Download - Low Frequency Background and Cosmology
Low Frequency Background and
CosmologyXuelei Chen
National Astronomical Observatories
Kashigar, September 10th 2005
Outline
• The angular power spectrum of the galactic synchrotron radiation (based on Chen, astro-ph/0409733)
• The evolution of 21cm signal during the dark age and the epoch of reionzation (based on Chen & Miralda-Escude, ApJ 602, 1 (2004)),
• The 21cm signature of the first starsThe 21cm signature of the first stars (based on Chen & Miralda-Escude, in preparation)
408 MHz skymap
Foreground for CMB and 21cm observation: galactic synchrotron
Tgal ~ 280 (/150MHz)-2.5 K @ NCP
(MHz) Tgal (K) z
200 140 6.1150 280 8.5151 770 1370 1900 1950 4400 27
Foreground Removal
Wang et al astro-ph/0501081
Synchrotron foreground is removable as long as it is smooth. Still, can we understand it physically?
A spherical cow model
Understand the synchrotron radiation at • high galactic latitude • small scale • random field • Fourier space
galactic synchrotron
power law distribution of cosmic ray electron
synchrotron emissivity
Total intensity along a line of sight
Angular power spectrum
Separable spatial and frequency variation
power spectrum
angular power spectrum (Limber approximation)
cosmic ray electron
B ~ microgauss, for 70-200 MHz, radiation from electron 0.1 GeV < E < 10 GeV
CR electron spectrum
Local measurement (Casadei & Bindi 2004):
Model
• B ~ 4 microG
• scale height ~ 1 kpc
• brightness temperature ~ 20 K at 408 MHz
Magnetic Field Variation
large scale magnetic field on the galactic plane (Beurmann, Kanbach, Bekhuijsen 1985)
small scale, out-of-galactic plane magnetic field
Magnetic Field in Turbulent ISM
Komolgorov turbulence
E(k)~k-5/3
Observation (Faraday Rotation): on small scale(0.01-100pc),
E(k)~k-5/3
on larger scale
E(k)~k-2/3
Han, Ferriere, Manchester (2004)
Cosmic Ray Variation
Injection-Diffusion model: cosmic ray electrons are injected at some points (SNR), propagate in random magnetic field, and diffuse out.
(Kobayashi et al 2004)
(Casadei & Binsi 2004)
scale height:
Solution of the Diffusion Equation
Fourier transformed
Steady State solution
power spectrum
Injection Rate
If SNe is Poisson,
V: effective volume where SNe occur, tSN: average interval for SNe within V
Result
WMAP
magnetic field induced
Field strength ~ OK
cosmic ray induced
WMAP:
Discussion
• Geometry
• Gaussianity
• Large scale field
• Variation of spectral index
• Correlation between magnetic field and cosmic ray
Theoretical simplification
Discussion
Observation:
• some observations with steeper angular spectrum
• extragalactic (unresolved point source) contribution
What to do next
• realistic geometry
• variation of spectral index
• include large scale field
• polarization
• multiwavelength cross correlation
• connection with dynamo and CR model
The Epoch of Reionization (EOR)
21cm probe of EOR
VLBI
21CMA
LOFAR
MWA
Related processes
• spontanous transition
F=1
F=0
• Lyman series scattering (Wouthousian-Field mechanism) Ly
• collision induced transition
• CMB induced transition
CMB
n=0
n=1
21cm
The spin temperature
Ly
collision
Thermal systems:
spin
atomic motion
CMBLy
photons
Chen & Miralda-Escude 2004
Simulation by Furlanetto, Sokasian, Hernquist, astro-ph/0305065
Modulation: density ionization fraction spin temperature
21cm tomography
Adiabatic Evolution of Temperatures
CMB
gas
spin
star formation
Star Formation and X-ray Heating of gas
spin temperature evolution
21cm brightness temperature
Chen & Miralda-Escude 2004
Heating of IGM:
• Shock• ionizing radiation (limited to HII)• Lyman alpha? (Madau, Meiksen, Rees
1997)• X-ray
X
possibility of absorption signal
Formation of first stars
QuickTime™ and aYUV420 codec decompressor
are needed to see this picture.
Frenk 2005• primodial density fluctuation grow to form dark matter halos, small halos form first
• gas fall in for sufficiently large halos (Jeans mass)
• gas cool by molecule or atomic H radiation to form first stars
• first stars may be very massive ~ a few hundred solar masses
Property of first stars
• pop I: disk stars Z~Zo
• pop II: halo stars Z~0.01 Zo
• pop III: ? Z<0.001 Zo
Tumlinson & Shull 2000 Bromm et al 2000
Comoving density
The Evolution of Lyman alpha background
Evolution of gas temperature
21cm signature of high-z objects: a quasar
Tozzi et al 2000
Lyman alpha photons emitted by the quasar couples spin temperature to the kinetic temperature
Ly alpha sphere around a first star
Heating function
Lyman alpha sphere
The 21cm signature of the first star
The 21cm brightness temperature around a first star
Typical size: a few arcsec
Typical width: 10 kHz
Typical dT: 20mK/2000K
Challenge for the future generation of radio astronomers!
The End