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Lyman Continuum Leakage in the Local Universe
Claus Sanch Tim Janice Sally Roderik Leitherer Borthakur Heckman Lee Oey Overzier STScI JHU JHU STScI UMich ON
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o Can normal (non-AGN) galaxies account for the ionizing photons required for re-ionization?
o Since H I is very effective in absorbing Lyman continuum photons, only a small fraction of the available photons will escape from the galaxy.
o What is fesc?
o Cosmological models suggest fesc ≈ 20% is needed for reionization.
Science Question
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o Local galaxies are not necessarily representative of the galaxy population at high redshift.
o However, they are excellent training sets for studying which factors determine fesc.
Galaxy Sample
Redshift Range
Spatial Information
Solid Angle
Ancillary Data
Observational Challenge
Sample Definition
IGM Properties
~0.0
~1
>3
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Sample 1: starburst galaxies with outflows at z ≈ 0
Lyman Continuum Leakage Candidates
Sample 2: Lyman-break analogs with weak Hα emission at z ≈ 0.2
Zackrisson et al. (2013)
Lyman Continuum Escape in Local Starburst Galaxies with Outflows
o Pilot study as a proof of concept; therefore the sample is driven by instrument requirements
o Preexisting UV spectra with F(1100) > 11014
o v > 10,000 km s1 > 30 Å
o Presence of O stars from UV spectra
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o The three galaxies were observed before with FUSE (Pellerin & Robert 2007; Grimes et al. 2009; Leitet et al. 2013)
o Several studies specifically targeted the Lyman continuum
Galaxy Sample
Galaxy
Type
E(BV)MW
(mag)vLSR
(km s1)D
(Mpc)MB
(mag)E(BV)int
(mag) 12+log O/H SFRUV
(M⊙yr1)
Tol 0440-381 HII 0.014 12,249 167 20.2 0.17 8.2 4
Tol 1247-232 HII 0.077 14,390 207 21.0 0.09 8.1 11
Mrk 54 BCG 0.013 13,450 191 22.2 0.14 8.6 12
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Grimes et al. (2009): Tol 0440-381
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o ISM lines in FUSE spectra are blueshiftedo Large-scale outflows driven by SNe and stellar windso Correlates with SFR/Mo Inhomogeneous ISM creates favorable conditions for
Lyman continuum escape
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o Spectral region around the Lyman break
o Corrected for intrinsic reddening
o Restframeo ~35 Å of Lyman
continuumo Geocoronal
Lyman lines denoted
o Dashed: modelo T=20 Myr, const.
SF, Kroupa IMF, Geneva models with rotation
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Lyman continuum escape fractions
Escape Fraction Tol 0440-381 Tol 1247-232 Mrk 54
fesc(gas only) 0.265±0.235 0.075±0.088 0.055±0.100fesc(gas+dust) 0.015±0.014 0.017±0.019 0.005±0.009
o Total escape fractions are of order 1% or lesso Consistent with optical depth and covering
factor suggested by CII 1036
Local Lyman-break Analogs as Candidates for Leakage
o Heckman et al. (2005): GALEX discovery of a sample of UV-bright galaxies with properties similar to those of LBG’s Lyman-break analogs (LBA’s)
o L ≈ 1011 L⊙
o SFR ≈ 101 – 102 M⊙yr1
o z ≈ 0.2
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Overzier et al. (2009): HST UV+ optical imaging actively star-forming galaxies
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o Heckman et al. (2011): HST COS spectroscopy of a subsample with compact cores
Properties of J0921+4509
o Hα derived SF rate significantly lower than UV+IR derived rate density bounded or inhomogeneous?
o Lyα profile shows blueshifted emission; models suggest low due to small covering factor
o IS absorption lines are optically thick, yet have residual intensity picket-fence model with covering factor of 60%
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o COS spectrum of J0921+4509 (Borthakur et al. 2014)o Young starburst dominated by OB stars; consistent with
profile fits to CIV 1550, NV 1240, and OVI 1035o Predict the expected Lyman-break from models
absolute fesc
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• Borthakur et al. (2014):
• Comparison of the observed break to models
• Dashed: 912+/912-
• Observed: 0.9 dex
• Model: 0.5 dex
• fesc = 0.37± 0.05
• Accounting for dust: fesc ≈ 1%
Take-away Pointso COS is well suited for Lyman continuum
observations in the local universe.o Local star-forming galaxies are excellent
training sets studying Lyman leakage.o The observed local star-forming galaxies
have dust corrected escape fractions of 1% or less.
o In the absence of dust absorption, the galaxies would have fesc compatible with the value required for reionization
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Back-up Slides
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Reliability of the Models
o Model predicts an intrinsic Lyman break of ~0.4 dex for a standard population
o Kroupa IMF, constant star formation, T=20 Myr, Geneva evolution models with rotation, spherically extended, expanding, blanketed non-LTE atmospheres
o How do model assumptions and uncertainties affect the prediction?
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Varying the duration of the SF episode has little effect for T>5 Myr as massive stars have reached equilibrium
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Varying the IMF exponent between 1.6 and 3 changes the flux level by a factor of ~100 but has a small effect on the break itself
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Extended model atmospheres accounting for winds decrease the break by up to 0.15 dex
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Evolution models with rotation have hotter, more luminous stars and increase the Lyman continuum flux by ~0.2 dex
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Chemical composition has a surprisingly small influence due to the compensating effects of atmospheres and evolution models
