ABSTRACT

Previous researchers have suggested that much of the cold interstellar gas in presentday elliptical galaxies is accreted from external sources. The strength of forbidden-line emission in elliptical galaxies provides a constraint on the enrichment history of the gas. Based on photoionization calculations, we conclude that the gas, if accreted, must originate in donor galaxies with metallicities > 0.5 Z⊙. This excludes primordial clouds and Magellanic Cloud-like objects as typical gas donors.

INTRODUCTION

Elliptical galaxies often contain modest quantities of interstellar gas that can be generated via normal mass loss by the galaxies' constituent stars, on timescales much shorter than a Hubble time (Faber and Gallagher 1976). Diffuse matter generated by such internal sources may be rapidly removed from the interstellar medium (ISM), however, if this material is heated to X-ray temperatures and expelled in a galactic wind, or compressed to form new stars in a cooling flow. In an alternative scenario, objects which feature significant cold interstellar gas may have acquired this matter by accretion from nearby galaxies or intergalactic clouds. Evidence in support of an external origin for the ISM in ellipticals includes a lack of correlation between interstellar and stellar masses (e.g., Knapp et al. 1985), and distinct kinematics for the gaseous and stellar components seen in some objects (e.g., Bertola et al. 1990).

ABUNDANCES AS A DISCRIMINATOR OF ISM ORIGIN

Stars in large elliptical galaxies are inferred to have average heavy element abundances ≳ 2 Z⊙, based on observational estimates and predictions from chemical evolution models (e.g., Bica et al. 1988).