Letter to the Editor

The cosmic microwave background radiation temperature at toward QSO 0347–3819^*

¹ Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
² Division of Theoretical Astrophysics, National Astronomical Observatory, Mitaka, Tokyo 181-8588, Japan
³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁴ Observatoire de Genève, 1290 Sauverny, Switzerland

Corresponding author: P. Molaro, molaro@oat.ts.astro.it

Received: 5 June 2001
Accepted: 28 November 2001

Abstract

From the analysis of the C⁺ fine-structure population ratio in the damped Lyα system at z_abs = 3.025 toward the quasar Q0347–3819 we derive an upper bound of K on the cosmic microwave background temperature (T_CMBR) regardless the presence of other different excitation mechanisms. The analysis of the ground state rotational level populations of H₂ detected in the system reveals a Galactic-type UV radiation field ruling out UV pumping as an important excitation mechanism for C⁺. The low dust content estimated from the Cr/Zn ratio indicates that the IR dust emission can also be neglected. When the collisional excitation is considered, we measure a temperature for the cosmic background radiation of T_CMBR = K. The results are in agreement with the T_CMBR =  K predicted by the hot Big Bang cosmology at z_abs = 3.025.