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

TL;DR

This paper uses 32 hours of uGMRT Band-2 data (120–250 MHz) supplemented by Band-3 (300–500 MHz) to study the DGSE in the ELAIS-N1 field, producing a 5.86 deg^2 image with RMS 237 μJy/beam and a 1027-source catalogue at 183 MHz. It employs both image-based (i-APS) and visibility-based (t-APS, via TGE) angular power-spectrum estimators to quantify the DGSE across multipoles ℓ ≈ 400–1000 and frequencies 120–500 MHz, finding consistent power-law behavior with a break in the MF-APS around ν_break ≈ 230 MHz. The MF-APS analysis yields a broken power-law with α_1 ≈ 1.0–1.3 and α_2 ≈ 5.3–6.4 and a derived synchrotron age t_syn ≈ 106 Myr for B ≈ 10 μG, signaling a low-energy cutoff in the cosmic-ray electron population and spectral curvature at low frequencies. The study also delivers a detailed cross-m catalogue comparison, flux- and position-precision assessments, and sub-mJy source-count corrections, highlighting the importance of wideband, low-frequency data for foreground modeling in 21-cm cosmology and setting the stage for deeper Band-2 observations. The results provide constraints on the Galactic foreground power spectrum and reinforce the consistency between i-APS and t-APS methods for DGSE characterization.

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 $237\,μ\mathrm{Jy}/\mathrm{beam}$ 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 $C_{\ell} = A({1000}/{\ell})^β$. Combining data from Band-2 and earlier Band-3 observations, we derived a spectral variation of $C_{\ell}$ in the form of $C_{\ell} = A{ν^{-2α}}{\ell^{-β}}$. Our result indicates a spectral break at $ν= 230\,{\pm}\,5$ 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 $α$ and power-law index $β$ are consistent within the frequency range $120-500$ MHz.

Figures (11)

• Figure 1: Primary-beam corrected 32 hours final image of ELAIS-N1 field after DD-calibration at 183 MHz (usable bandwidth $\sim$ 100 MHz). The final image covers an area of $5.86\,\text{deg}^2$ with the off-source RMS noise near the center is 237 ${\mu}$Jy/beam. Synthesized beam size is $11.45" {\times} 9.02"$.
• Figure 2: Left Panel: The background RMS noise is high near the bright sources and at the edge of FoV in the 32 hours final image of the ELAIS-N1 field. Right Panel: Cumulative area of primary-beam corrected (final) image as a function of $\sigma_L$, local RMS noise.
• Figure 3: The ratio of integrated to peak flux density ($S_{\text{int}}/S_{\text{peak}}$) as a function of SNR ($S_{\text{peak}}/\sigma_L$) for uGMRT sources. Resolved sources are presented in red circles, and compact sources are in sea-green triangles.
• Figure 4: Comparing the integrated flux densities of compact and isolated sources estimated with uGMRT at 183 MHz with other available catalogues after scaling them at 183 MHz frequency, including 150 MHz LoTSS (red, also in the sub-plot), 400 MHz GMRT (golden), 610 MHz GMRT (green), uGMRT 1250 MHz (purple), 150 MHz TGSS (black) and FIRST survey (blue). The black dashed line in both plots represents $S_{\text{uGMRT}}/S_{\text{others}} = 1$.
• Figure 5: Deviation in positions (RA $\&$ DEC) of the uGMRT sources at 183 MHz compared with the FIRST (blue), uGMRT 400 MHz (golden), GMRT 610 MHz (green), uGMRT 1250 MHz (purple), LoTSS (red) and TGSS 150MHz (orange) catalogue observations.