2005/08/01 by Andrew J. Fox, Blair D. Savage, Bart P. Wakker · 1 citation
Engineering · Physics and Astronomy · #Astrophysics and Star Formation Studies #Calibration and Measurement Techniques #Galaxies: Formation, Evolution, Phenomena #astro-ph
paper · pdf · doi:10.1086/491735
published as Astron.J.130:2418-2423,2005 · 7 pages, 3 figures (2 in color), accepted for publication in AJ
arxiv created 2005/08/01 · openalex publication_date 2005/10/26 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/31
To measure the column densities of interstellar and intergalactic gas clouds using absorption-line spectroscopy, the apparent optical depth (AOD) technique of Savage & Sembach can be used instead of a curve-of-growth analysis or profile fit. We show that the AOD technique, while an excellent tool when applied to data with good signal-to-noise ratio (S/N), will likely overestimate the true column densities when applied to data with low S/N. This overestimation results from the nonlinear relationship between the flux falling on a given detector pixel and the apparent optical depth in that pixel. We use Monte Carlo techniques to investigate the amplitude of this overestimation when working with data from the Far Ultraviolet Spectroscopic Explorer ( FUSE ) and the Space Telescope Imaging Spectrograph (STIS), for a range of values of S/N, line depth, line width, and rebinning. AOD measurements of optimally sampled, resolved lines are accurate to within 10% for FUSE LiF and STIS E140M data with S/N ≳ 7 per resolution element.