Microwave Background Temperature at a Redshift of 6.34 from H2O Absorption

TL;DR

This paper reports the first measurement of the cosmic microwave background temperature at a redshift of 6.34 using water absorption lines, confirming the expected increase with redshift in line with standard cosmological models.

Contribution

It provides a novel direct measurement of the microwave background temperature at high redshift using water absorption, extending observational constraints into the matter-dominated era.

Findings

Measured T_CMB at z=6.34 as 16.4-30.2 K

Confirmed T_CMB scales with (1+z) as predicted by standard cosmology

Detected water absorption influenced by starburst activity in a distant galaxy

Abstract

Distortions of the observed cosmic microwave background imprinted by the Sunyaev-Zel'dovich effect toward massive galaxy clusters due to inverse Compton scattering of microwave photons by high-energy electrons provide a direct measurement of the microwave background temperature at redshifts from 0 to 1. 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. To date, no deviations from the expected (1+z) scaling behavior of the microwave background temperature have been seen, but the measurements have not extended deeply into the matter-dominated era of the universe at redshifts z>3.3. Here we report the detection of sub-millimeter 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 Gyr. Radiative pumping of the upper level of the ground-state ortho-H2O(110-101) 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. The strength of this effect implies a microwave background temperature of 16.4-30.2 K (1-sigma range) at z=6.34, which is consistent with a background temperature increase with redshift as expected from the standard CDM cosmology.