constraints on the metagalactic hydrogen ionization rate from the lyman- a forest opacity

Shanghai, 16/03/05Shanghai, 16/03/05

Constraints on the Metagalactic Hydrogen Constraints on the Metagalactic Hydrogen Ionization Rate from the Lyman-Ionization Rate from the Lyman- Forest Forest

OpacityOpacity

Jamie BoltonJamie Bolton

Martin Haehnelt, Matteo Viel, Volker Springel

MNRAS, 2005, 357, 1178


OverviewOverview

Probes of the UV background intensity:

Motivation:

• Proximity effect (e.g. Scott et al. 2000)• Lyman continuum emission from LBGs (e.g. Steidel et al. 2001)• Modelling QSO population evolution (e.g. Haardt & Madau 1996)

• We use the Ly- forest opacity to determine HI for 2 < z < 4 with hydrodynamical simulations

• What is the intensity and spectral shape of the UV background?

• Constrain the sources responsible for reionizing the IGM• Probe the thermal history the IGM – implications for epoch of reionization


Obtaining Obtaining HIHI from simulations from simulations

• Hydrodynamical simulations of structure formation can be calibrated to reproduce popular parameters which influence the Ly forest opacity mb,h8,n,TIGM)

• Immerse box in a uniform UV background, keep its intensity as a free parameter.

• Rescale artificial spectra in post-processing to reproduce observed Ly- forest opacity (e.g. Rauch et al. 1997, Theuns et al. 1998)Earth

QSO


Estimates of Estimates of HI HI from simulationsfrom simulations

11212 10

s

HI


Lyman-Lyman- forest opacity forest opacityThe Fluctuating Gunn Peterson Approximation:The Fluctuating Gunn Peterson Approximation:

e.g. Rauch et al. 1997, McDonald & Miralda-Escudé 2000

2

)()(

)()1(

5.07.03212

127.0

226

0

zTh

zzHT

hz

mb

b

• Assume photoionization equilibrium and an effective equation of state for low density gas, T = T0-1 (Hui & Gnedin, 1997)


Fiducial Model ParametersFiducial Model Parameters

Cosmological parameters consistent with Spergel et al. (2003)

m = 0.26 ± 0.04bh2 = 0.024 ± 0.001

8 = 0.85 ± 0.05h = 0.72 ± 0.04

n = 0.95

Astrophysical parameters at z = [2, 3, 4]

TIGM = [11200,17800,12500] ± 5000 K = 1.3 ± 0.3

(Schaye et al. 2000)

eff = [0.130±0.021, 0.362±0.036, 0.805±0.070] (Schaye et al. 2003)


Resolution and box sizeResolution and box size

• Large volume required to include long wavelength perturbations and provide an adequate sample of the Universe.

• High resolution required to resolve small haloes.

• Minimum box size and resolution of 30 Mpc/h and 4003 gas particles required for marginal convergence of HI.

10 Mpc/h (Rauch et al. 1997)

30 Mpc/h (Bolton et al. 2005)


Scaling with Scaling with mm

• Lower m models have less gas in haloes, so a larger -12

is required to match the observedopacity.



• Significant departure from the predicted scaling of -12 with m

-0.5 when normalised to the fiducial model



JSB, Haehnelt, Viel & Springel, 2005

• Extra simulation with m=1; power spectrum normalised to have same fluctuation amplitude as the fiducial model at 30 kms-1 scale.

• The r.m.s fluctuation amplitude at a fixed velocity scale is more relevant than the geometrical scaling of HI with m

-0.5 from the Hubble parameter.


Scaling with Scaling with effeff


• We must assume a value of eff torescale the simulated spectra opacityand hence infer HI

• Systematic uncertainties stemming from the continuum fitting produce a wide range of estimates.

• A small change in eff can have adramatic effect on -12


Uncertainties (%) and ResultsUncertainties (%) and Results

ParameterParameter z=2.0 z=2.0 z=3.0z=3.0 z=4.0z=4.0

TT +50 -22+50 -22 +23 -14+23 -14 +35 -18+35 -18

mm +18 -13+18 -13 +19 -14+19 -14 +21 -15+21 -15

effeff +29 -19+29 -19 +18 -14+18 -14 +17 -13+17 -13

NumericalNumerical ±10±10 ±10±10 ±10±10

±1±1 +7 -9+7 -9 +12 -13+12 -13

bbhh22 +9 -8+9 -8 +9 -8+9 -8 +9 -8+9 -8

88 +6 -5+6 -5 ±6±6 +8 -7+8 -7

hh ±6±6 ±6±6 ±6±6

TotalTotal +62 -36+62 -36 +39 -30+39 -30 +49 -34+49 -34

]4,3,2[

]0.1,3.09.0,3.1[ 5.03.0

8.05.012

zat

Final values


Comparison to other observations Comparison to other observations

Our results with uncertainties (Bolton et al. 2005)



Rates from QSOs (Boyle et al. 2000) + IGM re-emission (Madau, Haardt & Rees 1999, updated)



Rates from galaxies (Bruzual & Charlot model), QSOs+IGM re-emission (Madau, Haardt & Rees 1999, updated)




Proximity effect (Scott et al. 2000) and emission from Lyman-break galaxies (Steidel et al. 2001)


Conclusions Conclusions

• Our data are consistent with a UV background with a substantial contribution from galaxies, and agree with other observational estimates for the metagalactic hydrogen ionization rate.

• The thermal state of the IGM is the biggest uncertainty when determining the ionization rate.

Bolton et al., 2005, MNRAS, 357, 1178

constraints on the metagalactic hydrogen ionization rate from the lyman- a forest opacity

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