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1. INTRODUCTION

In this review, I will discuss the development of Hi imaging in nearby galaxies, with emphasis on the galaxy outskirts, and take stock of the subject just before the start of the new surveys using novel instrumentation enabled by the developments in the framework of the Square Kilometer Array (SKA), which was originally partly inspired by Hi imaging (Wilkinson 1991). I refer to other reviews on more specific subjects when appropriate. Issues related to star formation are dealt with by Elmegreen and Hunter (this volume), and Koda and Watson (this volume).

In the late 1950s, it became clear that there was more to a galaxy than just its optical image. This was principally due to the prediction of the 21 cm Hi hyperfine structure line by Van de Hulst in 1944, and its detection in the Milky Way (Ewen and Purcell 1951; Muller and Oort 1951). A first rotation curve of the Milky Way was determined by Kwee et al. (1954). The first observations of M31 were done using the Dwingeloo 25 m telescope (van de Hulst et al. 1957), and a mass model for this galaxy considered by Schmidt (1957). A possible increase in the mass-to-light (M / L) ratio in the outermost parts of M31 prompted the photoelectric study of its light distribution by de Vaucouleurs (1958), who confirmed that the local M / L ratio at the last point measured by Van de Hulst et al. exceeds the expectation from a model with constant M / L ratio throughout this galaxy's disk, in contrast with an earlier model by Schwarzschild (1954). The outer Hi layer of the Milky Way was found to be warped (Burke 1957, Kerr 1957), and flaring (Gum et al. 1960). Dark matter in the Local Group was inferred by Kahn and Woltjer (1959), who attributed the high mass of the Local group to intergalactic gas, and explained the warp as due to our Milky Way moving through it.

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