Doppler bias: impact of peculiar velocities on color selection and the large scale structure of galaxy surveys

Lightcone selection effects on cosmic observables must be precisely accounted for in the next generation of surveys, including the Dark Energy Spectroscopic Instrument (DESI) survey. This will allow us to correctly model the data and extract subtle shifts from general-relativistic effects. We examine the effects of peculiar velocities on color selection in spectroscopic galaxy surveys, with a focus on their implications for the galaxy clustering dipole P1(k). Using DESI Emission Line Galaxy (ELG) targets, we show that peculiar velocities can shift spectral emission features into or out of filter bands, modifying galaxy colors and thereby changing galaxy selection. This phenomenon mimics the effect of evolution bias, and we refer to it as the Doppler bias, bD. The Doppler bias is of comparable size to the evolution bias at 0.8 < z < 1, where it is largest. This enhances the ELG-LRG (Luminous Red Galaxy) cross-correlation dipole by 25–50%. This could be detectable at the ∼ 6σ level for the full DESI survey. Additionally, we found that our bD estimate is impacted by the incompleteness of the parent ELG sample. Therefore, this work highlights the essential need for careful consideration of spectral-dependent biases caused by peculiar velocities during the selection phase of galaxy surveys, to enable unbiased analyses.