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