Detection of OH and wide H I absorption toward B0218+357

Abstract

We present deep Giant Metrewave Radio Telescope (GMRT) OH and Westerbork Synthesis Radio Telescope (WSRT) H I absorption spectra of the z = 0.6846 gravitational lens toward B0218+357. Both the 1665- and 1667-MHz OH lines are clearly detected for the first time, while a new wide absorption component was detected (at low significance) in the H I spectrum; the OH spectra yielded an OH column density of NOH = 2.3 × 1015 cm -2. The ratio of 1667-and 1665-MHz equivalent widths is ∼1.8 while the redshift of peak OH absorption is z = 0.68468 ± 0.000008 for both lines; this redshift agrees with that obtained from the H I line. The velocity spread (between nulls) of the H I absorption is ∼140 km s -1, while that of both OH lines is ∼100 km s-1; the H I and OH spectra are broadly similar in that they each have two principal narrow components and a wide absorption trough. We argue that the wide absorption is likely to arise from source components in the Einstein ring and derive a rotation velocity of ∼150 km s-1 at a distance of 1.5 kpc from the centre of the z ∼ 0.6846 galaxy.