HI and OH absorption at z= 0.89

Abstract

We report on WSRT observations of HI and OH absorption at a redshift of z = 0.885 towards the radio lens PKS 1830-21. The mm wave transitions of several molecular species have already been observed in absorption towards PKS 1830-21 at this redshift. At mm wavelengths the source structure is dominated by two extremely compact components, the northeast (NE) and southwest (SW) components. At lower frequencies the continuum emission is much more extended and there is also a broad Einstein ring connecting the NE and SW components. This larger extent of the continuum means that the HI and OH spectra sample a much larger region of the absorber than the mm wave spectra. The HI spectrum that we obtain is asymmetric, with a peak at −147 km/s with respect to the main molecular line redshift of z = 0.88582.Weak mm wave molecular absorption has also been detected towards the NE component at this same velocity. The HI absorption, however, covers a total velocity width of 300 km/sec, including velocities well to the red of the deep molecular features, and is fully resolved suggesting that it is spatially widespread. In OH we detect both the 1667 and the 1665 MHz transitions, and the velocity-integrated ratio of their optical depths is consistent with what is expected in thermal equilibrium. The OH spectrum has a velocity width comparable to that of the HI spectrum, suggesting that it too is widespread in the absorber. The lack of a prominent HI peak in the spectrum at the velocity corresponding to the SW component, suggests that the galaxy responsible for the absorption at z = 0.885 has a central molecular disk many kpc in size, and that HI may be deficient in this central region. Our observations are sensitive to the large scale kinematics of the absorber, and to first order the implied dynamical mass is consistent with the lens models of Nair et. al. (1993). Higher spatial resolution is however critical in order to better constrain the lensing models.