2019MNRAS.483.3910C

We present the Image-based Tapered Gridded Estimator (ITGE) to measure the angular power spectrum (C_l) of the sky signal directly from the visibilities measured in radio-interferometric observations. The ITGE allows us to modulate the sky response through a window function which is implemented in the image plane, and it is possible to choose a wide variety of window functions. In the context of the cosmological H I 21-cm signal, this is useful for masking out the sky signal from specific directions which have strong residual foregrounds. In the context of the ISM in external galaxies, this is useful to separately estimate the C_l of different parts of the galaxy. The ITGE deals with gridded data, hence it is computationally efficient. It also calculates the noise bias internally and exactly subtracts this out to give an unbiased estimate of C_l. We validate the ITGE using realistic VLA simulations at 1.4 GHz. We have applied the ITGE to estimate the C_l of HI 21-cm emission from different regions of the galaxy NGC 628. We find that the slope of the measured C_l in the outer region is significantly different as compared with the inner region. This indicates that the statistical properties of ISM turbulence possibly differ in different regions of the galaxy.