Clustering redshifts with the 21cm-galaxy cross-bispectrum

TL;DR

This paper investigates how the 21cm-galaxy cross-bispectrum can improve redshift calibration in cosmological surveys, overcoming foreground contamination issues that hinder traditional two-point correlation methods.

Contribution

It demonstrates that the bispectrum can effectively calibrate photometric galaxy redshift distributions for future surveys, especially when two-point functions are limited by foreground contamination.

Findings

Bispectrum calibrates redshift distributions accurately.

Calibration effectiveness depends on photometric redshift width and scale.

Scales k > 0.3 h/Mpc are crucial for constraints.

Abstract

The cross-correlation between 21-cm intensity mapping experiments and photometric surveys of galaxies (or any other cosmological tracer with a broad radial kernel) is severely degraded by the loss of long-wavelength radial modes due to Galactic foreground contamination. Higher-order correlators are able to restore some of these modes due to the non-linear coupling between them and the local small-scale clustering induced by gravitational collapse. We explore the possibility of recovering information from the bispectrum between a photometric galaxy sample and an intensity mapping experiment, in the context of the clustering-redshifts technique. We demonstrate that the bispectrum is able to calibrate the redshift distribution of the photometric sample to the required accuracy of future experiments such as the Rubin Observatory, using future single-dish and interferometric 21-cm observations, in situations where the two-point function is not able to do so due to foreground contamination. We also show how this calibration is affected by the photometric redshift width $\sigma_{z,0}$ and maximum scale $k_{\mathrm{max}}$. We find that it is important to reach scales $k \gtrsim 0.3\,h\,\mathrm{Mpc}^{-1}$, with the constraints saturating at around $k\sim 1\,h\,\mathrm{Mpc}^{-1}$ for next-generation experiments.