Lyman-alpha line and continuum radiative transfer in a clumpy interstellar medium

F. Duval; D. Schaerer; G. Östlin; Peter Laursen

doi:10.1051/0004-6361/201220455

Lyman-alpha line and continuum radiative transfer in a clumpy interstellar medium

F. Duval, D. Schaerer, G. Östlin, Peter Laursen

DARK

53 Citationer (Scopus)

Abstract

Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Lyα) line in starburst galaxies. Methods. Using our 3D Monte Carlo Lyα radiation transfer code, we have studied the radiative transfer of Lyα, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Lyα and continuum escape fraction, the Lyα equivalent width EW(Lyα), the Lyα line profile, and their dependence on the geometry of the gas distribution and the main input physical parameters. Results. The ISM clumpiness is found to have a strong impact on the Lyα line radiative transfer, leading to a strong dependence of the emergent features of the Lyα line (escape fraction, EW(Lyα)) on the ISM morphology. Although a clumpy and dusty ISM appears more transparent to radiation (both line and continuum) compared to an equivalent homogeneous ISM of equal dust optical depth, we find that the Lyα photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Lyα line (EWobs(Lyα)) is lower than the intrinsic one (EWint(Lyα)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Lyα photons escape more easily than the continuum, resulting in an enhanced EWobs(Lyα). The requirement for this to happen is that the ISM is almost static (galactic outflows ≤200 km s..1), extremely clumpy (with density contrasts >107 in HI between clumps and the interclump medium), and very dusty (E(B..V) > 0:30). When these conditions are fulfilled, the emergent Ly line profile generally shows no velocity shift and little asymmetry. Otherwise, the Ly line profile is very similar to the one expected for homogeneous media. Conclusions. Given the asymmetry and velocity shifts generally observed in star-forming galaxies with Lyα emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Lyα/UV transmission.

Originalsprog	Engelsk
Artikelnummer	A52
Tidsskrift	Astronomy & Astrophysics
Vol/bind	562
ISSN	0004-6361
DOI	https://doi.org/10.1051/0004-6361/201220455
Status	Udgivet - 6 feb. 2014

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10.1051/0004-6361/201220455

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title = "Lyman-alpha line and continuum radiative transfer in a clumpy interstellar medium",

abstract = "Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Lyα) line in starburst galaxies. Methods. Using our 3D Monte Carlo Lyα radiation transfer code, we have studied the radiative transfer of Lyα, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Lyα and continuum escape fraction, the Lyα equivalent width EW(Lyα), the Lyα line profile, and their dependence on the geometry of the gas distribution and the main input physical parameters. Results. The ISM clumpiness is found to have a strong impact on the Lyα line radiative transfer, leading to a strong dependence of the emergent features of the Lyα line (escape fraction, EW(Lyα)) on the ISM morphology. Although a clumpy and dusty ISM appears more transparent to radiation (both line and continuum) compared to an equivalent homogeneous ISM of equal dust optical depth, we find that the Lyα photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Lyα line (EWobs(Lyα)) is lower than the intrinsic one (EWint(Lyα)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Lyα photons escape more easily than the continuum, resulting in an enhanced EWobs(Lyα). The requirement for this to happen is that the ISM is almost static (galactic outflows ≤200 km s..1), extremely clumpy (with density contrasts >107 in HI between clumps and the interclump medium), and very dusty (E(B..V) > 0:30). When these conditions are fulfilled, the emergent Ly line profile generally shows no velocity shift and little asymmetry. Otherwise, the Ly line profile is very similar to the one expected for homogeneous media. Conclusions. Given the asymmetry and velocity shifts generally observed in star-forming galaxies with Lyα emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Lyα/UV transmission.",

author = "F. Duval and D. Schaerer and G. {\"O}stlin and Peter Laursen",

year = "2014",

month = feb,

day = "6",

doi = "10.1051/0004-6361/201220455",

language = "English",

volume = "562",

journal = "Astronomy and Astrophysics Supplement Series",

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TY - JOUR

T1 - Lyman-alpha line and continuum radiative transfer in a clumpy interstellar medium

AU - Duval, F.

AU - Schaerer, D.

AU - Östlin, G.

AU - Laursen, Peter

PY - 2014/2/6

Y1 - 2014/2/6

N2 - Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Lyα) line in starburst galaxies. Methods. Using our 3D Monte Carlo Lyα radiation transfer code, we have studied the radiative transfer of Lyα, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Lyα and continuum escape fraction, the Lyα equivalent width EW(Lyα), the Lyα line profile, and their dependence on the geometry of the gas distribution and the main input physical parameters. Results. The ISM clumpiness is found to have a strong impact on the Lyα line radiative transfer, leading to a strong dependence of the emergent features of the Lyα line (escape fraction, EW(Lyα)) on the ISM morphology. Although a clumpy and dusty ISM appears more transparent to radiation (both line and continuum) compared to an equivalent homogeneous ISM of equal dust optical depth, we find that the Lyα photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Lyα line (EWobs(Lyα)) is lower than the intrinsic one (EWint(Lyα)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Lyα photons escape more easily than the continuum, resulting in an enhanced EWobs(Lyα). The requirement for this to happen is that the ISM is almost static (galactic outflows ≤200 km s..1), extremely clumpy (with density contrasts >107 in HI between clumps and the interclump medium), and very dusty (E(B..V) > 0:30). When these conditions are fulfilled, the emergent Ly line profile generally shows no velocity shift and little asymmetry. Otherwise, the Ly line profile is very similar to the one expected for homogeneous media. Conclusions. Given the asymmetry and velocity shifts generally observed in star-forming galaxies with Lyα emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Lyα/UV transmission.

AB - Aims. We study the effects of an inhomogeneous interstellar medium (ISM) on the strength and the shape of the Lyman alpha (Lyα) line in starburst galaxies. Methods. Using our 3D Monte Carlo Lyα radiation transfer code, we have studied the radiative transfer of Lyα, UV, and optical continuum photons in homogeneous and clumpy shells of neutral hydrogen and dust surrounding a central source. Our simulations predict the Lyα and continuum escape fraction, the Lyα equivalent width EW(Lyα), the Lyα line profile, and their dependence on the geometry of the gas distribution and the main input physical parameters. Results. The ISM clumpiness is found to have a strong impact on the Lyα line radiative transfer, leading to a strong dependence of the emergent features of the Lyα line (escape fraction, EW(Lyα)) on the ISM morphology. Although a clumpy and dusty ISM appears more transparent to radiation (both line and continuum) compared to an equivalent homogeneous ISM of equal dust optical depth, we find that the Lyα photons are, in general, still more attenuated than UV continuum radiation. As a consequence, the observed equivalent width of the Lyα line (EWobs(Lyα)) is lower than the intrinsic one (EWint(Lyα)) for nearly all clumpy ISM configurations being considered. There are, however, special conditions under which Lyα photons escape more easily than the continuum, resulting in an enhanced EWobs(Lyα). The requirement for this to happen is that the ISM is almost static (galactic outflows ≤200 km s..1), extremely clumpy (with density contrasts >107 in HI between clumps and the interclump medium), and very dusty (E(B..V) > 0:30). When these conditions are fulfilled, the emergent Ly line profile generally shows no velocity shift and little asymmetry. Otherwise, the Ly line profile is very similar to the one expected for homogeneous media. Conclusions. Given the asymmetry and velocity shifts generally observed in star-forming galaxies with Lyα emission, we therefore conclude that clumping is unlikely to significantly enhance their relative Lyα/UV transmission.

U2 - 10.1051/0004-6361/201220455

DO - 10.1051/0004-6361/201220455

M3 - Journal article

SN - 0004-6361

VL - 562

JO - Astronomy and Astrophysics Supplement Series

JF - Astronomy and Astrophysics Supplement Series

M1 - A52

ER -

Lyman-alpha line and continuum radiative transfer in a clumpy interstellar medium

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