Suman Majumdar, T. Roy Choudhury, Somnath Bharadwaj
A growing bubble of ionized hydrogen (HII) around a very high redshift quasar
will have many anisotropic features in its shape mainly due to finite light
travel time (FLTT), neutral hydrogen density fluctuations in the IGM and
clustering of stellar sources. Detection of such a bubble in redshifted 21-cm
observations, will not only be a direct probe to the epoch of reionization but
will also provide us insight about the quasars' luminosity and age. We simulate
a growing HII bubble around a quasar at z ~ 8 in an IGM with mean neutral
fraction x_HI = 0.5, using a semi-numerical formalism. A targeted matched
filter bubble search on this simulated visibility data is performed with
spherical and anisotropic filters, considering 1000 hrs of GMRT observation. We
simulate our search at five different stage of growth of the target HII bubble.
We find that, in almost all realizations of our simulations the search results
over estimate the photon emission rate (N_phs) and the age (tau_Q) of the
quasar, due to the effect of clustering of stellar sources around the quasar.
The anisotropic filter, is motivated by the growth model of HII bubble itself,
gives a better match to the simulated bubble with respect to its spherical
counterpart. Search with anisotropic filter reveals an inherent degeneracy in
N_phs - tau_Q parameter space, which leads to a fundamental uncertainty in
quasar parameter estimation. This uncertainty also depends on the particular
state of anisotropy of the bubble. At very early stage of growth uncertainty in
tau_Q is very small but only a lower limit can be put on N_phs. On the contrary
at very late stage of growth, the N_phs can be estimated very precisely but
only a lower limit on tau_Q can be obtained. In most of the relevant N_phs -
tau_Q parameter space where the bubble appears compressed along the LoS, the
uncertainty on both N_phs and tau_Q are relatively small.
View original:
http://arxiv.org/abs/1111.6354
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