ar X iv : a st ro - p h / 99 08 35 3 v 1 3 1 A ug 1 99 9 The FIRST Efficient Lens Survey

We have found three lenses (two are new) by observing 34 likely FIRST radio lobes with APM galaxy counterparts. We expect to find ∼ 30 such lenses in over the next few years, which will significantly improve lensing constraints on galaxy structure and cosmology. Although ∼70% of lensed radio sources should be radio lobes (Kochanek & Lawrence 1990) few have been found. In fact, many radio searches (e.g., CLASS, see Rusin et al. in these proceedings) select only flat-spectrum sources, thereby eliminating the dominant type of lensed radio source. Lensed lobes can be found efficiently because they usually lie outside their host galaxy. If a galaxy is coincident with a radio lobe, it is likely to be a foreground lens, as in MG 1549+3047 (Lehár et al. 1993). Moreover, lensed lobes are ideal for studying galaxy mass distributions because, unlike most flat-spectrum sources, their extended structure can yield many lens model constraints. We searched the FIRST survey (Becker et al. 1995), then covering ∼ 3000 square degrees, for likely radio lobes with coincident galaxies in the APM catalog (McMahon & Irwin 1992). There were 845 double and colinear triple FIRST sources with an R < ∼ 20 mag APM galaxy within 2 ′′ of a presumed lobe's peak. The FIRST survey can't definitively identify components as lobes, nor can it resolve ∼ 1 ′′ lensed structures, so higher resolution observations were necessary. We obtained confirming VLA 6 cm observations (9 archival, 25 new) for those 34 candidates whose lobes were bright enough (100 mJy) to find rings similar to that in MG 1549+3047, using < 30 min integrations. We also observed 3 targets with MERLIN at 18 cm, and optical images or spectra were acquired for 4 targets. Most of our lobe candidates were ruled out as radio cores or independent radio sources. Three candidates survived our selection. MG 1549+3047 is a known lens, and two new lenses, J0823+39 and J1622+35 (see Figure 1), also have galaxies both at the core and a radio lobe. Keck LRIS spectra of J1622+35 give lens and source redshifts of z l = 0.32 and z s = 1.47, and the distorted structures seen by MERLIN are easily reproduced by simple lens models. About 10% of our VLA targets are lensed, compared to a success rate of only 0.2% for traditional lens searches. The sample can be expanded using larger radio surveys, fainter optical …