ABSTRACT

Many elliptical galaxies contain cold gas of external origin, which often forms a warped disk or ring. Recent observations of the atomic and the molecular gas in nearby ellipticals have shed considerable light on the properties of this accreted material. Resulting progress in understanding the details of the settling process are discussed, with attention to questions such as: Are any of the observed gas disks settled? How do the settling times and the observed properties of the gas disks depend on the structure and shape of the galaxy in which it resides? How much mass is delivered to the nucleus?

INTRODUCTION

Many elliptical galaxies contain significant amounts of cold gas. This is often detectable by its Hα emission which sometimes extends to 3–5 kpc from the center and appears as a warped disk (Demoulin–Ulrich, Butcher and Boksenberg 1984; Trinchieri and di Serego Alighieri 1991; Shields 1991; Macchetto and Sparks 1992; Buson et al. 1993). When seen nearly edge–on, such disks are seen as a dust lane across the image of the galaxy (Bertola and Galletta 1978; Hawarden et al. 1981; Ebneter and Balick 1985; Sadler and Gerhard 1985; Bertola 1987). These disks contain 103–105 M⊙ of ionized gas (Phillips etal 1986), 104–106 M⊙ of dust (Forbes 1991), and 106–108 M⊙ of neutral gas (Lees et al. 1991; Bregman, Hogg, and Roberts 1992). Large amounts of neutral hydrogen extending to tens of kpc have been found in only a half–dozen elliptical galaxies (van Gorkom 1992).