2018ApJ...860L..22G

We present the first detection of OH absorption in diffuse gas at z > 0, along with another eight stringent limits on OH column densities for cold atomic gas in galaxies at 0 < z < 0.4. The absorbing gas detected toward Q0248+430 (z_q = 1.313) originates from a tidal tail emanating from a highly star-forming galaxy G0248+430 (z_g = 0.0519) at an impact parameter of 15 kpc. The measured column density is N(OH) = (6.3 ± 0.8) x 10¹³(T_ex/3.5)(1.0/f_c^OH) cm^–2, where f_c^OH and T_ex are the covering factor and the excitation temperature of the absorbing gas, respectively. In our Galaxy, the column densities of OH in diffuse clouds are of the order of N(OH) ∼ 10^13–14 cm^–2. From the incidence (number per unit redshift; n₂₁) of H I 21 cm absorbers at 0.5 < z < 1 and assuming no redshift evolution, we estimate the incidence of OH absorbers (with log N(OH) > 13.6) to be n_OH = 0.008_–0.008^+0.018 at z ∼ 0.1. Based on this we expect to detect 10_–10⁺²⁰ such OH absorbers from the MeerKAT Absorption Line Survey (MALS). Using H I 21 cm and OH 1667 MHz absorption lines detected toward Q0248+430, we estimate (ΔF/F) = (5.2 ± 4.5) x 10^–6, where F≡g_p(α²/µ)^1.57, α is the fine structure constant, µ is the electron-proton mass ratio, and g_p is the proton gyromagnetic ratio. This corresponds to Δα/α(z = 0.0519) = (1.7 ± 1.4) x 10^–6, which is among the stringent constraints on the fractional variation of α.