First detection of CO in a high-redshift DLA

TL;DR

This paper reports the first detection of CO molecules in a high-redshift DLA, providing a new method to measure the cosmic microwave background temperature at z=2.41837, confirming the hot big-bang theory.

Contribution

It presents the first detection of CO in a high-redshift DLA and uses molecular excitation to precisely measure the CMB temperature at that epoch.

Findings

Detected CO, H_2, and HD molecules in a high-redshift DLA.

Measured CMB temperature at z=2.41837 as 9.15 K, consistent with big-bang predictions.

Found high metal enrichment and dust depletion patterns similar to the Galactic ISM.

Abstract

We present the first detection of carbon monoxide (CO) in a damped Lyman-alpha system (DLA) at z_abs =2.41837 toward SDSS J143912.04+111740.5. We also detected H_2 and HD molecules. The measured total column densities (in log units) of H I, H_2, and CO are 20.10\pm0.10, 19.38\pm0.10, and 13.89\pm0.02, respectively. The molecular fraction, f = 2N(H_2)/(N(HI)+2N(H_2)) = 0.27^{+0.10}_{-0.08}, is the highest among all known DLAs. The abundances relative to solar of S, Zn, Si, and Fe are -0.03\pm0.12, +0.16\pm0.11, -0.86\pm0.11, and -1.32\pm0.11, respectively, indicating a high metal enrichment and a depletion pattern onto dust-grains similar to the cold ISM of our Galaxy. The measured {N(CO)/N(H_2) = 3x10^{-6}} is much less than the conventional CO/H_2 ratio used to convert the CO emission into gaseous mass but is consistent with what is measured along translucent sightlines in the Galaxy. The CO rotational excitation temperatures are higher than those measured in our Galactic ISM for similar kinetic temperature and density. Using the C I fine structure absorption lines, we show that this is a consequence of the excitation being dominated by radiative pumping by the cosmic microwave background radiation (CMBR). From the CO excitation temperatures, we derive T_CMBR = 9.15 \pm 0.72 K, while 9.315 \pm 0.007 K is expected from the hot big-bang theory. This is the most precise high-redshift measurement of T_CMBR and the first confirmation of the theory using molecular transitions at high redshift.