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W. M. Lane, F. H. Briggs (Kapteyn Astronomical Institute), D. A. Turnshek, S. M. Rao (University of Pittsburgh)
The sightline towards the zem = 0.630 radio-loud quasar OI 363 (B 0738+313) presents a unique opportunity to study low redshift, high neutral column density absorbers. Recent observations using HST (Rao and Turnshek, 1998, ApJ, 500, 115), WSRT (Lane, et al., 1998, AJ, 116, 26), and the VLA have identified two HI 21cm/Damped Lyman-alpha(DLa) absorbers in the spectrum of this quasar, one at z=0.2212 and the other at z=0.0912. This second absorber is the lowest redshift DLa-identified system known. Because it lies at such a low redshift, it is possible to study this system in great detail at many wavelengths.
A short VLA observation of the z=0.0912 system shows a narrow 21cm absorption line, but no 21cm emission. The non-detection gives an HI mass limit of M \leq 9.5 \times 109 M\odot, if the gas is spread over 100 km s-1 (Ho=75 km s-1 Mpc-1). In order to study the absorbing gas in more detail, we have obtained VLBI 21cm absorption data with spatial resolution on scales of 10 h75-1 pc for the z=0.0912 system. An R-band image made with the WIYN telescope (Rao and Turnshek, 1998, ApJ, 500L, 115) reveals only one reasonably bright galaxy close to the quasar sightline. The second absorber is most likely either a galaxy which lies directly under the PSF of the optical image, or an LSB galaxy with a surface brightness below the detection limit in the image. Either option is in clear contrast to the standard paradigm of DLa systems arising in luminous, gas-rich, disk galaxies at low impact parameters.