# An Image-based Tapered Gridded Estimator (ITGE) for the angular power spectrum

## TL;DR

The paper introduces the ITGE, an efficient image-based estimator for measuring the angular power spectrum from radio interferometric data, useful for cosmology and galaxy studies, with validated results on simulated and real data.

## Contribution

The ITGE method enables direct, unbiased power spectrum estimation from gridded visibilities with flexible window functions and internal noise bias subtraction, validated on VLA data.

## Key findings

- Successfully applied to NGC 628, revealing different turbulence properties in galaxy regions.
- Accurately subtracts noise bias, ensuring unbiased power spectrum estimates.
- Validated with realistic simulations at 1.4 GHz.

## Abstract

We present the Image-based Tapered Gridded Estimator (ITGE) to measure the angular power spectrum $(C_{\ell})$ 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 HI 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 $C_{\ell}$ 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 $C_{\ell}$. We validate the ITGE using realistic VLA simulations at $1.4 {\rm GHz}$. We have applied the ITGE to estimate the $C_{\ell}$ of HI 21-cm emission from different regions of the galaxy NGC 628. We find that the slope of the measured $C_{\ell}$ 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.