[Paper Review] The Deep Full-Stokes Radio Sky

A. R. Taylor, S. Bhatnagar+9 more|arXiv (Cornell University)|May 1, 2014

Radio Astronomy Observations and Technology3 citations

TL;DR

This paper presents ultra-deep full-Stokes radio imaging of the sky using the GMRT and JVLA at 612 MHz and 5 GHz, achieving rms sensitivities of 10.3 μJy/beam and 1.05 μJy/beam, respectively. It demonstrates that μJy-level polarized emission arises from magnetic fields in starburst and normal galaxies, marking the transition from AGN-dominated to disk-dominated polarized sources below ~1 mJy flux density.

ABSTRACT

The new broad-band capabilities of large radio interferometers such as the GMRT and JVLA allow for long-integration mosaic imaging observations to create ultra-deep full-polarization images of the sky over wide frequency ranges. Achieving rms sensitivities of order 1 $μ$Jy, these observations explore the radio source population at flux densities well below the regime dominated by classical radio galaxies and Active Galactic Nuclei. We present initial results from radio sources revealed with deep mosaicking observations with the GMRT and JVLA at respectively 0.6 and 5 GHz, and evidence that the $μ$Jy sensitivity level marks the transition to detection of polarized emission from a population of sources dominated by emission from magnetic fields in the disks of starburst and normal galaxies.

Motivation & Objective

• To explore the polarized radio sky at flux densities below 1 mJy, where the population transitions from AGN-dominated to disk-dominated emission.
• To detect and characterize polarized emission from distant star-forming and normal spiral galaxies using deep mosaicking with GMRT and JVLA.
• To assess the role of internal depolarization in reducing fractional polarization at low frequencies and its implications for galaxy magnetic field studies.
• To establish the feasibility of detecting polarized emission from galaxy disks at μJy levels, a critical step toward SKA-era cosmology.
• To calibrate and correct wideband, wide-field polarimetry using advanced techniques like AW-projection and beam measurements for improved sensitivity.

Proposed method

• Conducted deep mosaicking observations with the GMRT (612 MHz, 30 hours per pointing, 7 pointings in hexagonal pattern) and JVLA (5 GHz, 60 hours total, 10 closely spaced pointings).
• Used CASA software for calibration, imaging, and mosaicking, applying absolute flux, bandpass, and polarization angle solutions via calibrator sources (3C286, J1549+506, J1624+5652).
• Applied off-axis gain and polarization leakage corrections using frequent calibrator scans to minimize beam response errors in central mosaic regions.
• Performed stacking analysis on 6 flux density bins (60 μJy to 10 mJy) to detect weak polarized signals below individual detection limits.
• Used Monte Carlo simulations to correct for non-linear bias in observed polarized intensity when deriving median fractional polarization.
• Projected future improvements via the GMRT's upcoming broad-band upgrade, enabling sub-μJy sensitivity and systematic study of distant galaxy magnetism.

Experimental results

Research questions

• RQ1 What is the nature of polarized radio emission at flux densities below 1 mJy, particularly in the context of galaxy disk magnetic fields?
• RQ2 Can polarized emission from star-forming and normal spiral galaxies be detected at μJy levels in deep full-Stokes surveys?
• RQ3 How does internal depolarization from thermal plasma affect fractional polarization at low frequencies (e.g., 612 MHz)?
• RQ4 What is the transition point in flux density where the polarized source population shifts from AGN-dominated to disk-dominated emission?
• RQ5 To what extent can wideband, wide-field polarimetry with modern interferometers like GMRT and JVLA enable the detection of faint polarized emission from distant galaxies?

Key findings

• The GMRT 612 MHz mosaic achieved an rms noise of 10.3 μJy/beam over 0.9 sq deg, detecting 2,800 sources above 50 μJy.
• The JVLA 5 GHz mosaic reached 1.05 μJy/beam noise over 0.13 sq deg, detecting 483 sources above 5 μJy.
• Polarized intensity imaging at 612 MHz revealed numerous sources, including classical double-lobed radio galaxies and fainter, compact polarized sources.
• Stacking analysis detected polarized signals down to 200 μJy in total intensity, indicating polarized emission from faint star-forming and normal galaxies.
• The median fractional polarization appears to decrease below ~1 mJy, possibly due to weak AGN components or internal depolarization in galaxy disks.
• Preliminary polarized source counts at 5 GHz show good agreement with predictions for disk galaxies based on nearby galaxy polarization properties, suggesting detection of distant disk emission is feasible at μJy levels.