The [CLC][ITAL]z[/ITAL][/CLC] = 1.6748 C [CSC]i[/CSC] Absorber toward PKS 1756+237

Abstract

We report the detection of the λ1560 and λ1657 ground-state C I absorption features in the zabs=1.6748 system toward the QSO PKS 1756+237. We find no associated C I* lines with a resulting 3 σ excitation temperature upper limit of Tex≤8.54(+0.65−0.56) K, which is consistent with the predicted cosmic microwave background radiation (CMBR) temperature of TCMBR=7.291 K. Because the redshifted CMBR populates the J=1 level and leaves little room for additional local excitation through either collisions or UV pumping, our data place 2-3 times more stringent limits on particle densities and UV fields than existing Copernicus observations of similar column density sight lines in the Milky Way. We also detect several Ni II lines and the weak Fe II λ1611 line. From the Ni/Fe column density ratio, we find evidence for dust at a dust-to-metals ratio similar to that seen toward warm Galactic disk clouds. Based on these findings and supported by our Lyα spectrum, we propose to reclassify this system as a damped Lyα absorber.