A Survey for High-redshift Gravitationally Lensed Quasars and Close Quasars Pairs. I. the Discoveries of an Intermediately-lensed Quasar and a Kpc-scale Quasar Pair at $z\sim5$

Abstract

We present the first results from a new survey for high-redshift $(z\gtrsim5)$ gravitationally lensed quasars and close quasar pairs. We carry out candidate selection based on the colors and shapes of objects in public imaging surveys, then conduct follow-up observations to confirm the nature of high-priority candidates. In this paper, we report the discoveries of J0025--0145 ($z=5.07$) which we identify as an {intermediately-lensed quasar, and J2329--0522 ($z=4.85$) which is a kpc-scale close quasar pair. The {\em Hubble Space Telescope (HST)} image of J0025--0145 shows a foreground lensing galaxy located $0\farcs6$ away from the quasar. However, J0025--0145 does not exhibit multiple lensed images of the quasar, and we identify J0025--0145 as an intermediate lensing system (a lensing system that is not multiply imaged but has a significant magnification). The spectrum of J0025--0145 implies an extreme Eddington ratio if the quasar luminosity is intrinsic, which could be explained by a large lensing magnification. The {\em HST} image of J0025--0145 also indicates a tentative detection of the quasar host galaxy in rest-frame UV, illustrating the power of lensing magnification and distortion in studies of high-redshift quasar host galaxies. J2329--0522 consists of two resolved components with significantly different spectral properties, and a lack of lensing galaxy detection under sub-arcsecond seeing. We identify it as a close quasar pair, which is the highest confirmed kpc-scale quasar pair to date. We also report four lensed quasars and quasar pairs at $2<z<4$, and discuss possible improvements to our survey strategy.