Probing Reionization with
Metal Absorption Lines
George Becker
Kavli Institute for Cosmology, Cambridge
Wal Sargent, Michael Rauch,
Bob Carswell, Alex Calverley
New Horizons for High Redshifts
July 27, 2011
Outline
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Survey results for high-ionization (C IV) and
low-ionization (O I) metals lines at z~6
•
Possible implications for hydrogen and
helium reionization
•
Nucleosynthesis in the reionization era
Metals in the IGM
(and in galaxies)
•
Tracers of
•
•
•
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Kawata & Rauch (2007)
C IV
Ellison et al
Star Formation
Galaxy Kinematics
Outflows
Galaxy -- IGM interactions
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Highly-ionized metals traced by C IV, Si IV, O VI
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Means to detect faint galaxies
Neutral metals traced by O I, Si II, C II, Fe II,
etc. -- DLAs
Direct reionization probe?
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ΔE(O I) = ΔE(H I)
QSO probes of the high-z IGM
Proximity zones
- Present at any redshift
- Biased regions?
Lyα forest
- Traces bulk of the IGM
- Saturates at z ~ 6
Metal Lines
- Trace dense regions
- Probe of star formation
- still useful at z > 6
IGM Metals at z~3
What we “know”
C IV
Ellison et al (2000)
Adelberger et al (2005)
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Metallicity [M/H] ~ -3, and decreases
towards lower densities.
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The densest 10% of the IGM is enriched.
The rest may or may not be pristine.
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Metals are found near star-forming
galaxies.
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Metals at z~3 could have either been
deposited recently by galactic winds from
LBGs, or at earlier epochs by lower-mass
galaxies.
z=6: The “Age of C IV”?
- B. Oppenheimer
Simulations suggest the
ionization balance at z~6
should highly favor C IV
Oppenheimer+ 2009
So where are the metals?
Number density and mass density of C IV both decline by ≥x4 at z > 5.3
z = 2-4.5
Songaila (2001)
D’Odorico+ (2010)
Simcoe (2011b)
z > 5.3
Simcoe+ (2011)
Becker+ (2009)
Number density
Comoving mass density
(dominated by rare, strong systems)
Becker+ (2009)
Ryan-Weber+ (2006,2009)
Simcoe+ (2006,2011)
Enrichment or ionization effect?
C IV host galaxy: See poster by Gonzalo Diaz!
Low-Ionization Metal Systems at z~6
z = 6.1312
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Strong low-ionization lines:
O I, C II, Si II, Fe II
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Weak or absent high-ionization lines:
Si IV, C IV
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Numerous
H I columns unknown
DLA at z=3.864
Wolfe+ (2005)
O I Survey
Expanded Survey:
17 QSOs at z > 5.8
Δz = 8.0
(ΔX = 40.)
Keck HIRES & ESI,
Magellan MIKE
10 systems
Becker+ (2011)
1. Similar dn/dX as z~2-4 DLAs and sub-DLAs
2. At z~6, strong O I is more common than strong C IV
Comparison with DLA evolution
0.4
O I systems
(95% confidence)
0.3
0.20
DLAs
Prochaska+ 09
Worseck+ preliminary!
DLA
(X)
0.15
0.10
0.05
0.00
2.0
2.5
3.0
3.5
4.0
z
4.5
5.0
5.5
6
W1526 (Å)
Velocity Widths
ed
t
ra
u
t
Sa
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At a given velocity widht, z~6 systems are weaker than lower-z
DLAs and sub-DLAs
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Scaling relation suggests that z~6 systems are have metallicities
that are ≥ 0.4 dex lower than lower-z counterparts
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Evolution in the “mass”-metallicity relation?
Sampling lower metallicities at larger radii?
Signature of a lower UV background?
z=3
z=6
C IV
C II
?
?
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•
‣
4x fewer DM halos
10 x lower UV background
Similar total cross section
He II & the UV Background
1. He II will remove C III → C IV photons
Without He II “sawtooth”
after He II
reionization
before He II
reionization
Photoionization
Heating
Madau & Haardt (2009)
2. He II reionization mostly occurs at z < 5
Becker et al (2011)
Possible scenarios at z~6
1. Low Enrichment
C IV
3. Enriched Regions Mostly Neutral
2. Lower ionization
C III
In all cases, would see the low ions (C II and O I),
and very little or no C IV.
C II
Nucleosynthesis in the reionization era
Fe and α elements
z=5.34: Keck/HIRES + VLT/X-Shooter
Becker+, in prep
Nucleosynthesis in the reionization era
Low-Ionization System
VMP DLAs (Cooke+ 2011)
DLAs with [M/H] < -1.0
sub-DLAs
[O/Fe]
[C/O]
Fe and α elements
• Enrichment timescales < 1 Gyr
• Relative abundances consistent
Becker+, in prep
[Si/Fe]
[C/Si]
with lower-redshift metal-poor
absorption systems (& metalpoor halo stars)
z > 5: optical & near-IR data
from Keck, Magellan, & VLT
• Very little scatter
• Evidence that stars that ended
the dark ages were “ordinary”
Pop II stars
see posters by Ryan Cooke
and Bryan Penprase!
Metals at z~7
ULAS J1120
VLT X-Shooter
5.0 hrs
(highly binned)
No intervening z~7 metals visible -- yet.
Prospects for an “O I forest”
(e.g., Oh 2002)
ULAS J1120+0641
fH I = 0.1, Z = 10−2.5 Z⊙
Current X-Shooter Data
Prospects for an “O I forest”
(e.g., Oh 2002)
ULAS J1120+0641
fH I = 0.1, Z = 10−2.5 Z⊙
Improved S/N
A “forest” of O I, Si II, and C II lines should be visible if the
IGM is significantly neutral AND metal enriched.
Summary
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Two trends are apparent at z~6
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High-ionization (C IV) systems decline at z > 5.3
Low-ionization (O I) systems have a roughly constant
number density from z < 4 to z~6
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May be due to a combination of enrichment and ionization
effects, and may reflect changes during both hydrogen and
helium reionization.
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Weak low-ionization lines suggest low-metallicity galaxies
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Further progress expected out to z~7 and beyond
Relative abundances (yields of massive stars) show no evolution
with redshift out to z~6. No evidence for exotic stellar
populations.