The Wisconsin H Mapper (WHAM): A Brief Review of Performance Characteristics and Early Scientific Results

Introduction

Warm (10 K), ionized hydrogen is a principal component of our Galaxy's interstellar medium, with a mass surface density approximately one third that of the H I, a vertical scale height nearly ten times that of the H I cloud layer, and a power budget equal to the kinetic energy injected into the interstellar medium by supernovae (Kulkarni & Heiles 1987; Reynolds 1991). Detailed studies of this component are thus required for an accurate understanding of interstellar matter and processes as well as for a proper interpretation of other astrophysical observations affected by this foreground screen of H II. Although originally detected in the 1960s with radio techniques (e.g., Hoyle & Ellis 1963; Bridle & Venugopal 1969), subsequent developments in Fabry-Perot spectroscopy soon showed that the primary source of information about the distribution, kinematics, and other properties of this gas is through the detection and study of interstellar emission lines at optical wavelengths (e.g., Reynolds, Scherb, & Roesler 1973; Reynolds 1980). However, the emission lines are weak, and a comprehensive exploration of this component had to await the development of modern, low noise imaging detectors, which increased dramatically the efficiency of spectrometers and imaging cameras for observations of the faint interstellar emission.

© Copyright Astronomical Society of Australia 1997