Simulating the z = 3.35 HI 21-cm visibility signal for the Ooty Wide
  Field Array(OWFA)
  The upcoming Ooty Wide Field Array (OWFA) will operate at $326.5 \, {\rm
MHz}$ which corresponds to the redshifted 21-cm signal from neutral hydrogen
(HI) at z = 3.35. We present two different prescriptions to simulate this
signal and calculate the visibilities expected in radio-interferometric
observations with OWFA. In the first method we use an input model for the
expected 21-cm power spectrum to directly simulate different random
realizations of the brightness temperature fluctuations and calculate the
visibilities. This method, which models the HI signal entirely as a diffuse
radiation, is completely oblivious to the discrete nature of the astrophysical
sources which host the HI. While each discrete source subtends an angle that is
much smaller than the angular resolution of OWFA, the velocity structure of the
HI inside the individual sources is well within reach of OWFA's frequency
resolution and this is expected to have an impact on the observed HI signal.
The second prescription is based on cosmological N-body simulations. Here we
identify each simulation particle with a source that hosts the HI, and we have
the freedom to implement any desired line profile for the HI emission from the
individual sources. Implementing a simple model for the line profile, we have
generated several random realizations of the complex visibilities. Correlations
between the visibilities measured at different baselines and channels provides
an unique method to quantify the statistical properties of the \HI signal. We
have used this to quantify the results of our simulations, and explore the
relation between the expected visibility correlations and the underlying HI
power spectrum.
