3D Lyαradiation transfer

Abstract

Aims. The development of a general code for 3D Lyα radiation transfer in galaxies to understand the diversity of Lyα line profiles observed in star-forming galaxies and related objects. Methods. Using a Monte Carlo technique, we developed a 3D Lyα 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 Lyα 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 Lyα profiles from galaxies. 1) Standard Lyα 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 Lyα profiles can be obtained in expanding (infalling) media from objects without any intrinsic Lyα emission, as a natural consequence of radiation transfer effects; 3) The formation and the detailed shape of Lyα profiles resulting from expanding shells has been thoroughly revised. In particular we find that, for sufficiently large column densities (N H > ∼ 10 20 cm −2 ), the position of the main Lyα emission peak is quite generally redshifted by approximately twice the expansion velocity. This is in excellent agreement with the observations of z ∼ 3 LBGs, which show that Lyα is redshifted by ∼2V exp , where V 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.

The Lyman alpha reference sample

“…For this purpose, we modeled the Lyα radiative transport inside the ISM of Mrk 1486 from a detailed study of our HST imaging and radiation transfer calculations performed using the 3D Lyα Monte Carlo code MCLyα of Verhamme et al (2006) and Schaerer et al (2011).…”

“…This low uncertainty on v exp is explained by the relatively high sensitivity of the output line profile to the mean expansion velocity v exp of the shells (Verhamme et al 2006;Duval et al 2014). We illustrate in Fig.…”

“…As shown in Verhamme et al (2006) and Duval et al (2014), the mean expansion velocity v exp of an ISM plays the most important role in the formation of the observed Lyα line profile of a galaxy. In particular, the observed wavelength of some redshifted bumps on top of an asymmetric Lyα line prove to be good tracers of the mean ISM expansion velocity v exp , such as the backscattering redshifted peak, as already pointed out by Verhamme et al (2006) for expanding shells 8 . As shown in Fig.…”

“…To fit the observed Lyα line of Mrk 1486 we used the latest version of the 3D Monte Carlo radiative transfer code MCLya of Verhamme et al (2006) and Schaerer et al (2011). For a given geometry of neutral hydrogen and dust, the code computes both the Lyα and adjacent UV continuum radiative transfer, including the physical details of the Lyα scattering, dust scattering, and dust absorption.…”

A young star-forming galaxy at z = 3.5 with an extended Lyman α halo seen with MUSE

“…In this section we model the Lyα emission using a radiative transfer code predicting the spectra emerging from spherically expanding shells (Verhamme, Schaerer & Maselli 2006). In our first approach, we fit the Lyα line using the usual library produced by the radiative transfer code, deriving the physical parameters that better describe the total and companion spectra.…”