Contribution of Bremsstrahlung Emission from Lyman-alpha Clouds to the Microwave Background Fluctuations

I calculate the contribution of Bremsstrahlung emission from Lyman-alpha absorption clouds to the brightness of the microwave sky. The calculation is based only on the assumption that the clouds below the Lyman-limit are in photoionization equilibrium with a UV background radiation, and avoids any uncertainty about the clumpiness of the gas. I predict a minimum fluctuation amplitude in the Rayleigh-Jeans regime of DeltaT/T = 10^{-5.5+-0.4}*J_21*(L/5cm)^2, which varies over characteristic angular scales of 1-100'', where L is the observed wavelength and J_21 is a weighted redshift average of the UV background intensity at the Lyman-limit in units of 10^{-21} erg cm^{-2} s^{-1} Hz^{-1} sr^{-1}. Detection of this signal can be used to map the intergalactic hydrogen distribution and to calibrate the UV background at high redshifts. Existing VLA observations constrain J_21<10^{1.3+-0.4}, unless some of the extended flat-spectrum sources which were detected are Lyman-alpha absorption systems.