3D Lyα radiation transfer

III. Constraints on gas and stellar properties of z ~ 3 Lyman break galaxies (LBG) and implications for high-z LBGs and Lyα emitters

¹ Geneva Observatory, University of Geneva, 51 Ch. des Maillettes, 1290 Versoix, Switzerland e-mail: anne.verhamme@unige.ch
² Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Avenue E. Belin, 31400 Toulouse, France
³ Institut d'Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
⁴ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany

Received: 25 February 2008
Accepted: 22 May 2008

Abstract

Aims. The aim of our study is to understand the variety of observed Lyα line profiles and strengths in Lyman Break Galaxies (LBGs) and Lyα emitters (LAEs), the physical parameters governing them, and hence derive constraints on the gas and dust content and stellar populations of these objects.

Methods. Using our 3D Lyα radiation transfer code including gas and dust, MCLya, we fit 11 LBGs from the FORS Deep Field with redshifts between 2.8 and 5. A simple geometry of a spherically expanding shell of H I is adopted.

Results. The variety of observed Lyα profiles is successfully reproduced. Most objects show outflow velocities of V_exp ~ 150-200 km s^-1; two objects are most likely quasi-static. The radial H I column density ranges from N_HI ~ 2 10¹⁹ to 7 10²⁰ cm^-2. Our Lyα profile fits yield values of ~ 0.05-0.2 for the gas extinction. We find indications for a dust-to-gas ratio higher than the Galactic value, and for a substantial scatter. The escape fraction of Lyα photons is found to be determined primarily by the extinction, and a simple fit formula is proposed. In this case a measurement of EW(Lyα)_obs can yield , if the intrinsic Lyα equivalent width is known (or assumed). Intrinsic EW(Lyα)_int ~ 50-100 Å are found for 8/11 objects, as expected for stellar populations forming constantly over long periods (10-100 Myr). In three cases we found indications of younger populations. Our model results also allow us to understand observed correlations between EW(Lyα)_obs and other observables such as FWHM(Lyα), , SFR(UV) etc. We suggest that most observed trends of Lyα, both in LBGs and LAEs, are driven by variations of N_HI and the accompanying variation of the dust content. Ultimately, the main parameter responsible for these variations may be the galaxy mass. We also show that there is a clear overlap between LBGs and LAEs: at z ~ 3 approximately 20-25% of the LBGs of Shapley et al. (2003, ApJ, 588, 65) overlap with ~23% of the LAEs of Gronwal et al. (2007, ApJ, 667, 79). Radiation transfer and dust effects explain the increase of the LAE/LBG ratio, and a higher percentage of LBGs with strong Lyα emission with increasing redshift.