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Lyman Alpha Spheres from the First Stars observed in 21 cm. Xuelei Chen (Beijing) Jordi Miralda Escud é (IEEC, Barcelona). First stars in the universe. Cooling of gas first took place from molecular hydrogen, at z ~30 in halos of mass ~ 10 6 M sun. Properties of first metal-free stars. - PowerPoint PPT Presentation
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Lyman Alpha Spheres from the First Stars
observed in 21 cm
Xuelei Chen (Beijing) Jordi Miralda Escudé
(IEEC, Barcelona).
First stars in the universe
• Cooling of gas first took place from molecular hydrogen, at z~30 in halos of mass ~ 106 Msun .
Properties of first metal-free stars
• Central gas cools only to T ≈ 200 K. Molecular hydrogen lines can be collisionally deexcited at density n > 104 cm-3, making the cooling rate independent of density and inhibiting fragmentation.
• Jeans mass ≈ 300 Msun .
• Accretion rate ≈ cs3/G ≈ 10-3 Msun/yr
• The first metal-free stars were massive, with L ≈ LEdd and T ≈ 105 K (Abel etal 2002, Bromm etal 2002, Schaerer 2002). Their lifetime is ~ 3 million years.
What is a first star?• All metal-free stars? Stars forming from matter
that has never been in other stars.• Another possible definition: a star forming at a
place and time where no light from another star has yet reached.– For CDMΛ model: first stars form at z ~ 40 from 6-
sigma fluctuations.
• Or: a star forming at a place and time where no light from other stars is substantially affecting any of its observable properties.
First ionized regions• Each metal-free star can produce about 105
ionizing photons per baryon it contains, creating an HII region of ~ 107 Msun of gas, of physical radius ~ 1 kpc at z=30. Probably only one metal-free star forms per halo.
• Star formation occurring after the HII region recombines and merges is probably from metal enriched gas.
Metal-free stars can increase the CMB optical depth by
only a few hundredths, if only one star forms per halo.
(Rozas et al. 2006)
How can we detect stars at the highest redshifts?
• Supernovae? Gamma-ray bursts?• 21 cm emission/absorption
on the CMB: )/1( sCMBHI TTT
• The spin temperature must be coupled to the kinetic temperature Tk to make HI observable in 21cm, either collisionally or through Lyman alpha photons (e.g., Madau, Meiksin, & Rees 1997).
• Initially, Tk < TCMB, HI seen in absorption. Lyα photons from stars increase Tk-Ts coupling. Later, X-rays heat the kinetic temperature.
c
kcCMBs yy
TyyTT
1
Evolution of kinetic temperature
• Typical X-ray emission of local starbursts:
1 keV per baryon.• Hard X-rays ( > 1keV)
heat the medium homogeneously; soft X-rays (such as the photospheric emission from metal-free stars) heat inhomogeneously.
Heating due to the scattering of Lyα photons itself is negligible
• Heating rate: IcnJ
TTkHn HBH
4
2 *
Injected photons:
Continuum photons:
What happens around one metal-free star?
• Lyα photons couple the spin and kinetic temperatures out to a radius much larger than the HII region.
• X-rays from the stellar photosphere heat the medium.
• X-ray ionizations also produce injected Lyα photons, which turn out to dominate for the surface temperatures of metal-free stars. These yields a dominant absorption signal from a ``Lyα sphere’’ around a metal-free star.
• Kinetic temperature is greatly heated just beyond the HII region, but further out it has been adiabatically cooled.
• 21cm absorption strongly dominates over the inner emission core.
Temperature and 21cm profiles
Detectability of single Lyα spheres• Angular size: θ ~ 10” (20 kpc at z=30)
– Required baseline: 100 km (at z=30)
– Signal temperature: δT ≈ 200 mK– Synchrotron background temperature: Tb≈4000 K (z=30)
covcov 20 t SNR fT
Tf
b
for t=1 year
• We need a large array of telescopes.• It may be better to look for clusters of Lyα spheres
on larger angular scales, or for a global signal.
Lyα background intensity• The coupling parameter yα
gets close to unity at z ≈ 25 everywhere because of the light background from all metal-free stars, so Lyα spheres lose their contrast.
• In addition, global temperature starts rising at z ≈ 25 due to X-rays, so absorption weakens, eventually turning to emission.
• 21 cm absorption must be searched at 30 – 40 MHz
Lyα spheres at z≈30 are strongly biased Average number of neighboring
star-forming halos
Conclusions
• The Lyα sphere of a metal-free star produces a strong 21cm absorption which is an unmistakable signature of a first star.
• Detection of Lyα spheres would tell us about formation history, mass function, clustering… of the first stars.
• Hard to detect! They are at very high redshift (very low frequency) and require ~ 100 km apertures.