Detecting HII bubbles in 21
reionization of the Universe, it is believed, occurred by the growth
of ionized regions (bubbles) in the neutral intergalactic medium (IGM).
We are studying the possibility of detecting these bubbles in
radio-interferometric observations of redshifted neutral hydrogen (HI)
21 cm radiation.
The signal (<1 mJy) will be buried in noise and
foregrounds, the latter being at least a few orders of magnitude
stronger than the signal. We have developed a visibility based
uses a filter to optimally combine the entire signal from a bubble
minimizing the noise and foreground contributions. This formalism makes
definite predictions on the ability to detect an ionized bubble or
conclusively rule out its presence in a radio-interferometric
We make predictions for the currently functioning GMRT and
a forthcoming instrument, the MWA at a frequency of 150 MHz
(corresponding to a redshift of 8.5). For both instruments, we show
it will be possible to detect a bubble of comoving radius R_b > 40
Mpc (assuming them to be spherical) in 100 hrs of observation and R_b
> 22 Mpc in 1000 hrs of observation, provided the bubble is at the
center of the field of view. In both these cases the filter effectively
removes the expected foreground contribution so that it is below the
signal, and the system noise is the deciding criteria.
We find that
there is a fundamental limitation on the smallest bubble that can be
detected arising from the statistical fluctuations in the HI
distribution. Assuming that the HI traces the dark matter we find that
it will not be possible to detect bubbles with R_b < 8 Mpc using the
GMRT and R_b < 16 Mpc using the MWA, however large be the
Detecting ionized bubbles in redshifted
21 cm maps, Datta, Kanan K.; Bharadwaj, Somnath;
Choudhury, T. Roy
Mon. Not. R. Astron. Soc. 382 809 (2007)
Simulating the impact of HI fluctuations on matched filter search for ionized bubbles in redshifted 21-cm maps, Datta, Kanan K.; Majumdar Suman; Bharadwaj, Somnath;
Choudhury, T. Roy
Mon. Not. R. Astron. Soc. 391 1900 (2008)
The impact of anisotropy from finite light traveltime on detecting ionized bubbles in redshifted 21-cm maps , Majumdar Suman; Bharadwaj, Somnath; Datta, Kanan K.;
Choudhury, T. Roy
Mon. Not. R. Astron. Soc. 413 1409 (2011)
Constraing quasar parameters through bubble detection in redshifted 21-cm maps, Majumdar Suman; Bharadwaj, Somnath; Choudhury, T. Roy
To be submitted in MNRAS
Ongoing and Future Research
1. Ooptimal strategy, blind search or
targeted search around known QSOs? Optimal z?
2. Can we combine other
observations like S-Z effect to increase the possibility of a detection?
A. We are currently using dark
matter N-body simulations with the assumption that the HI outside
the bubble traces the dark matter. A bubble is put in by
hand. The simulations have 256^3 particles on a 256^3 grid and
spacing of 2 Mpc. The simulations are evolved to a redshift z=6.
This shows a single channel image
256 Mpc x 256 Mpc area with a
bubble 20 Mpc radius at center
2 Mpc corresponds to 0.9' on the sky and 150 kHz in frequency
There are three major challenges for detecting HII bubbles.
a. Foregrounds. Current models
propose these have a very smooth frequency dependence. If so, our
earlier work shows that foregrounds do not pose a big problem for
b. HI fluctuations.
HI fluctuations outside the bubble we wish to detect will create
confusion. This is a big problem.
c. System Noise This decides the observation time
needed for a detection.
We are currently focusing on the two
latter effects only.
How does noise affect the image shown above? Can we still make out the
How do the HI fluctuations affect the visibilities?
Simulate HI fluctuation contribution to estimator, and compare
with earlier predictions.