Foreground contamination in Ly-alpha intensity mapping during the epoch of reionization

The intensity mapping of Ly-alpha emission during the epoch of reionization (EoR) will be contaminated by foreground emission lines from lower redshifts. We calculate the mean intensity and power spectrum of Ly-alpha emission at z~7, and estimate the uncertainties according to the relevant astrophysical processes. We find that the low-redshift emission lines from 6563 A H-alpha, 5007 A [OIII] and 3727 A [OII] will be the strong contaminants on the observed Ly-alpha power spectrum. We make use of both the star formation rate (SFR) and luminosity functions (LF) to estimate the mean intensity and power spectra of the three foreground lines at z~0.5 for H-alpha, z~0.9 for [OIII] and z~1.6 for [OII], as they will contaminate the Ly-alpha emission at z~7. The [OII] line is found to be the strongest. We analyze the masking of the bright survey pixels with a foreground line above some line intensity threshold as a way to reduce the contamination in the intensity mapping survey. We find that the foreground contamination can be neglected if we remove the pixels with fluxes above 1.4x10^-20 W/m^2.