SIMPLE: Simple Intensity Map Producer for Line Emission

We present the Simple Intensity Map Producer for Line Emission (SIMPLE), a public code for quickly simulating mock line-intensity maps, and an analytical framework for modeling intensity maps including observational effects. SIMPLE can be applied to any spectral line sourced by galaxies. The SIMPLE code is based on lognormal mock catalogs of galaxies including positions and velocities and assigns luminosities following the luminosity function. After applying a selection function to distinguish between detected and undetected galaxies, the code generates an intensity map, which can be modified with anisotropic smoothing, noise, a mask, and sky subtraction, and calculates the power spectrum multipoles. We show that the intensity autopower spectrum and the galaxy-intensity cross-power spectrum agree well with the analytical estimates in real space. We derive and show that the sky subtraction suppresses the intensity autopower spectrum and the cross-power spectrum on scales larger than the size of an individual observation. As an example application, we make forecasts for the sensitivity of an intensity mapping experiment similar to the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) to the cross-power spectrum of Ly$α$-emitting galaxies and the Ly$α$ intensity. We predict that HETDEX will detect the galaxy-intensity cross-power spectrum on scales of $0.04\, h\,\mathrm{Mpc}^{-1} < k < 1\, h\,\mathrm{Mpc}^{-1}$.