H.K. Vedantham, L.V.E. Koopmans, A.G. de Bruyn, S.J. Wijnholds,
B. Ciardi and M.A. Brentjens, "Chromatic effects in the 21 cm
global signal from the cosmic dawn", Monthly Notices of the Royal
Astronomical Society, V437, no. 2, pp1056-1069, January 2014
abstract:
The redshifted 21 cm brightness distribution from neutral hydrogen is
a promising probe into the cosmic dark ages, cosmic dawn and
re-ionization. Low Frequency Array's (LOFAR) Low Band Antennas (LBA)
may be used in the frequency range 45 to 85 MHz (30 > z > 16) to
measure the sky-averaged redshifted 21 cm brightness temperature as a
function of frequency, or equivalently, cosmic redshift. These low
frequencies are affected by strong Galactic foreground emission that
is observed through frequency-dependent ionospheric and antenna beam
distortions which lead to chromatic mixing of spatial structure into
spectral structure. Using simple models, we show that (i) the
additional antenna temperature due to ionospheric refraction and
absorption are at an ~1 percent level - two-to-three orders of
magnitude higher than the expected 21 cm signal, and have an
approximate v-2 dependence, (ii) ionospheric refraction
leads to a knee-like modulation on the sky spectrum at v ~= 4 times
plasma frequency. Using more realistic simulations, we show that in
the measured sky spectrum, more than 50 percent of the 21 cm signal
variance can be lost to confusion from foregrounds and chromatic
effects. To mitigate this confusion, we recommend modelling of
chromatic effects using additional priors and interferometric
visibilities rather than subtracting them as generic functions of
frequency as previously proposed.
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