ELAIS-N1 deep field uGMRT Band-2: constraints on diffuse Galactic synchrotron emission power spectrum

TL;DR

This study uses deep uGMRT Band-2 observations of the ELAIS-N1 field to analyze the diffuse Galactic synchrotron emission, revealing a spectral break at 230 MHz and constraining cosmic-ray electron properties.

Contribution

First detailed analysis of DGSE power spectrum at low frequencies combining image and visibility-based estimators, identifying a spectral break and low-energy cutoff in cosmic-ray electrons.

Findings

Spectral break at 230 MHz indicating synchrotron aging.

Consistent spectral indices across two analysis methods.

Constraints on cosmic-ray electron age and energy distribution.

Abstract

We present high sensitivity, low radio frequency continuum observations of the ELAIS-N1 field with 32 hours of observations of the uGMRT Band-2 ($120-250$ MHz) covering $5.86\,\text{deg}^2$ area, achieving a central off-source RMS noise of $237\,\mu\mathrm{Jy}/\mathrm{beam}$ with a resolution of $11.45''$ at the central frequency of 183 MHz. A radio source catalogue of 1027 sources statistically matches with similar observations at different frequencies within the sensitivity range of the uGMRT. The calibrated data is further used to characterise the dominant foreground, the Diffuse Galactic Synchrotron Emission (DGSE), in angular scale and frequency regime. We derived the angular power spectrum (APS) of DGSE in two ways: image-based estimator (i-APS) and visibility-based Tapered Gridded Estimator (TGE; hereafter as t-APS). We assess the characteristics of DGSE with a power-law form of $C_{\ell} = A({1000}/{\ell})^{\beta}$. Combining data from Band-2 and earlier Band-3 observations, we derived a spectral variation of $C_{\ell}$ in the form of $C_{\ell} = A{\nu^{-2{\alpha}}}{\ell^{-{\beta}}}$. Our result indicates a spectral break at $\nu = 230\,{\pm}\,5$ MHz, corresponding to a synchrotron age of $t_\text{syn} = 106\,{\pm}\,1$ Myr for the cosmic-ray electrons (CRe). This break result suggests a low-energy cutoff in the CRe population, leading to spectral curvature at low frequencies. Using both of the techniques, i-APS and t-APS, we find that the mean spectral index $\alpha$ and power-law index $\beta$ are consistent within the frequency range $120-500$ MHz.