g >= 5 as b(g) = 1.31 +/- 0.10, 2.22 +/- 0.10, and 3.52 +/- 0.20 at 0.1 < z <= 0.33, 0.33 < z <= 0.67, and 0.67 < z <= 1, respectively. The group catalogue provides the estimation of group halo mass and therefore allows us to detect the dependence of bias on group mass with high significance. It also allows us to compare the measured bias with the theoretically predicted one using the estimated group mass. We find excellent agreement for the two high-redshift bins. However, it is lower than the theory by similar to 3 sigma for the lowest redshift bin. Another interesting finding is the significant impact of the thermal Sunyaev Zel'dovich. It contaminates the galaxy group-CMB lensing cross-correlation at similar to 30 per cent level, and must be deprojected first in CMB lensing reconstruction.