Clues on star formation, galaxy evolution and cosmic reionization

Report
Lyman Continuum Leakage
in the Local Universe
Claus
Leitherer
STScI
9/19/2014
Sanch
Borthakur
JHU
Tim
Heckman
JHU
Janice
Lee
STScI
Claus Leitherer:
Lyman Continuum Leakage
Sally
Oey
UMich
Roderik
Overzier
ON
1
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Lyman Continuum Leakage
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Science Question
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.
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Galaxy Sample
Redshift
Range
Spatial
Information
Solid
Angle
Ancillary Observational
Data
Challenge
Sample
Definition
IGM
Properties
~0.0
~1
>3
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.
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Lyman Continuum Leakage Candidates
Zackrisson et al. (2013)
Sample 1:
starburst galaxies with
outflows at z ≈ 0
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Sample 2:
Lyman-break analogs
with weak Hα
emission at z ≈ 0.2
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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) > 11014
o
v > 10,000 km s1   > 30 Å
o
Presence of O stars from UV spectra
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Galaxy Sample
Galaxy
Type
E(BV)MW
(mag)
vLSR
(km s1)
D
(Mpc)
MB
(mag)
E(BV)int
(mag)
12+log O/H
SFRUV
(M⊙yr1)
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
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
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Grimes et al. (2009): Tol 0440-381
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o
o
o
o
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ISM lines in FUSE spectra are blueshifted
Large-scale outflows driven by SNe and stellar winds
Correlates with SFR/M
Inhomogeneous ISM creates favorable conditions for
Lyman continuum escape
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o
o
o
o
o
o
o
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Claus Leitherer:
Lyman Continuum Leakage
Spectral region
around the
Lyman break
Corrected for
intrinsic
reddening
Restframe
~35 Å of Lyman
continuum
Geocoronal
Lyman lines
denoted
Dashed: model
T=20 Myr, const.
SF, Kroupa IMF,
Geneva models
with rotation
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Lyman continuum escape fractions
Escape Fraction
fesc(gas only)
fesc(gas+dust)
o
o
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Tol 0440-381
0.265±0.235
0.015±0.014
Tol 1247-232
0.075±0.088
0.017±0.019
Mrk 54
0.055±0.100
0.005±0.009
Total escape fractions are of order 1% or less
Consistent with optical depth and covering
factor suggested by CII 1036
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Local Lyman-break Analogs as
Candidates for Leakage
o
o
o
o
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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)
L ≈ 1011 L⊙
SFR ≈ 101 – 102 M⊙yr1
z ≈ 0.2
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Overzier et al.
(2009): HST
UV+ optical
imaging

actively starforming galaxies
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o
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Heckman et al. (2011): HST COS spectroscopy of a
subsample with compact cores
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Properties of J0921+4509
o
o
o
Hα derived SF rate significantly lower than
UV+IR derived rate  density bounded or
inhomogeneous?
Lyα profile shows blueshifted emission; models
suggest low  due to small covering factor
IS absorption lines are optically thick, yet have
residual intensity  picket-fence model with
covering factor of 60%
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o
o
o
COS spectrum of J0921+4509 (Borthakur et al. 2014)
Young starburst dominated by OB stars; consistent with
profile fits to CIV 1550, NV 1240, and OVI 1035
Predict the expected Lyman-break from models 
absolute fesc
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Claus Leitherer:
Lyman Continuum Leakage
•
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%
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Take-away Points
o
o
o
o
COS is well suited for Lyman continuum
observations in the local universe.
Local star-forming galaxies are excellent
training sets studying Lyman leakage.
The observed local star-forming galaxies
have dust corrected escape fractions of 1%
or less.
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
o
o
Model predicts an intrinsic Lyman break of ~0.4
dex for a standard population
Kroupa IMF, constant star formation, T=20 Myr,
Geneva evolution models with rotation,
spherically extended, expanding, blanketed
non-LTE atmospheres
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
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