Query : 2009A&A...502..791A

2009A&A...502..791A - Astronomy and Astrophysics, volume 502, 791-801 (2009/8-2)

Origin of Lyα absorption in nearby starbursts and implications for other galaxies.


Abstract (from CDS):

Despite the privileged position that Lyman-α (Lyα) emission line holds in the exploration of the distant universe and modern observational cosmology, the origin of the observed diversity of Lyα profiles still needs to be thoroughly explained. Observations of nearby star forming galaxies bring their batch of apparent contradictions between Lyα emission and their physical parameters, and call for a detailed understanding of the physical processes at work. One of the most metal-poor galaxies known, IZw 18, is of particular interest in this context. We fit the Lyα spectrum of IZw 18 to understand the origin of the damped absorption profile and its spatial variations across the NW region, and establish a general picture of the physical parameters governing the Lyα strength and profile both in local and in high-z galaxies. We used a 3D Lyα radiation transfer code to model Hubble Space Telescope (HST) observations of IZw 18. Observational constraints of relevant parameters such as dust or HI column density were derived from previous studies and from the present analysis. Different geometrical configurations of the source and the neutral gas were explored. The integrated Lyα profile of NW region of IZw 18 is reproduced using the observed small amount of dust (E(B-V)≃0.05) and a spherical HI shell with NHI=6.5x1021cm–2. Such a high column density makes it possible to transform a strong Lyα emission (EWLyα=60Å) into a damped absorption even with low extinction. When a slab geometry is applied and a given line of sight is chosen, the Lyα profile can be successfully reproduced with no dust at all and NHI=3x1021cm–2. The spatial variations of the profile shape are naturally explained by radiation transfer effects, i.e. by scattering of Lyα photons, when the observed surface brightness profile of the source is taken into account. In the case of outflowing inter stellar medium (ISM), as commonly observed in Lyman break galaxies (LBGs), a high NHI and dust content are required to observe Lyα in absorption. For nearly static neutral gas as observed in IZw 18 and other low-luminosity galaxies, only a small amount of dust is required provided a sufficiently high NHI covers the galaxy. We also show how geometrical and apertures effects affect the Lyα profile.

Abstract Copyright:

Journal keyword(s): galaxies: starburst - galaxies: ISM - ultraviolet: galaxies - radiative transfer - galaxies: individual: I Zw 18

CDS comments: Paragraph 4.2 FORJ0332-3557 is [CVJ2005] For J0332-3557 Lensed source in Simbad.

Simbad objects: 6

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Number of rows : 6
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2023
1 [CVJ2005] For J0332-3557 Lensed source G 03 32 59.0 -35 57 51           ~ 9 1
2 SBSG 0335-052 bCG 03 37 44.04 -05 02 38.5     16.65     ~ 455 1
3 Mrk 116 PaG 09 34 02.1 +55 14 25           ~ 1075 1
4 Mrk 36 SBG 11 04 58.298 +29 08 16.54   15.50 15.29     ~ 218 0
5 Mrk 829 GiP 14 50 56.495 +35 34 18.27   14.5 14.83     ~ 201 0
6 NAME MS 1512-cB58 AGN 15 14 22.2751 +36 36 25.674     20.64   20.35 ~ 322 0

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