Radiative Transfer with Finite Elements II. Ly-alpha Line Transfer in Moving Media

A finite element method for solving the resonance line transfer problem in moving media is presented. The algorithm works in three spatial dimensions on unstructured grids which are adaptively refined by means of an a posteriori error indicator. Frequency discretization is implemented via a first-order Euler scheme. We discuss the resulting matrix structure for coherent isotropic scattering and complete redistribution. The solution is performed using an iterative procedure, where monochromatic radiative transfer problems are successively solved. The present implementation is applicable for arbitrary model configurations with an optical depth up to 10^(3-4). Results of Ly-alpha line transfer calculations for a spherically symmetric model, a disk-like configuration, and a halo containing three source regions are discussed. We find the characteristic double-peaked Ly-alpha line profile for all models with an optical depth > 1. In general, the blue peak of the profile is enhanced for models with infall motion and the red peak for models with outflow motion. Both velocity fields produce a triangular shape in the two-dimensional Ly-alpha spectra, whereas rotation creates a shear pattern. Frequency-resolved Ly-alpha images may help to find the number and position of multiple Ly-alpha sources located in a single halo. A qualitative comparison with observations of extended Ly-alpha halos associated with high redshift galaxies shows that even models with lower hydrogen column densities than required from profile fitting yield results which reproduce many features in the observed line profiles and two-dimensional spectra.Comment: 13 pages, accepted for publication in A&

Lyman alpha emission in high-redshift galaxies

A significant fraction of the high-redshift galaxies show strong Lyman emission lines. For redshifts z>5, most known galaxies belong to this class. However, so far not much is known about the physical structure and nature of these objects. Our aim is to analyse the Lyman alpha emission in a sample of high-redshift UV-continuum selected galaxies and to derive the physical conditions that determine the Lyman alpha profile and the line strength. VLT/FORS spectra with a resolution of R ~ 2000 of 16 galaxies in the redshift range of z = 2.7 to 5 are presented. The observed Lyman alpha profiles are compared with theoretical models. The Lyman alpha lines range from pure absorption (EW = -17 Angstroem) to strong emission (EW = 153 Angstroem). Most Lyman alpha emission lines show an asymmetric profile, and three galaxies have a double-peaked profile. Both types of profiles can be explained by a uniform model consisting of an expanding shell of neutral and ionised hydrogen around a compact starburst region. The broad, blueshifted, low-ionisation interstellar absorption lines indicate a galaxy-scale outflow of the ISM. The strengths of these lines are found to be determined in part by the velocity dispersion of the outflowing medium. We find star-formation rates of these galaxies ranging from SFR(UV) = 1.2 to 63.2 Msun uncorrected for dust absorption. The Lyman alpha emission strength of our target galaxies is found to be determined by the amount of dust and the kinematics of the outflowing material.Comment: 11 pages, 6 figures. A&A accepte

A REVIEW OF HEIDELBERG RADIATIVE TRANSFER EQUATION SOLUTIONS

Analytical and numerical solutions of the radiative transfer equation recently obtained in Heidelberg are reviewed. In particular, a multidimensional finite-element algorithm for unstructured grids that can be adaptively refined is discussed in detail

Radiative transfer with finite elements

A finite element method for solving the monochromatic radiation transfer equation including scattering in three dimensions is presented. The algorithm employs unstructured grids which are adaptively refined. Adaptivity as well as ordinate parallelization reduce memory requirements and execution time and make it possible to calculate the radiation field across several length scales for objects with strong opacity gradients. An a posteriori error estimate for one particular quantity is obtained by solving the dual problem. The application to a sample of test problems reveals the properties of the implementation