ABSTRACT

Many bright elliptical galaxies are active in the sense of having compact radio cores of high brightness temperature (‘engines’) and/or a LINER–like optical emission spectrum. Nuclear activity is very common in the most luminous galaxies (brighter than absolute magnitude MB ∼ – 21) and essentially absent in those less luminous than MB ∼ -19.

ACTIVE NUCLEI IN EARLY–TYPE GALAXIES

The presence of LINER emission (Phillips et al. 1986) and the fact that many E and S0 galaxies have arcsecond–scale central radio sources suggest that some kind of active nucleus lurks at the centre of most galactic bulges brighter than MB ∼ -19 (H0= 100 km s-1 Mpc-1).

Wrobel and Heeschen (1991), however, argue that the central radio emission in many S0 galaxies may be associated with star formation rather than an active nucleus. Furthermore, the emission–line luminosity in E and S0 nuclei correlates more closely with the luminosity of the parent galaxy than with other indicators of activity such as radio emission (Sadler et al. 1989), suggesting that the dominant ionization mechanism may be linked to the underlying stellar population rather than to an active nucleus. Optical spectroscopy and arcsecond–scale radio maps, therefore, may not provide an unambiguous test for the presence of low–level ‘central engines’ in these galaxies.