H1517 656: The birth of a BL lacertae object?

Abstract

H1517+656 is an unusual source, even for a BL Lac object. It is one of the most luminous BL Lac objects known, with extreme emission properties at radio, optical, and X-ray frequencies. Furthermore, in our WFPC2 snapshot survey we discovered a series of peculiar arcs describing a 2″.A radius ring surrounding the source. This paper describes follow-up observations with additional WFPC2 bands and the STIS long-pass filter, which have revealed this structure to be the remnants of a very recent galaxy merger. Population synthesis modeling has shown that regions of the arcs have stellar populations with age < 20 Myr. In addition, the circularity of the arcs indicates that the plane of the collisi..