A 21 Centimeter Absorber Identified with a Spiral Galaxy: Hubble Space Telescope Faint Object Spectrograph and Wide-Field Camera Observations of 3CR 196

Abstract

We present imaging and spectroscopy of the quasar 3CR 196 (z_e = 0.871), which has 21 cm and optical absorption at z_a = 0.437.

We observed the region of Lyα absorption in 3CR 196 at z_a = 0.437 with the Faint Object Spectrograph on the Hubble Space Telescope. This region of the spectrum is complicated because of the presence of a Lyman limit and strong lines from a z_a ≈ z_e system. We conclude that there is Lyα absorption with an H I column density greater than 2.7 × 10¹⁹ cm^-2 and most probably 1.5 × 10²⁰ cm^-2. Based on the existence of the high H I column density along both the optical and radio lines of sight, separated by more than 15 kpc, we conclude that the Lyα absorption must arise in a system comparable in size to the gaseous disks of spiral galaxies.

A barred spiral galaxy, previously reported as a diffuse object in the recent work of Boissé & Boulade, can be seen near the quasar in an image taken at 0" 1 resolution with the Wide Field Planetary Camera 2 on the HST. If this galaxy is at the absorption redshift, the luminosity is approximately L_*, and any H I disk should extend in front of the optical quasar and radio lobes of 3CR 196, giving rise to both the Lyα and 21 cm absorption.

In the z_a ≈ z_e system we detect Lyman lines and the Lyman limit, as well as high ion absorption lines of C III, N V, S VI, and O VI. This absorption probably only partially covers the emission-line region. The ionization parameter is approximately 0.1. Conditions in this region may be similar to those in broad absorption line QSOs.