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On the (Non-)Enhancement of the LyαEquivalent Width by a Multiphase Interstellar
MediumP. Laursen, F. Duval & G. Ostlin
-2013 ApJ 766 124
Christian Herenz
MUSE Science Meeting @ Leibniz-Institut fur Astrophysik in Potsdam (AIP)
August 28, 2013
Introduction
• Lyα equivalent width
EWLyα =
∫ &λLyα
.λLyα
F (λ)− F≈λLyαcont.
F≈λLyαcont.
dλ (1)
depends on galacitic parameters such as IMF andmetallicity⇒ suggested as a probe for these quantities.((in)famous EWLyα = 240A limit for “normal” stellarpopulations)
• EWLyα ∼ escape fraction of Lyα photons relative tocontinuum photons.
• Reonant nature of Lyα photons = no direct Lyα escape.F≈λLyα
cont. prone to dust extinction.
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 areduction in continuum luminosity.
The Neufeld Scenario - Numerically Revisted
• “Brute Force” 3DMonte-Carlo Lyα radiativetransfer simulations (MoCaLaTA) of the NeufeldScenario on adaptive gridwith cells either cloud orinter-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
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 isconsidered later.
Fiducial Model Examined - I
fc = nclrgal.σcl =34
Ncl
(rcl
rgal.
)2τd = NHσd
ZZ0
σd = 4× 10−22 cm−2 Z0 ' 0.25Z�
Fiducial Model Examined - III
ICM neutral hydrogen density Probability of photon emittedinside a cloud
Towards a Realistic Model
Approach
⇒ Define typical (unusual & unrealistic) ranges of parametersto be expected in LAEs (using observational constraints, alsofrom nearby SBGs).
⇒ Randomly sample sets of parameters in these ranges
⇒ Set up model “galaxies” spanned in this multi-dimensionalparameter space P & run high-res RT
⇒ Likelihood map for b.
Discussion
• Neufeld Scenario allows for significant b only under veryspecial circumstances?
• Reports of “unusual” large Lyα EW do exist - how toexplain.• Measurment Errors & Missinterpretations.• Stellar Populations.• Delayed escape of Lyα→ (homogenous dust free
rgal. = 8kpc→ tLyαesc. ≈ 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)
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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