Active Galaxies in High Density Environments
A radio survey of 401 galaxies in small groups with local galaxy space densities ranging from 102 to 106 Mpc-3 has detected 41 sources above a flux limit of 1.5 mJy at 18 cm. These radio sources fall into two catagories according to the optical morphology of the host galaxies. Radio sources in elliptical galaxies occur only in the first-ranked (in optical luminosity) galaxies. They are compact nuclear sources with no detectable extended emission. Those in spirals are more extended and show no preference for optical rank. Our interpretation is that the emission in spiral galaxies is a result of star formation activity, possibly triggered by interactions, and that the sources in elliptical galaxies result from inflow onto a central compact object. This inflow is a direct result of the large scale galaxy environment.
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