The OH line contamination of 21 cm intensity fluctuation measurements for z=1~4

The large-scale structure of the Universe can be mapped with unresolved intensity fluctuations of the 21 cm line. The power spectrum of the intensity fluctuations has been proposed as a probe of the baryon acoustic oscillations at low to moderate redshifts with interferometric experiments now under consideration. We discuss the contamination to the low-redshift 21 cm intensity power spectrum generated by the 18 cm OH line since the intensity fluctuations of the OH line generated at a slightly higher redshift contribute to the intensity fluctuations observed in an experiment. We assume the OH megamaser luminosity is correlated with the star formation rate, and use the simulation to estimate the OH signal and the spatial anisotropies. We also use a semi-analytic simulation to predict the 21 cm power spectrum. At z=1 to 3, we find that the OH contamination could reach 0.1% to 1% of the 21 cm rms fluctuations at the scale of the first peak of the baryon acoustic oscillation. When z>3 the OH signal declines quickly, so that the contamination on the 21-cm becomes negligible at high redshifts.