A conspicuous tangential alignment of galaxies in a STIS Parallel Shear Survey field: A new dark-lens candidate?^*

¹ Institut für Astrophysik und Extraterrestrische Forschung, Universität Bonn, Auf dem Hügel 71, Bonn, Germany
² ST-ECF, European Southern Observatory, Karl Schwarzschild Str. 2, Garching b. München, Germany
³ Institut d'Astrophysique de Paris, 98bis boulevard Arago, Paris, France
⁴ Observatoire de Paris, DEMIRM, 61 avenue de l'Observatoire, 75014 Paris, France
⁵ Max-Planck Institut für Astrophysik, Karl Schwarzschild Str. 1, Postfach 1317, Garching b. München, Germany
⁶ University of Pennsylvania, Dept. of Physics and Astronomy, Philadelphia, USA

Corresponding author: J.-M. Miralles, miralles@astro.uni-bonn.de

Received: 7 February 2002
Accepted: 19 March 2002

Abstract

We report the serendipituous discovery of a conspicuous alignment of galaxies in a field obtained through the STIS Parallel Shear Survey. This project collects randomly distributed fields to investigate the cosmic shear effect on this scale. Analyzing the parallel observations having the Seyfert galaxy NGC 625 as primary target, we recognized over the whole field of view a strong apparent tangential alignment of galaxy ellipticities towards the image center. The field shows several arclet-like features typical for images of massive galaxy clusters, but no obvious over-density of bright foreground galaxies. We also find a multiple image candidate. On the basis of the possible strong and weak lensing effect within the data, we discuss whether this could be compatible with a massive halo with no clear optical counterpart.