3D Ly alpha radiation transfer III. Constraints on gas and stellar properties of z similar to 3 Lyman break galaxies (LBG) and implications for high-z LBGs and Ly alpha emitters

摘要

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 Lya 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 Lya profiles is successfully reproduced. Most objects show outflow velocities of V-exp similar to 150-200 km s(-1); two objects are most likely quasi-static. The radial H I column density ranges from N-HI similar to 2 x 10(19) to 7 x 10(20) cm(-2). Our Ly alpha profile fits yield values of E(B - V) similar to 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 Lya 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)(obs) can yield E(B - V), if the intrinsic Lya equivalent width is known (or assumed). Intrinsic EW(Ly alpha)(int) similar to 50-100 angstrom are found for 8/11 objects, as expected for stellar populations forming constantly over long periods (greater than or similar to 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)(obs) and other observables such as FWHM(Ly alpha), E(B - V), SFR(UV) etc. We suggest that most observed trends of Lya, both in LBGs and LAEs, are driven by variations of NHI 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 similar to 3 approximately 20-25 of the LBGs of Shapley et al. (2003, ApJ, 588, 65) overlap with similar to 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 Lya emission with increasing redshift.