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 dependent3. No deviations from the expected (1 + z) scaling behaviour of the microwave background temperature have been seen4, 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-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. This implies a microwave background temperature of 16.4–30.2 K (1σ range) at z = 6.34, which is consistent with a background temperature increase with redshift as expected from the standard ΛCDM cosmology4.
Highlights
The absorption of photons from the CMB radiation field appreciably populates the H2O 110 level
These two processes result in an excitation temperature Tex of the H2O(110–101) line that is lower than TCMB, such that the line becomes observable in absorption against the CMB
H2O-bearing region associated with HFLS3 to the black-body CMB radiation field at TCMB(z = 6.34), the models suggest that 77.2% of the molecules will be in the 101 ground state and 20.3% will be in the upper 110 state, and all H2O transitions have an excitation temperature Tex equal to TCMB
Summary
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 intense infrared radiation field from the starburst preferentially de-populates the 110 level through radiative pumping, resulting in a deficit in the 110 level compared with 101 relative to a thermal distribution In combination, these two processes result in an excitation temperature Tex of the H2O(110–101) line that is lower than TCMB, such that the line becomes observable in absorption against the CMB. Article log N(H2O) (cm–2) Max absorption SQ (mJy) SQ H2O(110–101) over CMB (mJy) (line + continuum) a Absorption depth into the CMB
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