ABSTRACT

We present r – i colors of GHz–Peaked Spectrum (GPS) Radio Galaxies. We find that most GPS radio galaxies have r – i colors in the range of 0.2 – 0.4, typical of passively evolving elliptical galaxies. However, several have much redder colors in the range 1–2. We suggest several possible explanations for the very red colors, including (1) large amounts of dust in the galaxy, (2) a significant post-starburst population, and (3) a highly reddened active galactic nucleus. The red colors are consistent with a hypothesis in which significant mass transfer has occurred, producing a dense, clumpy, and possibly dusty ISM in the GPS host galaxy.

INTRODUCTION

GPS radio galaxies are characterized by the following properties: (1) a simple, convex, radio spectrum, peaking near 1 GHz, (2) mostly compact (sub kpc) radio structure, (3) low radio and optical polarization, and (4) low variability. We (O'Dea, Baum, and Stanghellini 1991) have suggested that the GPS galaxies have a dense, clumpy ISM (probably acquired externally via cannibalism) that confines and depolarizes the radio source. Our r and i band optical imaging has shown that the GPS galaxies are often in strongly interacting systems (Stanghellini et al. 1993).

THE R – I COLORS

We find that most GPS radio galaxies have integrated r – i colors in the range of 0.2 – 0.4, typical of passively evolving elliptical galaxies. This suggests that most GPS galaxies are not currently undergoing a significant starburst (though they might have in the past), unless the starburst population is highly obscured. However, several galaxies have much redder r – i colors in the range 1–2.