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The 21cm signature of the First Stars

The 21cm signature of the First Stars. Xuelei Chen 陳學雷 National Astronomical Observatory of China. KIAS workshop on cosmology and structure formation , Seoul, Korea, Sept 20, 2006. z ~ 1000. z ~ 30. z ~ 6. z ~ 0. The Cosmic History. ?. small perturbations. gravity.

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The 21cm signature of the First Stars

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  1. The 21cm signature of the First Stars Xuelei Chen 陳學雷 National Astronomical Observatory of China KIAS workshop on cosmologyand structure formation, Seoul, Korea,Sept 20, 2006

  2. z ~ 1000 z ~ 30 z ~ 6 z ~ 0 The Cosmic History ?

  3. small perturbations gravity collapse to form dark halos gas accreate and cool form stars and galaxy Hierachical structure formation, formation of first objects, and reionization Barkana & Loeb 2001

  4. 21cm: Historical Review 21cm: hyperfine structure of neutral H atom • van de Hulst (1945): theoretical prediction • Ewen & Purcell; Muller & Oort (1951): detection in the Milky Way, discovery of spiral structure of Milky Way • Field, ... (late 1950s): theoretical understanding of 21cm brightness determined by spin temperature and the role of Ly  photons • 1960, 1970s: observation of nearby galaxies, discovery of dark matter in galaxies, search for neutral InterGalactic Medium (null) • 1980s: search for pancake clouds predicted by Zeldovich’s hot dark matter (neutrino) model (null) • 1990: search for high redshift galaxy, loss of interest • Madau, Meikesen, Rees (1997) probe reionization, revival of interest

  5. The 21cm tomographic probe Probe the reionization process with 21cm tomography (Madau, Meiksen & Rees 1997) Furlanetto, Sokasian, Hernquist 2003

  6. Ongoing 21cm projects 21CMA/PAST MWA Carilli 2005

  7. 21CMA/PAST • interferometers • clustered dipoles • typical baseline of a few km, collecting area 105 m2 MWA prototype LOFAR prototype

  8. The physics of 21cm line • spontanous transition n=2 • collision induced transition Ly  • CMB induced transition n=1 • Lyman series scattering • (Wouthousian-Field mechanism) Ly  n=0 F=1 CMB 21cm F=0

  9. collision atomic motion Ly  Ly  photons CMB spin Spin Temperature Thermal systems:

  10. Lyman alpha photons • injected photons: photons emitted/scattered at Ly alpha frequency, produced by recombination • Continuum photons: UV photon between Ly alpha and Ly beta, redshift to Ly alpha frequency • Once enter Ly alpha frequency (Doppler core), resonant scattering & confined locally. At Ly alpha frequency, color temperature equals to kinetic temperature • leaking by 2 photon process • higher Lyman series

  11. Modulation of 21cm signal • density (cosmic web, minihalo) • ionization fraction (galaxy, cosmic HII region) • spin temperature: temperature, density, Ly alpha flux When Ts >> Tcmb: emission saturates • dark age: density & peculiar velocity • first light: density & spin temperature • reionization: density & ionization

  12. Models of 21cm • global edge: due to reionization or emission/absorption transition • 21cm forest: HII region, cosmic web, minihalo • tomography: bubble model of HII region • tomography: density modulation due to cosmic web • tomography: spin temperature due to collision (dark age) • tomography: spin temperature due to Ly : large scale, • individual quasar, galaxy, first star

  13. Reionization Model Expansion of ionized (HII) region photon production rate: calculate the bound fraction of baryons in star forming halos (M>Mmin), then assume each baryon produce a number of photons Evolution of ionization fraction

  14. The bubble model Furlanetto et al 2004 For an isolated region, condition for ionization: but so ionized bubble if distribution of bubble Zahn 2006

  15. star formation Evolution of global spin temperature z>150: Tk=Tcmb=Ts 50<z<150: Ts=Tk<Tcmb collisional coupling 25<z<50: Tk<Ts= Tcmb no coupling 15<z<25: Tk<Ts <Tcmb Ly alpha coupling 10<z<15: Tk>Ts >Tcmb Ly alpha coupling CMB spin gas

  16. 21cm brightness temperature spin temperature evolution Chen & Miralda-Escude 2004 The temperature of gas Heating of IGM: • Shock • ionizing radiation • Lyman alpha? No • (Madau, Meiksen, Rees 1997, Chen & Miralda-Escude 2004, Hirata 2006, chuzhoy & Shapiro 2006, Rybicki 2006, Meiksen 2006, • Pritchard & Furlanetto 2006) • X-ray

  17. The absorption signatures • z>200, CMB and gas has about the same temperature, no 21cm signal • 200>z>40, gas temperature < CMB temperature, absorption modulated by density • z<40, before the presence of Lyman alpha background: absorption, density modulation in mini-halos • Lyman alpha modulation, temperature modulation, density modulation, ionization modulation...

  18. high redshift fluctuation large scale spin-temperature variation induced by first galaxies density fluctuation during dark age Barkana & Loeb 2004 Barkana & Loeb 2005

  19. H H2 Formation of the first stars Star could form in a dark halo only if • halo gravity exceeds gas pressure (Jeans mass) • gas in the halo can cool • molecule H cooling ~102-3 K • atomic cooling ~ 104 K Simulations (Abel 2000, Bromm 2000) indicate first star may form in halos of 105-6 solar mass, one or a few per halo, with masses of a few hundredsolar.

  20. Lyman alpha sphere around first stars first stars: 100 solar mass metal free star radiating at Eddington limit (Bromm et al 2001) Chen & Miralda-Escude 2006 • life time of the star ~ 3 Myr, Hubble time ~ 108 yr • light propagation time ~ size of Lya sphere (10 kpc) • halo virial radius ~ 0.1 kpc NOT TO SCALE

  21. Radiative Transfer For stellar mass of 25, 50, 100, 200, 400, 800 Msun.

  22. continuum Lyman alpha photons • Line photons confined to HII region (or where it is produced) by resonance scattering • continuum photon: decrease as r-2

  23. The secondary Lyman alpha photons induction by X-ray photons: recombination, excitation, cascade photon production rate ~ energy rate/E frequency shift rate ~ H  flux ~ photon production rate / frequency shift rate: Neglected order 1 correction factor and higher Lyman series

  24. Ly  sphere profile with only continuum Ly photons, weak absorption with injected Lyman photons very strong absorption

  25. Formation Rate of first stars • Minimal mass requirement: • Tvir > 2000 K • One star formed per halo • Star died after 3 Myr, so exist only in halos just formed This breaks down at low redshift: (1) halo destruction (2) feed back

  26. Biase & correlation function of halos (not stars) PS ST

  27. The effect of heating no heating with heating

  28. Cross Section Map • Ly alpha background reduce contrast of Ly alpha sphere • If gas heated above CMB, no absorption signal • absorption signal much stronger than emission

  29. Foreground X. Wang et al astro-ph/0501081

  30. Observablity measurement error system temperature dominated by galactic foreground: covering factor signal to noise ratio

  31. Observablity difficult to achieve high SNR: require almost-filled array 105 1 10 -5

  32. Signal To Noise Ratio Assume: z=20, M=400, t=1.5 Myr, 1 year intergration covering factor=1

  33. Some Numbers Very Challenging, far beyond the capability of current generation 21cm experiments. But Not Impossible!

  34. Summary • Redshifted 21cm observation provides powerful probe for dark age, first light, and reionization • The 21cm signal is modulated by density, ionization fraction and spin temperature • The spin temperature is determined by CMB temperature, kinetic temperature, density, and Ly  background flux • Models of 21cm fluctuation due to ionized region, density fluctuation, first star • X-ray from first stars maybe important in heating and in creating “injected Lyman alpha photons”. This may be used to distinguish different models of first objects

  35. Thanks

  36. Overlap of Lyman alpha spheres • cumulative number of halos within a certain distance • caveat: may not form at the same time • first star clusters: bigger Lyman alpha sphere • the shape of Ly alpha sphere may be irregular due to nearby first star and density fluctuation

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