Multi-tracer intensity mapping: cross-correlations, line noise & decorrelation

Abstract

Line intensity mapping (LIM) is a rapidly emerging technique for constraining cosmology and galaxy formation using multi-frequency, low angular resolution maps. Many LIM applications crucially rely on cross-correlations of two line intensity maps, or of intensity maps with galaxy surveys or galaxy/CMB lensing. We present a consistent halo model to predict all these cross-correlations and enable joint analyses, in 3D redshift-space and for 2D projected maps. We extend the conditional luminosity function formalism to the multi-line case, to consistently account for correlated scatter between multiple galaxy line luminosities. This allows us to model the scale-dependent decorrelation between two line intensity maps, a key input for foreground rejection and for approaches that estimate auto-spectra from cross-spectra. This also enables LIM cross-correlations to reveal astrophysical properties of the interstellar medium inacessible with LIM auto-spectra. We expose the different sources of luminosity scatter or “line noise” in LIM, and clarify their effects on the 1-halo and galaxy shot noise terms. In particular, we show that the effective number density of halos can in some cases exceed that of galaxies, counterintuitively. Using observational and simulation input, we implement this halo model for the Hα, [Oiii], Lyman-α, CO and [Cii] lines. We encourage observers and simulators to measure galaxy luminosity correlation coefficients for pairs of lines whenever possible. Our code is publicly available at https://github.com/EmmanuelSchaan/HaloGen/tree/LIM. In a companion paper, we use this halo model formalism and code to highlight the degeneracies between cosmology and astrophysics in LIM, and to compare the LIM observables to galaxy detection for a number of surveys.

Astrophysics & Cosmology from Line Intensity Mapping vs Galaxy Surveys

“…In this section, we briefly summarize the halo model results derived in ref. [17]. More discussion and interpretation of these results can be found in §2 of that paper.…”

“…For cosmology, the comparison of LIM observables and galaxy detection as tracers of the matter density field was tackled in a formal and intuitive way in [1], focusing on a single pixel from an intensity map and under some simplifying approximations. The formalism we build in the companion paper [17] is perfectly suited to extend this study. We thus compare LIM and galaxy detection as tracers of the matter density field, as a function of Fourier scale, including the effects of cosmic variance, pixel-tpixel correlations, luminositydependent clustering bias and redshift-space distortions (RSD).…”

“…As in the companion paper [17], we assume throughout a flat ΛCDM cosmology from ref. [33], with Ω CDM = 0.267, Ω b = 0.0493, H 0 = 67.12 km/s/Mpc, A S = 2.3 × 10 −9 , n s = 0.9624, N eff = 3.046 with massless neutrinos.…”

The THESAN project: predictions for multi-tracer line intensity mapping in the Epoch of Reionization

“…In fact, the larger the number of emission lines used the better the estimate of the spectrum. Importantly, 𝑟 12 also determines the ability to use one line as a tracer of the other, in order to subtract foregrounds/interlopers (Schaan & White 2021), with values near one indicating good subtraction while values near zero indicate ineffective subtraction. Moreover, since the lines considered in this paper all trace slightly different gas phases in the ISM of star-forming galaxies, the correlated abundance of these lines can help constrain the ISM of high-redshift galaxies (Sun et al 2019).…”

“…It is therefore important to model the luminosities of multiple tracers arising from high-𝑧 galaxies in order to make accurate predictions for these experiments. Most current theoretical and simulation-based modelling involves using scaling relations that relate the line luminosity to the star-formation rate of the galaxy (Gong et al 2012(Gong et al , 2017Fonseca et al 2017;Heneka & Cooray 2021;Schaan & White 2021). However, most of these scaling relations come from observational estimates and/or theoretical calculations of emission line properties of low-𝑧 galaxies.…”