EDP Sciences
Free access
Volume 491, Number 1, November III 2008
Page(s) 89 - 111
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:200809648
Published online 09 July 2008

A&A 491, 89-111 (2008)
DOI: 10.1051/0004-6361:200809648

3D Ly$\alpha$ radiation transfer

III. Constraints on gas and stellar properties of z ~ 3 Lyman break galaxies (LBG) and implications for high-z LBGs and Ly$\alpha$ emitters
A. Verhamme1, D. Schaerer1, 2, H. Atek3, and C. Tapken4

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

Aims. The aim of our study is to understand the variety of observed Ly$\alpha$ line profiles and strengths in Lyman Break Galaxies (LBGs) and Ly$\alpha$ 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 Ly$\alpha$ 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 Ly$\alpha$ profiles is successfully reproduced. Most objects show outflow velocities of $V_{\rm exp}$ ~ 150-200 km s-1; two objects are most likely quasi-static. The radial H I column density ranges from $N_{\rm HI}$ ~ 2 $\times$ 1019 to 7 $\times$ 1020 cm-2. Our Ly$\alpha$ profile fits yield values of E(B-V) ~ 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 Ly$\alpha$ photons is found to be determined primarily by the extinction, and a simple fit formula is proposed. In this case a measurement of EW(Ly$\alpha$)$_{\rm obs}$ can yield E(B-V), if the intrinsic Ly$\alpha$ equivalent width is known (or assumed). Intrinsic EW(Ly$\alpha$)$_{\rm int}$ ~ 50-100 Å are found for 8/11 objects, as expected for stellar populations forming constantly over long periods ($\ga$10-100 Myr). In three cases we found indications of younger populations. Our model results also allow us to understand observed correlations between EW(Ly$\alpha$)$_{\rm obs}$ and other observables such as FWHM(Ly$\alpha$), E(B-V), SFR(UV) etc. 
We suggest that most observed trends of Ly$\alpha$, both in LBGs and LAEs, are driven by variations of $N_{\rm HI}$ 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 Ly$\alpha$ emission with increasing redshift.

Key words: line: profiles -- radiative transfer -- galaxies: starburst -- galaxies: ISM -- galaxies: high-redshift -- ultraviolet: galaxies

© ESO 2008