Microwave background temperature at a redshift of 6.34 from H2O absorption
Dominik A. Riechers, Axel Weiss, Fabian Walter, Christopher L. Carilli, P. Cox, Roberto Decarli, R. Neri
Abstract
Abstract Distortions of the observed cosmic microwave background provide a direct measurement of the microwave background temperature at redshifts from 0 to 1 (refs. 1,2 ). Some additional background temperature estimates exist at redshifts from 1.8 to 3.3 based on molecular and atomic line-excitation temperatures in quasar absorption-line systems, but are model dependent 3 . No deviations from the expected (1 + z ) scaling behaviour of the microwave background temperature have been seen 4 , but the measurements have not extended deeply into the matter-dominated era of the Universe at redshifts z > 3.3. Here we report observations of submillimetre line absorption from the water molecule against the cosmic microwave background at z = 6.34 in a massive starburst galaxy, corresponding to a lookback time of 12.8 billion years (ref. 5 ). Radiative pumping of the upper level of the ground-state ortho-H 2 O(1 10 –1 01 ) line due to starburst activity in the dusty galaxy HFLS3 results in a cooling to below the redshifted microwave background temperature, after the transition is initially excited by the microwave background. This implies a microwave background temperature of 16.4–30.2 K (1 σ range) at z = 6.34, which is consistent with a background temperature increase with redshift as expected from the standard ΛCDM cosmology 4 .