We forecast the prospects for cross-correlating future line intensity mapping
(LIM) surveys with the current and future Ly-α forest data. We use large
cosmological hydrodynamic simulations to model the expected emission signal for
the CO rotational transition in the COMAP LIM experiment at the 5-year
benchmark and the Ly-α forest absorption signal for various surveys,
including eBOSS, DESI, and PFS. We show that CO×Ly-α forest can
significantly enhance the detection signal-to-noise ratio of CO, with a 200
to 300% improvement when cross-correlated with the forest observed in the
Prime Focus Spectrograph (PFS) survey and a 50 to 75% enhancement for the
currently available eBOSS or the upcoming DESI observations. We compare to the
signal-to-noise improvements expected for a galaxy survey and show that
CO×Ly-α is competitive with even a spectroscopic galaxy survey in
raw signal-to-noise. Furthermore, our study suggests that the clustering of CO
emission is tightly constrained by CO×Ly-α forest, due to the
increased signal-to-noise ratio and the simplicity of Ly-α absorption
power spectrum modeling. Any foreground contamination or systematics are
expected not to be shared between LIM surveys and Ly-α forest
observations; this provides an unbiased inference. Our findings highlight the
potential benefits of utilizing the Ly-α forest to aid in the initial
detection of signals in line intensity experiments. For example, we also
estimate that [CII]×Ly-α forest measurements from EXCLAIM and
DESI/eBOSS, respectively, should have a larger signal-to-noise ratio than
planned [CII]×quasar observations by about an order of magnitude. Our
results can be readily applied to actual data thanks to the observed quasar
spectra in eBOSS Stripe 82, which overlaps with several LIM surveys.Comment: Codes and the produced data are available at
https://github.com/qezlou/lal