Radial population synthesis and the ionization of gas in elliptical galaxies

Extract

A remarkable discovery of recent years has been the detection of various kinds of interstellar matter (ISM) in elliptical galaxies. The presence of dust patches, ionized gas, and hot X-ray gas have proven to be quite common in ellipticals which were once thought to be simple structures, devoid of gas and dust. A review of ISM in ellipticals has recently been published by Forbes (1991). However, the origin and fate of ISM in ellipticals is not fully understood. Its origin may be stellar mass-loss, condensation of hot X-ray gas pervading the galaxy, or merging with gas-rich galaxies. Recently, the merger picture receives much attention, since (i) the ionized gas usually is dynamically decoupled from the stars, and (ii) the great variety in radial line-strength gradients found among ellipticals also favours merging as formation process (cf. Gorgas et al. 1990). To study the origin and fate of ISM in ellipticals we are currently undertaking an optical survey of a complete sample of ellipticals (B⁰_T < 12, cf. Goudfrooij et al. 1990). An important result of our extensive CCD imaging program is that a relevant fraction of the sample objects exhibits dust patches within Hα+[NII] line-emitting filaments. This common occurrence can be easily accounted for if the dust and gas have an external origin. In these cases, the extended line emission often has a peculiar distribution and is more sharply peaked at the nucleus than is the stellar continuum. Furthermore, all of these ellipticals exhibit a compact flat-spectrum radio source in their nucleus, suggesting that this nuclear activity also has an external origin. In this respect it would be interesting to know the excitation mechanism of the gas.