Estimation of the Cosmic Microwave Background Temperature from Atomic C I and Molecular CO Lines in the Interstellar Medium of Early Galaxies

Abstract

The linear increase of the cosmic microwave background (CMB) temperature with cosmological redshift, $T_{\textrm{CMB}}=T_{{0}}(1+z)$, is a prediction of the standard cosmological $\Lambda$CDM model. There are currently two methods to measure this dependence at redshift $z>0$ and, what is equally important, to estimate of the CMB temperature $T_{0}$ at the present epoch. The first method is based on the Sunyaev–Zeldovich effect for a galaxy cluster. However, this method is limited to redshifts $z\lesssim 1$ and only the deviations from the standard relation can be measured with it. The second method is based on the analysis of the populations of atomic and molecular energy levels observed in the absorption spectra of quasars. This method allows $T_{\textrm{CMB}}(z)$ to be measured directly. We present new estimates of $T_{\textrm{CMB}}(z_{i})$ in the redshift range $1.7\leq z_{i}\leq 3.3$ based on the analysis of the excitation of CO rotational levels and C I fine-structure levels in 15 absorption systems. We take into account the collisional excitation of CO and C I with hydrogen atoms and $\textrm{H}_{2}$ and the radiative pumping of C I by the interstellar ultraviolet radiation. Applying this corrections leads to a systematic decrease in the previously obtained estimates of $T_{\textrm{CMB}}(z_{i})$ (for some systems the magnitude of the effect is ${\sim}10\%$). Combining our measurements with the measurements of $T_{\textrm{CMB}}(z)$ in galaxy clusters we have obtained a constraint on the parameter $\beta=+0.010\pm 0.013$, which characterizes the deviation of the CMB temperature from the standard relation, $T_{\textrm{CMB}}=T_{{0}}(1+z)^{1-\beta}$, and an independent estimate of the CMB temperature at the present epoch, $T_{0}=2.719\pm 0.009$ K, which agrees well with the estimate from orbital measurements, $T_{0}=2.7255\pm 0.0006$ K.