We present g (F475W), r (F625W), and I (F814W) images of the O2R binary quasar FIRST J1643+3156 (z=0.586) observed with the Wide Field Channel (WFC) of the Advanced Camera for Surveys (ACS) as part of the ACS/GTO program. This small-separation system represents an ideal laboratory for studying the morphology and evolution of the host galaxies of a radio-loud and -quiet active galaxy in similar environments. The pair consists of a compact steep-spectrum source (MI = -24.0) and a radio-quiet source (MI = -23.2) separated by 2.3" (15 kpc). The host galaxy of the radio-loud quasar is highly disturbed. Three distinct morphological features are detected : a bright arc extending ~3 kpc west of the nucleus, a large diffuse knot located ~8 kpc to the south, and a group of filaments to the north-east of the nucleus. Narrow, distorted filaments are also observed between the two quasars. By comparison, the host galaxy of the radio-quiet source appears smooth and unperturbed. Nearest neighbor statistics of the field reveal an overdensity of sources in the images suggesting that the binary is part of a group of galaxies. Color models are presented and the implication of this system in context of the triggering and evolution of radio activity in active galaxies is discussed. ACS was developed under NASA contract NAS 5-32865, and this research is supported by NASA grant NAG5-7697.
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