Lyman α radiative transfer in a multiphase medium

By M. Hansen, S. Oh. 2005

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Hydrogen Lyman a (Lya) is our primary emission-line window into high-redshift galaxies. Despite an extensive literature, Lya radiative transfer in the most realistic case of a dusty, multiphase medium has received surprisingly little detailed theoretical attention. We investigate Lya resonant scattering through an ensemble of dusty, moving, optically thick gas clumps. We treat each clump as a scattering particle and use Monte Carlo simulations of surface scattering to quantify continuum and Lya surface scattering angles, absorption probabilities, and frequency redistribution, as a function of the gas dust content. This atomistic approach speeds up the simulations by many orders of magnitude, making possible calculations which are otherwise intractable. Our fitting formulae can be readily adapted for fast radiative transfer in numerical simulations. With these surface scattering results, we develop an analytic framework for estimating escape fractions and line widths as a function of gas geometry, motion, and dust content. Our simple analytic model shows good agreement with full Monte Carlo simulations. We show that the key geometric parameter is the average number of surface scatters for escape in the absence of absorption, N 0 , and we provide fitting formulae for several geometries of astrophysical interest. We consider the following two interesting applications. (i) Equivalent widths (EWs). Lya can preferentially escape from a dusty multiphase interstellar medium if most of the dust lies in cold neutral clouds, which Lya photons cannot penetrate. This might explain the anomalously high EWs sometimes seen in high-redshift/submillimetre sources. (ii) Multiphase galactic outflows. We show the characteristic profile is asymmetric with a broad red tail, and relate the profile features to the outflow speed and gas geometry. Many future applications are envisaged.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e9351d1042fb4e539b5df838ec2ba22bcdc0a13a

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A characteristic scale for cold gas (2016, background): “Radiative transfer in a multi-phase medium, with scattering off numerous optically thick clouds (Neufeld, 1991; Hansen & Oh, 2006) has distinct and unique properties.”
Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies – I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission (2010, background): “While the presence of dust may enhance Lyα relative to the continuum (Neufeld 1991; Hansen & Oh 2006; Finkelstein et al. 2008), the results in Fig.”
3D Lyman-alpha radiation transfer. I. Understanding Lyman-alpha line profile morphologies (2006, methodology): “…new numerical algorithms, mostly based on Monte Carlo techniques (Spaans 1996, Ahn et al. 2001, 2002; 2003, Ahn 2004; Zheng & Miralda-Escudé 2002; Richling et al. 2001; Richling 2003; Kobayashi & Kamaya 2004; Cantalupo et al. 2005; Dijkstra et al. 2005ab; Hansen & Oh 2006; Tasitsiomi 2006).”
The impact of The IGM on high-redshift Lyα emission lines (2007, background): “Furthermore, the spectrum is typically asymmetric and contains more photons redward of this peak (Ahn 2004; Hansen & Oh 2006; Tasitsiomi 2006; Verhamme et al. 2006).”
THE HETDEX PILOT SURVEY. II. THE EVOLUTION OF THE Lyα ESCAPE FRACTION FROM THE ULTRAVIOLET SLOPE AND LUMINOSITY FUNCTION OF 1.9 (2010, background): “…the neutral gas inside a galaxy and in its immediate surround- ings also play an important role regarding the escape of Lyα photons (Verhamme et al. 2006; Dijkstra et al. 2006; Hansen & Oh 2006; Dijkstra et al. 2007; Verhamme et al. 2008; Adams et al. 2009; Laursen et al. 2010; Zheng et al. 2010).”
EXTREME FEEDBACK AND THE EPOCH OF REIONIZATION: CLUES IN THE LOCAL UNIVERSE (2011, background): “Clues from Lyman Alpha The properties of the Lyα profile are highly sensitive to the kinematics and distribution of the outflowing gas and dust in star-forming galaxies (e.g. Steidel et al. 2010; Hansen & Oh 2006; Verhamme et al. 2006, 2008; Kornei et al. 2010).”
Lyα COOLING EMISSION FROM GALAXY FORMATION (2010, background): “…case approximates a multiphase medium in which cold neutral clumps embedded in a hot medium are either so compact that their covering factor is negligible, or in which they simply reflect the Lyα photons and contribute only a small effective Lyα optical depth (e.g., Neufeld 1991; Hansen & Oh 2006).”
THE STRUCTURE AND KINEMATICS OF THE CIRCUMGALACTIC MEDIUM FROM FAR-ULTRAVIOLET SPECTRA OF z ≃ 2–3 GALAXIES (2010, background): “…recent theoretical treatments of Lyα radiative transfer in the context of galactic outflows (Verhamme et al. 2006, 2008; Schaerer & Verhamme 2008; Hansen & Oh 2006; Dijkstra et al. 2006b,a; Zheng & Miralda-Escudé 2002), and even in the highlyidealized geometric configurations considered in…”
ON THE REDSHIFT EVOLUTION OF THE Lyα ESCAPE FRACTION AND THE DUST CONTENT OF GALAXIES (2010, background): “…et al. 2010a), dust geometry (Scarlata et al. 2009), Hi content and kinematics (Kunth et al. 1998; Mas-Hesse et al. 2003; Shapley et al. 2003; Tapken et al. 2007), and geometry/neutral–ionized gas topology (Neufeld 1991; Giavalisco et al. 1996; Hansen & Oh 2006; Finkelstein et al. 2008, 2009).”
Studying reionization with Lyα emitters (2007, background): “A 2-phase structure may be necessary for the transmission of Lyα photons in the presence of dust (Neufeld 1991; Hansen & Oh 2006).”
Atmospheric Escape and the Evolution of Close-In Exoplanets (2018, background): “The Lyman-α cross-section at line centre is σα,c = 5.9×10−14 cm2 for 104 K gas (e.g. Hansen & Oh 2006).”
Lyα Emitting Galaxies as a Probe of Reionisation (2014, background): “Dust can increase the EW of the Lyα line in a ‘clumpy’ medium that consists of cold clumps containing neutral hydrogen gas and dust, embedded within a (hot) ionised, dust free medium (Neufeld 1991; Hansen & Oh 2006).”
Lyα Radiation from Collapsing Protogalaxies. I. Characteristics of the Emergent Spectrum (2005, background): “If the medium is clumpy, then Ly photons may still escape by scattering of (dusty) cold clouds embedded in a tenuous medium (Neufeld 1991; Haiman & Spaans 1999; Hansen & Oh 2006).”