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On the (Non-)Enhancment of the Lyα Equivalent Width by a Multiphase Interstellar Medium

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On the (Non-)Enhancment of the Lyα Equivalent Width by a Multiphase Interstellar Medium

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Talk at MUSE Science Meeting @ Leibniz-Institut für Astrophysik in Potsdam (AIP) presenting a recent Paper by P. Laursen et al. http://dx.doi.org/10.1088/0004-637X/766/2/124

Talk at MUSE Science Meeting @ Leibniz-Institut für Astrophysik in Potsdam (AIP) presenting a recent Paper by P. Laursen et al. http://dx.doi.org/10.1088/0004-637X/766/2/124

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On the (Non-)Enhancment of the Lyα Equivalent Width by a Multiphase Interstellar Medium

  1. 1. On the (Non-)Enhancement of the Lyα Equivalent Width by a Multiphase Interstellar Medium P. Laursen, F. Duval & G. ¨Ostlin - 2013 ApJ 766 124 Christian Herenz MUSE Science Meeting @ Leibniz-Institut f¨ur Astrophysik in Potsdam (AIP) August 28, 2013
  2. 2. Introduction • Lyα equivalent width EWLyα = λLyα λLyα F(λ) − F ≈λLyα cont. F ≈λLyα cont. dλ (1) depends on galacitic parameters such as IMF and metallicity ⇒ suggested as a probe for these quantities. ((in)famous EWLyα = 240 ˚A limit for “normal” stellar populations) • EWLyα ∼ escape fraction of Lyα photons relative to continuum photons. • Reonant nature of Lyα photons = no direct Lyα escape. F ≈λLyα cont. prone to dust extinction.
  3. 3. Notation • EWin . . . galaxies intrinsic EW • EWem . . . emitted EW • EWobs = (1 + z) · EWem . . . observed EW Boost: b = EWem EWin Boost does not imply an increase in Lyα luminosity, rather a reduction in continuum luminosity.
  4. 4. The Neufeld Scenario Neufeld D. 1991, ApJ 370, L85
  5. 5. The Neufeld Scenario - Numerically Revisted • “Brute Force” 3D Monte-Carlo Lyα radiative transfer simulations ( MoCaLaTA) of the Neufeld Scenario on adaptive grid with cells either cloud or inter-cloud. • Cell-Parameters: nH, nd, T & vbulk • Galaxy-Parameters: Ncl, rcl & ncl = Ncl/Vcl ∝ Ncl/r3 gal. • Related Quantitites: Z, fc Typical Model: 103 − 105 clouds, each with 102 − 103 cells
  6. 6. Fiducial Model Fiducial Model: Ncl = 6500, rgal = 5 kpc, nH = 1 cm−3, Tcl = 104 K (WNM), later TICM = 106 K (HIM) Vary one Parameter at a time, “realism” of those parameters is considered later.
  7. 7. Fiducial Model Examined - I fc = nclrgal.σcl = 3 4 Ncl rcl rgal. 2 τd = NHσd Z Z0 σd = 4 × 10−22 cm−2 Z0 0.25Z
  8. 8. Fiducial Model Examined - II cloud velocity dispersion terminal outflow expansion velocity
  9. 9. Fiducial Model Examined - III ICM neutral hydrogen density Probability of photon emitted inside a cloud
  10. 10. Towards a Realistic Model Approach ⇒ Define typical (unusual & unrealistic) ranges of parameters to be expected in LAEs (using observational constraints, also from nearby SBGs). ⇒ Randomly sample sets of parameters in these ranges ⇒ Set up model “galaxies” spanned in this multi-dimensional parameter space P & run high-res RT ⇒ Likelihood map for b.
  11. 11. Discussion • Neufeld Scenario allows for significant b only under very special circumstances? • Reports of “unusual” large Lyα EW do exist - how to explain. • Measurment Errors & Missinterpretations. • Stellar Populations. • Delayed escape of Lyα → (homogenous dust free rgal. = 8kpc → t Lyα esc. ≈ 100 kyr) • AGN activity • Viewing Angle • Cooling Radiation • Star Formation Stochasticity (only becomes relevant SFR 1 M yr−1 ) • Inhomogenous Escape (only if galaxies are resolved)
  12. 12. Summary & Conclusion • Neufeld Scenario can be reproduced in RT simulations. • However, as soon as more realistic “ingredients” are introduced (random motions of clouds, expansion flows, nHI,ICM ...) EWLyα boosts are unlikely in this scenario. However, if • Z Z • nHI,cl much larger than typical • Most Lyα photons launched in ICM (i.e. at edges of clouds) • (almost) no “outflows” & no random motions significant boosts could be reached. • These explanations are deemed less likely than other scenarios for observed high Lyα EWs.

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