Canceling Out Intensity Mapping Foregrounds

Intensity mapping of the 21 cm line has arisen as a powerful probe of the high-redshift Universe, but its potential is limited by extremely bright foregrounds and high source confusion. We propose a new analysis which can help solve both problems. From the combination of an intensity map with an overlapping galaxy survey, we construct a new one-point statistic which is unbiased by foregrounds and contains information left out of conventional analyses. We show that our method can measure the HI mass function with unprecedented precision using observations similar to recent 21 cm detections.

Figure 1

Sample CVR analysis. (Left panel) Sample CVIDs for a model of Parkes-2dF data with noise and residual foregrounds modeled by a 10 mK Gaussian PDF, for voxels with $N_{\det }=0$ (blue curve) and 1 (orange curve). Solid curves show the theoretical expectation, and points show a toy simulation as described in the text. Histograms are normalized to sum to unity. See below for details of the astrophysical modeling. (Right panel) CVR computed from the signal model only (solid curve) along with that estimated from the simulation. The gray band shows the predicted $1\sigma$ error from the model. Note that CVR errors are highly correlated, leading to an offset at moderate $\mathcal{T}$. Note also that we plot only the real part of the complex CVR.

Figure 2

(Left column) Forecasted CVR constraints on the HIMF of all galaxies, (center column) those undetected by the optical survey, and (right column) those detected optically. Black solid lines show our fiducial model [Eqs. (1), (9), and (10) with ALFALFA parameters], the other lines show $1\sigma$ prior uncertainties for the ALFALFA priors (red solid) and the $10\times$ ALFALFA priors (blue dashed). Shaded regions show the $1\sigma$ regions after our Fisher forecast, both with the ALFALFA (red) and $10\times$ ALFALFA priors (blue). The upper row shows the case with modest foreground cleaning (${\sigma }_{\mathrm{FG}}=35\mathrm{mK}$), the lower row the one with stronger cleaning (${\sigma }_{\mathrm{FG}}=10\mathrm{mK}$).