Radio and optical observations of 0218+357 - The smallest Einstein ring?

Abstract

We present VLA radio observations and optical imaging and spectroscopy of 0218+357, which has been suggested to be the smallest known Einstein radio ring by Patnaik et al. (1991), preprint. The optical spectrum is rather red (F_v_ is proportional to V<SUP>alpha</SUP>^~-3.5) and shows no strong features. We have a tentative detection of the 4000 A break and the G band which give a redshift of z = 0.68. The identification is a m_r_~20 resolved galaxy, with an absolute magnitude of M_R_~-23.8 or M_B_~ -22.0 which is consistent with the radio galaxy host being a brightest cluster member. The ring is detected for the first time at 22.4 GHz, and has basically similar structure at 5, 15, and 22.4 GHz. The ring is highly polarized at 8.4 GHz. Component B has varied in the radio and was ~15% brighter in our data than in Patnaik et al's. There is very powerful (P_1.4_~10^26^ W Hz^-1^) amorphous radio structure extending out to ~11" (70 kpc) from the core. By analogy with previous analyses of Einstein radio rings we suggest that the observed double core and ring structure may be produced by a combination of an off-center radio core with extended radio structure (a jet?) which crosses behind the center of the lensing galaxy. Based on the small angular size of the ring, we estimate a small value for the central radial velocity dispersion of the lens σ~110 km s^-1^ which leads to a rather faint absolute magnitude (MB^0^_T_~-17.4 for an elliptical via the Faber-Jackson relationship or MB^0^_T_~- 19.1 for a spiral via the Tully-Fisher relationship). The high value of the observed RM and the difference in RM between A and B may require unusual parameters for the interstellar medium (ISM) of the lens. This is one of the remaining puzzles in the object. In summary, the radio source shares some of the optical and radio properties of BL Lac objects and compact double radio sources and we cannot currently rule out the possibility that the source is not lensed. However, based on the core and ring properties we suggest that the object may well be lensed. In this case, the lensed object is most likely a powerful radio source (a BL Lac object or a Fanaroff-Riley Class I edge- darkened radio galaxy) at a redshift of 0.68, and the lens is a faint, low mass object at an intervening redshift. The small angular scale of the lensed image implies that very small scale structure in the radio source is being lensed. Proper motions in the radio "jet" may be magnified and may be visible as changes in the structure of the Einstein ring.