Detection of OH and wide HI 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 HI spectrum; the OH spectra yielded an OH column density of N_OH= 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 HI 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 HI 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.