Extracting non-Gaussian information of line-intensity maps with weighted skew spectra

Abstract

The bispectrum of line-intensity maps carries cosmological and astrophysical information beyond the power spectrum, but its large data vector hampers the analysis of survey data. Skew spectra address this by compressing the bispectrum into a handful of power-spectrum-like observables, cross-spectra of the observed field with filtered squares of it. We present the first application of optimally weighted skew spectra to line-intensity mapping of galaxy-hosted emission, where all clustering and shot-noise terms are luminosity weighted and the mean intensity is unknown. We perform idealized real-space Fisher forecasts for a SPHEREx-like survey of H$α$ emission at $0.2 \lesssim z \lesssim 4.6$, in its deep fields and in a noisier all-sky configuration. First, the compression carries over to intensity maps: on perturbative scales and for a fixed redshift evolution of the emitters, four skew spectra, one of them matched to the shot noise, recover the bispectrum constraints on biases, shot noise and cosmological parameters to within a few percent. Second, the joint analysis of the power spectrum and the skew spectra measures the ratios of the second-order biases to the linear bias in each redshift bin. Within a bin the biases are degenerate with the unknown mean intensity, but their ratios are not, so their redshift evolution is measured without a model for the emitters. For emission that depends on halo mass alone, these ratios are determined by luminosity-weighted averages of the halo biases, so their evolution can test the halo-emission connection, including dependence on assembly history or environment. The bias ratios are not accessible to the tree-level real-space power spectrum alone. At $z \lesssim 3$ an all-sky SPHEREx-like configuration can detect departures of order $0.1$ in the bias ratios at $3σ$, while deeper fields perform better in the highest-redshift bins.