accurate 21cm signal forecasts from simulations of
TRANSCRIPT
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Accurate 21cm signal forecasts from simulations of inhomogeneous reionization
Girish Kulkarni – IoA Cambridge — July 15, 2015
Tirth Roy Choudhury (NCRA Pune), Martin Haehnelt (IoA Cambridge), Ewald Puchwein (IoA Cambridge)
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Cosmological 21cm signal modelling
• Accurate simulations are especially important due to needle-in-a-haystack nature of the problem
• On scales , power spectrum of 21cm signal predicted to be ~ flat (McQuinn+ 2007; Zahn+ 2011; Friedrich+ 2011)
• But all simulations have a drawback: limited dynamical range
k = 0.1–0.3 Mpc�1
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Lyman Limit Systems• Gas expected to have
rich small-scale structure, at least down to the Jeans scale.
• This acts as a sink for ionisation photons
• But importantly, small-scale structure can self-shield and become 21cm bright.
• This can affect large-scale power, but are missed by simulations
Kulkarni+ 2015
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Issue: what is the effect of unresolved self-shielded structure on 21cm predictions?
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SPH Simulations of the IGM• P-Gadget-3:
2048^3 gas particles in a 160 Mpc/h box (Bolton et al.)
• Mass resolution is ~4*10^6 Msun
• Mean inter-particle separation is ~53 comoving kpc.
z=7
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Hybrid Excursion Set Method
• For “radiative transfer,” use excursion set method on the gas distribution (Mesinger & Furlanetto 2007, Choudhury+ 2009, Mesinger+ 2011)
• A point is ionised if sources/sinks > 1 in a sphere centred on it
• Gives large scale morphology but still misses self-shielding (Majumdar+ 2014)
z=7
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Accounting for Sub-structure• To get sub-structure, we calibrate simulation to
given reionization history.
• For a given self-shielding criterion, solve global reionization evolution for photo-ionization rate (Choudhury+ 2015)
dQ
dt=
nion
nH� QM
trec
H II H I H II
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Get consistent radiative transfer
Choudhury+ 2015
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Self-shielded regions are now resolved
z=7
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Self-shielded regions are now resolved
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21cm brightness temperature
z=7
21cm-bright self-shielded regions are now resolved in HII bubbles
Tb ⇡ 22 mK xHI�gas
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Effect on the power spectrum
x
ion
= 0.7
decrease on
large scales
increase on
large scales
LOFAR
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LOFAR
Effect on the power spectrum
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LLSs dominate towards end of EoR
LOFARLOFAR
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“Redshift” dependence of this effect
zHM12 ~ 14 zHM12 ~ 6.5 zHM12 ~ 14 zHM12 ~ 6.5
Power is reduced by ~15%
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“Redshift” dependence of this effect
zHM12 ~ 14 zHM12 ~ 6.5
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Implications for Observations
• Look at other 1-point statistics now to consider observability (Watkinson and Pritchard 2014)
• These are important for constraining reionization models (Furlanetto+ 2004, Harker+ 2009)
• Smooth maps over ~ 2 cMpc
• Instrument noise for SKA (1000 hours) is ~3 mK (Watkinson and Pritchard 2014)
�2
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Variance and Skewness
zHM12 ~ 14 zHM12 ~ 6.5
“Zero-Tb spike”
zHM12 ~ 14 zHM12 ~ 6.5
LLSs suppress strong evolution of moments
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Conclusions
• Presented method to calibrate photon budget in high resolution simulations
• LLSs reduce large scale power throughout most of the reionization history by ~15%.
• This suppresses strong evolution of 21cm moments
• This makes constraining epoch of reionization using LOFAR and SKA more difficult