[Paper Review] 3D Lyman-alpha radiation transfer. III. Constraints on gas and stellar properties of z~3 Lyman break galaxies (LBG) and implications for high-z LBGs and Lyman-alpha emitters(LAEs)

Anne Verhamme, D. Schaerer+2 more|ArXiv.org|May 23, 2008

Galaxies: Formation, Evolution, Phenomena96 references160 citations

TL;DR

This study uses 3D Lyman-alpha radiation transfer modeling to constrain gas and stellar properties of z~3 Lyman break galaxies (LBGs), showing that observed Lyα line profiles and equivalent widths are primarily shaped by H i column density and dust extinction (E(B-V) ~ 0.05–0.2). The key result is that most LBGs have intrinsic Lyα equivalent widths of 50–100 Å, with variations due to radiation transfer and dust, explaining the overlap between LBGs and Lyα emitters (LAEs) and trends in Lyα strength with redshift and SFR.

ABSTRACT

The Aim of our study is to understand the variety of observed Lyman-alpha (Lya) line profiles and strengths in Lyman Break Galaxies (LBGs) and Lya emitters (LAEs), the physical parameters governing them, and hence deriving constraints on the gas and dust content and stellar populations of these objects. Using our 3D Lya radiation transfer code including gas and dust (Verhamme et al. 2006), we fit 11 LBGs from the FORS Deep Field with redshifts between 2.8 and 5 observed by Tapken et al. (2007). A simple geometry of a spherically expanding shell of HI is adopted. RESULTS : The variety of observed Lya profiles is successfully reproduced. Most objects show outflow velocities of 150-200 km/s; two objects are most likely quasi-static. The radial HI column density ranges from NH=2.10^{19} to 7.10^{20} cm^{-2}. Our Lya 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 Lya photons is found to be determined primarily by the extinction, and a simple fit formula is proposed. Intrinsic EW(Lya)~50-100 Angstroms are found for 8/11 objects, as expected for stellar populations forming constantly over long periods (> 10-100 Myr). In three cases we found indications for younger populations. Correlations between the observed EW(Lya) and other observables such as FWHM(Lya), E(B-V),SFR(UV) etc, are reproduced. We also show that there is a clear overlap between LBGs and LAEs. 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. [shortened]

Motivation & Objective

• To understand the physical origins of the observed diversity in Lyα line profiles and strengths in high-redshift Lyman break galaxies (LBGs) and Lyα emitters (LAEs).
• To constrain gas and dust content, stellar populations, and outflow properties in z~3 LBGs using detailed radiation transfer modeling.
• To determine whether observed correlations between Lyα equivalent width, FWHM, extinction, and SFR arise from intrinsic differences or radiative transfer effects.
• To assess the relationship between LBGs and LAEs, particularly the overlap in their observed properties at z~3.
• To test whether Lyα equivalent width can be used as a reliable SFR indicator after correcting for radiation transfer and dust effects.

Proposed method

• 3D Lyα radiation transfer code is applied to 11 z~3 LBGs from the FORS Deep Field with redshifts 2.8–5.
• A spherically symmetric, expanding H i shell geometry is assumed to model Lyα transfer and dust extinction.
• Radiative transfer includes resonant scattering, dust absorption (with E(B-V) as free parameter), and anisotropic escape of Lyα photons.
• Model fits are performed to observed Lyα line profiles and equivalent widths, using intrinsic Lyα EW as a free parameter.
• The model incorporates dust-to-gas ratio variations and tests for deviations from the Galactic dust law.
• A simple empirical fit formula is derived for the Lyα escape fraction as a function of E(B-V).

Experimental results

Research questions

• RQ1 What physical parameters—particularly H i column density and dust extinction—determine the diversity of observed Lyα line profiles and equivalent widths in z~3 LBGs?
• RQ2 To what extent do radiation transfer and dust effects explain the observed correlations between Lyα properties, FWHM, extinction, and SFR in LBGs and LAEs?
• RQ3 How do the intrinsic Lyα equivalent widths of LBGs compare to their observed values, and what do they imply about star formation histories?
• RQ4 What is the relationship between LBGs and LAEs at z~3, and can the observed overlap and increasing LAE/LBG ratio with redshift be explained by radiative transfer and dust?
• RQ5 Can the observed Lyα equivalent width be used to infer E(B-V) if the intrinsic Lyα EW is known?

Key findings

• The observed diversity in Lyα line profiles and strengths in z~3 LBGs is successfully reproduced by a 3D radiation transfer model with spherically symmetric outflows.
• Most LBGs exhibit outflow velocities of 150–200 km s⁻¹, with two objects consistent with quasi-static conditions.
• H i column densities range from N_Hi ~ 2×10¹⁹ to 7×10²⁰ cm⁻², and dust extinction (E(B-V)) ranges from ~0.05 to 0.2, with evidence for a dust-to-gas ratio higher than Galactic.
• The Lyα escape fraction is primarily governed by dust extinction, and a simple fit formula is proposed to relate EW_obs to E(B-V) when intrinsic EW is known.
• Intrinsic Lyα equivalent widths are found to be ~50–100 Å for 8 out of 11 LBGs, consistent with continuous star formation over 10–100 Myr, with three objects suggesting younger populations.
• There is a clear overlap between LBGs and LAEs at z~3: ~20–25% of LBGs with EW(Lyα)_{obs} > 20 Å match LAEs brighter than R_AB = 25.5 mag, and radiation transfer/dust effects naturally explain the increasing LAE/LBG ratio with redshift.