VLT narrow-band photometry in the Lyman continuum of two galaxies at z

Limits to the escape of ionizing flux

¹ Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France e-mail: [akio.inoue;veronique.buat;denis.burgarella;jean-michel.deharveng]@oamp.fr
² Department of Physics, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
³ Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
⁴ Subaru Mitaka Office (Subaru Telescope), National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan e-mail: iwata@optik.mtk.nao.ac.jp

Received: 2 August 2004
Accepted: 18 January 2005

Abstract

We have performed narrow-band imaging observations with the Very Large Telescope, aimed at detecting the Lyman continuum (LC) flux escaping from galaxies at . We do not find any significant LC flux from our sample of two galaxies in the Hubble Deep Field South, at and 3.275. The corresponding lower limits on the  flux density (per Hz) ratio are 15.6 and 10.2 (3-σ confidence level). After correction for the intergalactic hydrogen absorption, the resulting limits on the relative escape fraction of the LC are compared with those obtained by different approaches, at similar or lower redshifts. One of our two objects has a relative escape fraction lower than the detection reported by Steidel et al. in a composite spectrum of  galaxies. A larger number of objects is required to reach a significant conclusion. Our comparison shows the potential of narrow-band imaging for obtaining the best limit on the relative escape fraction at . Stacking a significant number of galaxies observed through a narrow-band filter would provide constraint on the galactic contribution to the cosmic reionization.