Are Two z ∼ 6 Quasars Gravitationally Lensed?

Several high-z (z > 5.7) quasars have been found during the course of Sloan Digital Sky Survey. The presence of such very high-z quasars is expected to put constraints on early structure formation. On one hand, it is suggested that these most luminous objects at high redshift are biased toward highly magnified objects by gravitational lensing. To clarify the effect of gravitational lensing on high-z quasars, we began an imaging survey of intervening lensing galaxies. Indeed, our previous optical image showed that SDSSp J104433.04+012502.2 at z = 5.74 is gravitationally magnified by a factor of 2. In this paper, we report our new optical imaging of two other high-z quasars, SDSSp J103027.10+052455.0 at z = 6.28 and SDSSp J130608.26+035626.3 at z = 5.99. Since we found neither an intervening galaxy nor a counter image with i′ < 25.4-25.8 around each quasar, we conclude that they are not strongly magnified regardless that a lens galaxy is dusty.