Download Signatures of the first stars in the 21cm Emission and Absorption

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 6sigma 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
T
on the CMB:
  HI (1  TCMB / Ts )
• 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).
TCMB   y  yc Tk
Ts 
1  y  yc
• 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 Xrays (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:
J

 2TT*
I
cnH
Hn H k B
4
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)
SNR   t f cov
T
Tb
 20 f cov
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.