EDP Sciences
Free access
Volume 460, Number 2, December III 2006
Page(s) 397 - 413
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20065554

A&A 460, 397-413 (2006)
DOI: 10.1051/0004-6361:20065554

3D Ly$\alpha$ radiation transfer

I. Understanding Ly$\alpha$ line profile morphologies
A. Verhamme1, D. Schaerer1, 2, and A. Maselli3

1  Observatoire de Genève, 51 Ch. des Maillettes, 1290 Sauverny, Switzerland
    e-mail: daniel.schaerer@obs.unige.ch
2  Laboratoire d'Astrophysique (UMR 5572), Observatoire Midi-Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
3  Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany

(Received 5 May 2006 / Accepted 26 July 2006)

Aims.The development of a general code for 3D Ly$\alpha$ radiation transfer in galaxies to understand the diversity of Ly$\alpha$ line profiles observed in star-forming galaxies and related objects.
Methods.Using a Monte Carlo technique, we developed a 3D Ly$\alpha$ radiation transfer code that allows for prescribed arbitrary hydrogen density, ionisation, temperature structures, dust distributions, arbitrary velocity fields, and UV photon sources. As a first test and application we examined the Ly$\alpha$ line profiles predicted for several simple geometrical configurations and their dependence on the main input parameters.
Results.Overall, we find line profiles reaching from doubly peaked symmetric emission to symmetric Voigt (absorption) in static configurations with increasing dust content, and asymmetric red- (blue-) shifted emission lines with a blue (red) counterpart ranging from absorption to emission (with increasing line/continuum strength) in expanding (infalling) media. In particular we find the following results to be interesting for the interpretation of Ly$\alpha$ profiles from galaxies. 1) Standard Ly$\alpha$ absorption line fitting of global spectra of galaxies may lead to an underestimation of the true hydrogen column density in certain geometrical conditions; 2) Normal (inverted) P-Cygni-like Ly$\alpha$ profiles can be obtained in expanding (infalling) media from objects without any intrinsic Ly$\alpha$ emission, as a natural consequence of radiation transfer effects; 3) The formation and the detailed shape of Ly$\alpha$ profiles resulting from expanding shells has been thoroughly revised. In particular we find that, for sufficiently large column densities ( $N_{\rm H} \ga 10^{20}$ cm-2), the position of the main Ly$\alpha$ emission peak is quite generally redshifted by approximately twice the expansion velocity. This is in excellent agreement with the observations of $z \sim 3$ LBGs, which show that Ly$\alpha$ is redshifted by ~ $2V_{\rm exp}$, where $V_{\rm exp}$ is the expansion velocity measured from the interstellar absorption lines blueshifted with respect to the stellar redshift. This finding also indicates that large-scale, fairly symmetric shell structures must be a good description of the outflows in LBGs.

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

© ESO 2006