Ionized bubbles in the epoch of reionization and redshifted 21cm signal

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Ionized bubbles in the epoch of reionization and
redshifted 21-cm signal
Kanan K. DattaDept. of Physics
MeteorologyCenter for Theoretical Studies
(CTS)IIT KharagpurIndia
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Collaborators

• Somnath Bharadwaj (CTS IIT KGP)
• T. Roy Choudhury (IoA, Cambridge)
• Suman Majumdar (CTS IIT KGP)

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The Epoch of Reionization
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The Epoch of Reionization Major challenges in
modern cosmology

• When?
• How ?
• How long ?
• What are the sources ?
• From the analysis quasar absorption spectra
  Reionization finished at z 6 .
• From the CMB observations Reionization occurred
  around z 10.
• These two measurements are somewhat
  contradictory.

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Detecting Ionized bubbles
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21 cm tomography

• Perhaps the most promising prospects of studying
  reionization at various stages is through future
  21 cm observations.
• HI is most abundant elements in the Universe and
  distributed all over the space. Large scale
  structure can be probed.
• It is line emission. So this basically involves
  observing the redshifted 21 cm line from the
  neutral hydrogen (HI) at higher redshifts.
• Track neutral fraction at any redshift.

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Simulation

• HI 21-cm images on slices through the center of
  the bubbles

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HI signal from ionized bubbles
The bubble is assumed to be embedded in an
uniform intergalactic medium.A planar section
through the bubble will be observed for a
particular frequency bin of observation
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Visibility in Radio-Experiments

• The Visibility recorded in radio-interferometric
  obser-
• vations is a combination of three separate
  contributions

Where is baseline, is
separation between two antennae, is observing
frequency.
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Simulating visibilities
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HI signal from ionized bubble
The visibility due to a spherical ionized bubble
embeded into uniform neutral hydrogen at the
center of the field of view is given as
The peak value of the signal is
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HI signal from ionized bubble
10 Mpc bubble, redshift 8.5, frequency150 MHz,
neutral fraction1
The angular size of bubble 5(R/10) arc min. The
signal from the bubble remains confined within
baseline lt700 for 10 Mpc bubbles. For any bubble
of radius R, the signal will be restricted within
baseline U 700(10/R). The signal extends over
0.56(R/10) MHz in frequency. The strength of the
signal is proportional to the neutral fraction.
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Challenges in detecting ionized bubbles

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System noise

• It is expected to be a Gaussian random variable
  in each baseline and frequency channel
• The predicted rms. Noise contribution is

• Tsys is dominated by sky contribution Tsky with
  major contribution coming from our own galaxy.
• Tsys also varies with diections of observation.

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Foregrounds

• Galactic Synchrotron radiation
• Extra galactic points sources
• These are expected to be two major foreground
  components and 104 times larger than the signal
  from the bubbles.
• The statistical properties of the foregrounds are
  quantified using two visibility correlation

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HI fluctuations
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GMRT
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MWA Marchison Widefield Array
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HI fluctuations, noise and foreground
contribution to the total visibility

• HI fluctuations

Noise
Foregrounds
The signal is 1,00,000 and 100 times smaller
than foregrounds and 100 h noise !!!
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Simulating the signal detection from ionized
bubbles

• We need to combine the signal from bubble
  optimally in all baselines and frequency
  channels.
• We use matched filter to do that.

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Filtering the Signal

• To detect the signal from an ionized bubble of
  radius R_b at redshift z_c we introduce an
  estimator

In continuum limit
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Sources of Confusion

• Variance of the estimator is sum of the
  contributions from the noise, foregrounds and HI
  fluctuations

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FILTERS
Matched filter

• The signal to noise ratio (SNR) is maximum if
  we use the filter matched with the signal we are
  looking for ie.,

The signal to noise has the value
The filter
The filter subtracts out any frequency
independent component from the frequency range
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Results

• Filtered Signal with 3-sigma of the variance

Detection of bubble of radius lt8 Mpc for GMRT and
lt16 Mpc for MWA is not possible because of HI
fluctuations.
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Size determination
With 1000 h of observations
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Position determination
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Bubble detection in Patchy reionization
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• Thank You

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Challenges ..

• Radio frequency Interference (RFI)
• Signal from FM radio, satellites, cell phones and
  other communication devices
• Radio wave propagation effect through plasma
  (Faraday Rotation)
• Ionosphereic distortion etc.