Boosting line intensity map signal-to-noise ratio with the Ly-a forest cross-correlation

We forecast the prospects for cross-correlating future line intensity mapping (LIM) surveys with the current and future Ly-alpha forest measurements. Using large cosmological hydrodynamic simulations, we model the emission from the CO rotational transition in the CO Mapping Array Project LIM experiment at the 5-yr benchmark and the Ly-alpha forest absorption signal for extended Baryon Acoustic Oscillations (BOSS), Dark energy survey instrument (DESI), and Prime Focus multiplex Spectroscopy survey (PFS). We show that CO xLy-alpha forest significantly enhances the detection signal-to-noise ratio (S/N) of CO, with up to 300 per cent improvement when correlated with the PFS Ly-alpha forest survey and a 50-75 per cent enhancement with the available eBOSS or the upcoming DESI observations. This is competitive with even CO xspectroscopic galaxy surveys. Furthermore, our study suggests that the clustering of CO emission is tightly constrained by CO x Ly-alpha forest due to the increased sensitivity and the simplicity of Ly-alpha absorption modelling. Foreground contamination or systematics are expected not to be shared between LIM and Ly-alpha forest observations, providing an unbiased inference. Ly-alpha forest will aid in detecting the first LIM signals. We also estimate that [C II] x Ly-alpha forest measurements from Experiment for Cryogenic Large-Aperture Intensity Mapping and DESI/eBOSS should have a larger S/N than planned [C II] x quasar observations by about an order of magnitude.