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Large Scale Simulations: properties of the 21cm signal

Garrelt Mellema Stockholm Observatory. Large Scale Simulations: properties of the 21cm signal. Collaborators: Martina Friedrich, Ilian Iliev, Paul Shapiro, Ue-Li Pen, Yi Mao & the LOFAR EoR Key Project team. Contents. The 21cm signal Simulation methodology Image cubes

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Large Scale Simulations: properties of the 21cm signal

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  1. Garrelt Mellema Stockholm Observatory Large Scale Simulations: properties of the 21cm signal Collaborators: Martina Friedrich, Ilian Iliev, Paul Shapiro, Ue-Li Pen, Yi Mao & the LOFAR EoR Key Project team. Reionization@Ringberg

  2. Reionization@Ringberg Contents • The 21cm signal • Simulation methodology • Image cubes • Statistical properties: global signal, power spectra, redshift space distortions, skewness. • Helium photo-ionization • Conclusions

  3. Reionization@Ringberg  time θx z The Redshifted 21cm Signal • The measured signal is the differential brightness temperature • Depends on: • xHI: neutral fraction • δ: overdensity • Ts: spin temperature The image cube: images stacked in frequency space • For Ts»TCMB, the dependence on Ts drops out. • The signal is line emission: carries spatial, temporal, and velocity information. θy

  4. Reionization@Ringberg Simulation Methodology • To better learn the properties of the 21cm signal we are studying it with detailed large scale cosmological simulations. • Three steps: • Evolution of IGM density (δ) & (proto-)galaxies from a cosmological simulation. • Assign EUV luminosity to (proto-)galaxies. • Transfer EUV radiation through the IGM (xHI). • For large scale simulations galaxy formation is unresolved and baryons and dark matter have the same distribution: • Cosmological N-body simulation (for DM). • Transfer of EUV radiation can be done in postprocessing mode. • See talk by Ilian Iliev this afternoon.

  5. Reionization@Ringberg 21cm LOS Reionization History ’Slice of Sight’ • From the calculated evolution of the ionized densities we can construct reionization histories along the line of sight. • Frequency/redshift direction contains evolutionary, geometrical and velocity information. redshift Simulation: • Box size: 114/h=163 Mpc • Mhalo > 108 M • Radiative feedback on halos 108 < Mhalo < 109 M (‘self-regulation’)

  6. Reionization@Ringberg Statistical Measurements • The sensitivity of the upcoming EoR experiments will be too low to image 21cm from reionization pixel by pixel: Statistical measurements needed. • First goal: to reliably detect signatures from reionization (and separate them from foreground and instrumental effects). • Second goal: to interpret them in terms of astrophysics (source population and properties). • Luckily, the 21cm line signal is rich in properties: • Global signals: rms fluctuations. • Angular properties: power spectra • Frequency properties: Kaiser effect • Non-Gaussianity: skewness

  7. Reionization@Ringberg Global Signals • An interferometer measures the change of the 21cm (rms) fluctuations. • Peak of these falls around 60-70% ionized (depends on spatial/frequency resolution). • Simulations shown: • 114/h Mpc • 114/h Mpc & clumping • 37/h Mpc • 37/h Mpc & clumping Fluctuations No fluctuations

  8. Reionization@Ringberg Power Spectra Angular Power Spectra • Information about the length scales can be obtained from the power spectra. • Simulations show clear trends of shifting power to larger scales as reionization progresses, and a flattening of the power spectra. • Note that the angular power spectrum is measured directly by an interferometer, the multipole l is equivalent to √(u2+v2) in a visibility map. 114/h Mpc volume

  9. Reionization@Ringberg Redshift Space Distortions • Due to the peculiar velocity field, the signal can be displaced from its cosmological redshift. • `Kaiser effect´ or `velocity compression´: due to infall, signal concentrates at the high density peaks. • It is clearly seen in the simulation results and gives ~30% increase in fluctuations (and up to a factor of 2).

  10. Reionization@Ringberg RSD Effects on the Image Cube No velocity • Adding the redshift space distortions visibly increases the fluctuations in the neutral medium. • The effect remains noticeable even at LOFAR-like resolution (3’, 440 kHz). • Simulation shown: 114/h Mpc volume. With velocity

  11. Reionization@Ringberg RSD Effects on the Image Cube No velocity • Adding the redshift space distortions visibly increases the fluctuations in the neutral medium. • The effect remains noticeable even at LOFAR-like resolution (3’, 440 kHz). • Simulation shown: 114/h Mpc volume. With velocity

  12. Reionization@Ringberg Power spectrum • Since the velocity gradients responsible for the distortions are related to the density field, one can write the total (3D) power P(k) in terms of a polynomial in μ=cos(θk), the angle between the LOS and the k vectors (see e.g., Barkana & Loeb 2005). • P(k)= (Pδδ - 2Pxδ + Pxx) +2(Pδδ - Pxδ)μ2 + Pδδμ4 • Since Pδδ goes with μ4, it should be possible to separate it from the dependence on the ionized fraction x, and thus directly find the density power spectrum.

  13. Reionization@Ringberg And in Fourier Space... Frequency-spatial plane Spatial-spatial plane kx,ky, no distortions kx,kz, no distortions • 3D Fourier transform of (part) of the image cube shows the effect of redshift distortions. • Without distortions the contours are spherical in the spatial-spatial plane and in the frequency-spatial plane. • With distortions the power in the spatial-spatial plane is still isotropic, but the frequency-spatial plane shows elliptical contours. k (h/Mpc)‏ k (h/Mpc)‏ k (h/Mpc)‏ kz (Hz-1)‏ kx,ky, with distortions kx,kz, with distortions k (h/Mpc)‏ k (h/Mpc)‏ k (h/Mpc)‏ kz (Hz-1)‏

  14. Reionization@Ringberg <x>=0.2 <x>=0.7 k (h/Mpc)‏ k (h/Mpc)‏ kz (Hz-1)‏ kz (Hz-1)‏ Reionization and Redshift Space Distortions Angular Power Spectra • Since reionization affects the high density regions first, the redshift space distortions become much less noticeable as reionization progresses. • This can be seen in the angular power spectra, as well as in the image cube power spectra. Solid lines: without redshift distortions Dashed lines: with redshift distortions

  15. Reionization@Ringberg Skewness • The probability distribution function for 21cm is non-Gaussian. The signal shows a clear evolution of skewness with increasing ionization. • Finite resolution modifies skewness, but does not remove it. • Skewness may offer an alternative wayto detect the signal, if remnants of foreground subtractions and other effects are dominantly Gaussian (Harker et al. 2009). Mean signal Skewness Mean signal and its skewness for three different reionization simulations, Harker et al. (2009)

  16. Reionization@Ringberg Helium Photo-Ionization • We are working on adding helium to our code C2-Ray (Martina Friedrich). • Comparing results to Cloudy and other codes (SPHRAY, Gabriel Altay; CRASH) on Test Problem 1 of the Cosmological Comparison Project.

  17. Reionization@Ringberg Time = 500 Myrs

  18. Reionization@Ringberg Time = 4 Gyrs

  19. Reionization@Ringberg Fractions • For a mix of H and He, the photons need to be distributed over the species. • There are several recipes given in the literature for which fraction of photons gets absorbed by species 1 given a mix of species 1 and 2 (and by analogy for more than 2 species): • 1. Lidz (Altay?): • 2. CRASH: • 3. Bolton:

  20. Reionization@Ringberg Fractions • Which one is correct? • For τ1,τ2 →0, all recipes are equivalent. • Consider τ1,τ2 → ∞andτ2/τ1= 2 : • Recipe 1: f1=0.333 • Recipe 2: f1=0.5 (for all τ2/τ1)‏ • Recipe 3: f1=0.0 (for all τ2/τ1)‏ • This suggests that recipe 1 is the correct one. • Monte Carlo experiments also show that recipe 1 is the correct one. • Note: for the test problem results the actual choice does not make a large difference.

  21. Reionization@Ringberg Conclusions • The 21cm signal has a rich set of properties which should help in recognizing it in the data of upcoming EoR experiments. • The later stages of the EoR are characterized by an increase followed by a decline of the rms fluctuations, and the reverse behaviour for the skewness. • The angular power spectra show a characteristic evolution to flatter structures. • Redshift space distortions important, but less easily measured once reionization gets going. • He-photoionization: In a mix of absorbing species, photons should be distributed according to the ratios of their optical depths.

  22. Reionization@Ringberg Reionization with Multifrequency Datasets, Stockholm, August 17-21, 2009 Meeting on combining information of different datasets for studying reionization: 21cm, Ly-a, QSOs, CMB, backgrounds. Invited speakers: Ger de Bruijn, Gil Holder, Ilian Iliev, Nobunari Kashikawa, Antony Lewis, Adam Lidz, Miguel Morales, Jochen Weller Organizers: Garrelt Mellema, Göran Östlin, Saleem Zaroubi, Axel Brandenburg Venue: AlbaNova Centre, Stockholm, Sweden http://agenda.albanova.se/conferenceDisplay.py?confId=1186

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