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

Neufeld D. 1991, ApJ 370, L85

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 - II

cloud velocity dispersion terminal outflow expansion velocity

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.