The Closest Damped Lyα System

Abstract

A difficulty of studying damped Lyα systems is that they are distant, so one knows little about the interstellar medium of the galaxy. Here we report on a damped Lyα system in the nearby galaxy NGC 4203, which is so close (v_helio=1117 km s^-1) and bright (B₀=11.62) that its H I disk has been mapped. The absorption lines are detected against Ton 1480, which lies only 1.9 arcmin (12 h₅₀ kpc) from the center of NGC 4203. Observations were obtained with the Faint Object Spectrograph on the Hubble Space Telescope (G270H grating) over the 2222-3277 Å region with 200 km s^-1 resolution. Low-ionization lines of Fe, Mn, and Mg were detected, leading to metallicities of -2.29, less than -0.68, and greater than -2.4, which are typical of other damped Lyα systems but well below the stellar metallicity of this type of galaxy. Most notably, the velocity of the lines is 1160+/-10 km s^-1, which is identical to the H I rotational velocity of 1170 km s^-1 at that location in NGC 4203, supporting the view that these absorption-line systems can be associated with the rotating disks of galaxies. In addition, the line widths of the Mg lines give an upper limit to the velocity dispersion of 167 km s^-1 at the 99% confidence level.