On The Origin of Lyman-alpha Absorption in Nearby Starbursts and Implications for Other Galaxies - Astrophysics > Cosmology and Nongalactic AstrophysicsReport as inadecuate




On The Origin of Lyman-alpha Absorption in Nearby Starbursts and Implications for Other Galaxies - Astrophysics > Cosmology and Nongalactic Astrophysics - Download this document for free, or read online. Document in PDF available to download.

Abstract: Abridged Despite the privileged position that Lyman-alpha Lya emissionline holds in the exploration of the distant universe and modern observationalcosmology, the origin of the observed diversity of lya profiles remains to bethoroughly explained. Observations of nearby star forming galaxies bring theirbatch of apparent contradictions between Lya emission and their physicalparameters, and call for a detailed understanding of the physical processes atwork. IZw 18, one of the most metal-poor galaxies known is of particularinterest in this context.We use a 3D Lya radiation transfer code to model Hubble Space Telescope HSTobservations of IZw 18 and to fit its Lya spectrum. Different geometricalconfigurations of the source and the neutral gas are explored.The integrated Lya profile of NW region of IZw 18 is reproduced using theobserved small amount of dust EB-V ~ 0.05 and a spherical HI shell withNHI = 6.5 x 10^21 cm^-2. Such a high column density makes it possible totransform a strong Lya emission EWLya = 60 A into a damped absorption evenwith a small extinction. When a slab geometry is applied and a given line ofsight is chosen, the Lya profile can be successfully reproduced with no dust atall and NHI = 3 x 10^21 cm^-2. The spatial variations of the profileshape are naturally explained by radiation transfer effects. In the case ofoutflowing Inter Stellar Medium ISM, as commonly observed in Lyman BreakGalaxies LBGs, a high NH and dust content are required to observe Lya inabsorption. For nearly static neutral gas as observed in IZw 18 and other lowluminosity galaxies only a small amount of dust is required provided asufficiently high NH covers the galaxy.



Author: Hakim Atek 1, Daniel Schaerer 2 and 3, Daniel Kunth 1 1 Institut d'Astrophysique de Paris, 2 Observatoire de Geneve, 3 Labor

Source: https://arxiv.org/







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