A straight gravitational image in Abell 2390 - A striking case of lensing by a cluster of galaxies

In this paper we present the results obtained on the photometry and spectroscopy of a very linear arclike object found in the center of the rich cluster of galaxies Abell 2390 (z = 0.231). The main feature in its spectrum is an emission line at 7130 A that exists all along the arc. The identification of this emission line as O II λ3727 leads to a redshift of 0.913 for the structure. This is the third arc candidate confirmed spectroscopically as a gravitationally lensed background galaxy. We report the presence of several arclets in the neighborhood of the arc, and a faint object whose redshift and spectrum are about the same as those of the arc. We discuss the detection of a velocity gradient inside the arc. A first attempt to model the lens is also proposed, and the difficulties of reproducing a "straight arc" are emphasized.