Program: GN-2009B-Q-2

This proposal aims at obtaining a reliable redshift determination for the BL Lac object 3C66A. It is motivated by the recent detection of this blazar at very high energy (VHE, i.e. E >100 GeV) gamma-rays by VERITAS. The redshift of 3C66A, despite a value z=0.444 is normally cited in the literature and catalogs, is still judged as uncertain. This raises several questions, because, if that redshift value were correct, 3C66A's emitted gamma-ray flux should be heavily absorbed by infrared photons of the extragalaxtic background light (EBL). On the other hand, 3C66A belongs to the class of low-energy peaked (LBL) blazars, while most blazars detected at VHE gamma-rays belong to the high-energy peaked (HBL) class. A reliable redshift determination for 3C66A would thus provide a solid ground to understand the emission properties of this object at gamma-ray frequencies, as well as to better constrain the absorption mechanism of gamma-rays by the EBL.