All sky angular power spectrum: I. Estimating brightness temperature
  fluctuations using TGSS 150 MHz survey
  Measurements of the Galactic synchrotron emission is relevant for the 21-cm
studies from the Epoch of Reionization. The study of the synchrotron emission
is also useful to quantify the fluctuations in the magnetic field and the
cosmic ray electron density of the turbulent interstellar medium (ISM) of our
Galaxy. Here, we present the all-sky angular power spectrum $(C_{\ell})$
measurements of the diffuse synchrotron emission using the TIFR GMRT Sky Survey
(TGSS) at 150 {\rm MHz}. We estimate $C_{\ell}$ using visibility data both
before and after subtracting the modelled point sources. The amplitude of the
measured $C_{\ell}$ falls significantly after subtracting the point sources,
and it is also slightly higher in the Galactic plane for the residual data. The
residual $C_{\ell}$ is most likely to be dominated by the Galactic synchrotron
emission. The amplitude of the residual $C_{\ell}$ falls significantly away
from the Galactic plane. We find the measurements are quite symmetric in the
Northern and Southern hemispheres except in the latitude range $15-30^{\circ}$
which is the transition region from the disk dominated to diffuse halo
dominated region. The comparison between this interferometric measurement with
the scaled version of the Haslam rms map at 150 {\rm MHz} shows that the
correlation coefficient $(r)$ is more than 0.5 for most of the latitude ranges
considered here. This signifies the TGSS survey is quite sensitive to the
diffuse Galactic synchrotron radiation.
