# Tomographic Angular Cross Correlation of Emission-Line Galaxies and CMB Lensing

Probing the early era of dark energy

## Motivation

Emission-Line Galaxies (ELGs) will form the bulk of the DESI data, with a plan to observe 17 million of them by the end of Year 5. These galaxies map will map $0.6 < z < 1.6$, corresponding to the era when the cosmic acceleration (due to dark energy) was just ramping up.

By cross-correlating the Planck CMB lensing map with the ELG overdensity map, we construct a more unbiased map of the large-scale structures in which these ELGs reside. Because the noises in these maps are uncorrelated, the cross power spectrum, $C_{\ell}^{g \kappa}$ is not affected by the systematics that affect the auto power spectra, $C_{\ell}^{gg}$ and $C_{\ell}^{\kappa \kappa}$ respectively.