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Foreground emission makes it difficult to detect the highly-redshiftedcosmological 21 cm signal at any frequency. However, at low frequenciesforegrounds are likely to become optically thick, which would make itcompletely impossible to see a 21 cm signal behind them. To find out whichregions of the sky might be optically thick for the highest redshifts of the 21cm signal, we fit the measurements from LWA1 and the Haslam 408 MHz map with atwo-component spectral model and calculate the frequency-dependent foregroundoptical depth point-by-point across the sky. Limitations of the current dataprevent us from making any strong conclusions at high statistical significance,but there is suggestive evidence ($\sim1\sigma$) that as much as 25% of the skycould be obscured for the highest redshift 21 cm signals.

The Optical Depth of Foregrounds for the Highest Redshift 21 cm Signals