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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.
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Lyman Alpha Spheres from the First Starsobserved 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: • 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.
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: Continuum photons: Injected 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.
Temperature and 21cm profiles • 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.
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) 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 yαgets 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.