Abundances of neutral argon
and
ionization properties of DLA galaxies
G. Vladilo
INAF - Trieste Astronomical Observatory
Friday, December 6, 13
The ESO UVES Advanced Data Products Quasar Sample IV. On the deficiency of Argon in DLA systems
Tayyaba Zafar, Giovanni Vladilo, Miriam Centurión, Céline Péroux,
Valentina D’Odorico, Kumail Abbas, Paolo Molaro, Attila Popping
ESO-Garching (Germany), INAF-Trieste (Italy), CNRS-LAM (France),
Univ. Punjab (Pakistan), ICRAR (Australia)
EUADP: sample of 250 quasars spectra collected to study DLAs & sub-DLAs
see Zafar, Popping & Péroux, 2013, A&A, 556, A140
Friday, December 6, 13
Why Argon ?
Interstellar studies indicate that the abundance of ArI
is sensitive to ionization conditions, rather than dust depletion
(Sofia & Jenkins 1998, Jenkins 2013)
IP(Ar o) = 15.76 eV
At hν > 13.6 eV
(photoionization rate)/(recombination rate) ~1 order of magnitude higher for ArI than for HI
Hard photons with hν ≫ 13.6 eV can leak through HI layers and efficiently ionize ArI
In a previous work we took advantage of these characteristics
to employ ArI abundances measured in DLA systems as a probe of ionization conditions
during the epoch of cosmological reionization of HeII,
when the IGM becomes transparent to photons with hν > 54.4 eV
Friday, December 6, 13
ArI as a tracer of ionization evolution
Paper I, Vladilo et al. 2003, A&A 402, 487
zabs=3.39 Q0000-26
General deficiency of
Ar in DLA systems
[Ar/α]
Tentative evidence of a
trend with redshift
z
The cosmological reionization of HeII is expected to be completed around z ~ 3
Is Ar ionized at z > 3 ?
Friday, December 6, 13
Expanding the dataset of ArI lines in DLA systems
Search for ArI lines in 140 DLAs at z > 1.7
in the framework of the EUADP
Challenging task:
ArI lines λ1048 Å, λ1066 Å
in Lyα forest
Absorption lines fitted with Voigt profiles
Parameters of the profiles constrained from fits of other lines
(SiII, FeII, SII)
Friday, December 6, 13
zabs=3.2655 in QSO J1113-1533
Friday, December 6, 13
zabs=2.3377 in LBQS 1232+0815
zabs=2.9669 in QSO J1356-1101
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zabs=3.0572 in QSO B2332-094
The ArI column-density database
EUADP quasar spectra:
Our analysis: 5 new
measurements + 4 limits
Previous work: 9
measurements + 2 limits
Merged sample with data
from other surveys:
19 measurements + 15 limits
Most are classic DLAs
A few “proximate DLAs”
are also present
(relative velocity with respect zQSO
Δv < 3000 km s-1)
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HI, ArI, SII, SiII, FeII, ZnII column densities → abundance ratios
Results:
Ar abundances in DLA systems
Friday, December 6, 13
Deficiency of argon in DLA systems
<[Ar/H]>=-1.76 ± 0.35 dex
Lower abundance than the typical metallicity of DLA systems: [M/H] ≃ -1.1 dex
<[Ar/S]>=-0.61 ± 0.21 dex
<[Ar/Si]>=-0.64 ± 0.25 dex
Scatter larger than in other metal-to-metal ratios in DLA systems
We adopt log (Ar/H) + 12 = 6.60 (Jenkins 2013)
Slightly higher than in Sofia & Jenkins (1998)
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Trends of [Ar/α] ratios in DLA systems
We use S & Si as reference “metals”
Advantages:
1) can be measured in DLAs at z > 1.7
2) α-capture elements (common nucleosynthetic history with Ar)
3) little or negligible dust depletion
We investigate trend versus zabs, N(HI) and metallicity
[S/H] used as a dust-free indicator of metallicity
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[Ar/α] versus zabs
Tentative evidence for a positive trend with redshift
More scatter than in the original sample of Paper I
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[Ar/α] versus N(HI)
Weak evidence for a positive trend with HI column density,
but with a large scatter
Also found in Paper I
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[Ar/α] versus metallicity
Tentative evidence for a negative trend with metallicity
Reported here for the first time
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Correlation analysis
The statistical evidence for correlation is generally weak
e.g., Spearman’s rank correlation coefficient ρSp ~ 0.3 to 0.7
However, the trends found from S and Si data are consistent
The analysis is complicated by the presence of at least three different quantities
affecting Ar abundances (zabs, NHI and metallicity)
Given the size of the sample, it is difficult to study the effect of one quantity in
presence of variations of the other two quantities
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Correlation analysis
Also the inhomogeneity of the sample can contribute
to the large scatter observed in each trend
e.g., if we exclude proximate DLAs, we obtain slightly larger ρSp
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Nature of the argon deficiency
Dust depletion ?
Nucleosynthesis ?
Ionization ?
Friday, December 6, 13
Dust depletion of argon
Is a fraction of Ar atoms hidden
in interstellar dust grains?
Theoretical arguments suggest that argon is not incorporated in
dust grains (Sofia & Jenkins, 1998)
Ar is a “noble gas”.
Studies of condensation of solids indicate that Ar is extremely
volatile (e.g., Lodders 2003)
Chemical pathways for incorporation of Ar in ices are possible,
in principle, but only in molecular gas (Duley 1980)
Molecular fraction is generally negligible in DLA systems
(e.g., Ledoux et al. 2010, Srianand et al. 2012)
The DLAs of our sample are not molecular
Exception: zabs=4.224 towards QSO 1443+2724; Noterdaeme et al. 2008
Friday, December 6, 13
Empirical test on the existence of dust depletion:
[Ar/S] versus [Zn/Fe]
Studies of local ISM indicate that [Zn/Fe] is an indicator of dust depletion:
Zn is volatile, whereas Fe is refractory
No trend is found between [Ar/S] and [Zn/Fe]
Taking also into account the above mentioned theoretical arguments,
we assume that dust depletion of argon is negligible
Friday, December 6, 13
Nucleosynthesis effects
Studies of galactic chemical evolution indicate that
α-capture elements tend to track each other
Argon is measured in Galactic and extragalactic
HII regions, but not in late-type stars
Measurements in metal-poor HII regions indicate that [Ar/O] ≃ 0
(Henry & Worthey 1999, Izotov & Thuan 1999)
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Nucleosynthesis effects
From the O/H, S/O and Ar/O data in Izotov & Thuan (1999),
we find a small deficiency of [Ar/S]
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Nucleosynthesis effects
However, the behaviour is very different from what we see in our sample
Friday, December 6, 13
Nucleosynthesis effects
Nucleosynthesis cannot explain the observed level of argon
deficiency, especially at high metallicity
We need to invoke another effect
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Ionization effects
Predictions of idealized models of DLA ionization,
constrained by measurements of the AlIII/AlII ratio
(Vladilo et al. 2001,2003)
Dotted line: model S2 (soft radiation field; two-regions)
Dashed line: model H1 (hard radiation field; single region)
Friday, December 6, 13
Ionization effects
At variance with dust depletion and nucleosynthesis,
ionization effects can explain:
1) the observed level of Ar underabundance
2) the existence of a positive trend with N(HI)
We now assume that the observed deficiency of argon
is due to ionization processes
and investigate the possible sources of ionization
Friday, December 6, 13
Sources of argon ionization
Are they internal or external to DLA host galaxies ?
Can they provide an explanation for the observed trends
of [Ar/α] versus redshift and metallicity ?
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Ar ionization requires hard photons → QSO-like energy distribution
The most natural source of ionization is the UV/X-ray quasar background,
which is particularly strong at the redshift of our sample
The redshift evolution of the quasar background provides a natural explanation
for the existence of a positive trend [Ar/α] versus zabs
Synthesis models of the evolving spectrum of the UV/X-ray diffuse background
(Haardt & Madau 2012) show a severe decline of the quasar contribution at z > 3
Friday, December 6, 13
We do see a hint of the expected trend with z,
but with a large scatter
Which is the origin of the scatter ?
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Scatter of the redshift trend:
(1) Ar ionization is modulated by HI self-absorption
log N(HI) = 21.75
Examples
log N(HI) = 21.37
By excluding all DLA systems with log N(HI) > 21.3,
the Pearson’s correlation coefficient of [Ar/S] versus zabs
increases from +0.30 to +0.52
Friday, December 6, 13
Scatter of the redshift trend:
(2) inhomogeneity of the sample
Results can be affected, for instance, by the presence of
proximate DLAs and
host galaxies with different SFRs
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Proximate DLA systems
Δv < 3000 km s-1
If argon ionization is dominated by quasar photons,
we expect proximate DLAs to show the highest ionization,
owing to their vicinity to the background quasar
Friday, December 6, 13
DLA host galaxies with different SFRs
Is the SFR the key for interpreting the trend with metallicity?
SFR → Δ[M/H] in Δt ≲ 1 Gyr
SFR → SNRs → porosity of hot gas → transparency to hard IGM photons
→ emission of hard photons from local hot gas
Low metallicity
& low porosity of hot gas
High metallicity
& high porosity of hot gas
If DLA host galaxies have a low gravitational potential,
this would help the disruption of neutral gas by bubbles of hot gas
Friday, December 6, 13
Alternative interpretation of the trend with metallicity:
Bias driven by the observed trend of [M/H] versus log N(HI) in DLAs ?
High [M/H] → low N(HI) → low self-shielding → high Ar ionization
[S/H]
[Ar/S]
log N(HI)
Friday, December 6, 13
[S/H]
Comparison with argon deficiency in the Milky Way
Warm Galactic ISM: <[Ar/O]>=-0.4 ± 0.1 dex (Jenkins 2013)
DLA systems:
<[Ar/α]>=-0.6 ± 0.2 dex (this work)
The difference is remarkable, because the MW lines of sight
have lower HI (log NHI < 20) than DLAs
and therefore are less self-shielded
The higher level of Ar ionization in DLAs suggests that
ionization sources must be more effective in DLAs than in the Milky Way
This is consistent with the scenario of DLA ionization by quasar radiation
since the quasar background at z ~3 is much higher than at z ~ 0
Friday, December 6, 13
Probing the end of HeII reionization
Is Ar ionized at z > 3 ?
This work
Paper I
zabs=3.39 Q0000-26
zabs=4.22 Q1443+27
With respect to Paper I, we have a few more systems at z ≳ 3.
They show evidence of Ar deficiency, but there is a gap in the data
at z ≳ 3.4, and the only system at z > 4 is molecular.
Hard ionizing photons seem to be present inside DLAs at z ≳ 3
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Conclusions
The underabundance of Ar in DLAs is confirmed
<[Ar/α]>=-0.6 ± 0.2 dex
Detailed analysis of the [Ar/α] ratios suggests that
the deficiency is due to ionization effects
All together, the data are consistent with a scenario of
DLA ionization by quasar photons,
modulated by the porosity of hot gas in the host galaxies,
and by HI absorption in the DLA neutral layers
Ar appears to be ionized also at z ≳ 3, or perhaps even z ≳ 4,
suggesting that the photons with hν > 54 eV,
expected to leak after HeII reionization, are already present at z ≳ 3 to 4
A larger sample of high-z DLAs is required to corroborate the above scenario
and track the lasts stages of HeII reionization
Friday, December 6, 13
Interpretation of the trend with metallicity:
central role of SFR ?
Clues from the Milky Way ISM distribution
Galactic ISM cartoon by Cox (2005)
Hatched green: CNM & WNM. Yellow: diffuse WIM.
Orange: HIM that contains small regions of hot, young SNRs in red.
Grey areas: Galactic worms where superbubbles break out into the halo
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zabs=2.9092 in QSO B2342+3417
Friday, December 6, 13