[Paper Review] A Wideband Polarization Survey of the Extragalactic Sky at 2-4 GHz: A Science White Paper for the VLA Sky Survey

Sui Ann Mao, Julie Banfield+24 more|arXiv (Cornell University)|Jan 9, 2014

Astronomy and Astrophysical Research2 references8 citations

TL;DR

This white paper proposes a wideband polarimetric survey of the extragalactic sky at 2–4 GHz using the upgraded VLA to map the magneto-ionic medium in AGNs, galaxies, and absorption line systems via Faraday rotation and depolarization analysis. By leveraging wideband spectro-polarimetry and RM synthesis with advanced modeling, it enables the detection of over 220,000 polarized sources and resolves degeneracies in Faraday structure, advancing understanding of cosmic magnetic fields and source evolution.

ABSTRACT

A VLA Sky Survey of the extragalactic sky at S band (2-4 GHz) with polarization information can uniquely probe the magneto-ionic medium in a wide range of astrophysical environments over cosmic time. For a shallow all-sky survey, we expect to detect over 4 million sources in total intensity $>$ 0.45 mJy beam$^{-1}$ and over 2.2$ imes$10$^5$ sources in polarized intensity. With these new observations, we expect to discover new classes of polarized radio sources in very turbulent astrophysical environments and those with extreme values of Faraday depth. Moreover, by determining reliable Faraday depths and by modeling depolarization effects, we can derive properties of the magneto-ionic medium associated with AGNs, absorption line systems and galaxies, addressing the following unresolved questions: (1) What is the covering fraction, the degree of turbulence and the origin of absorption line systems? (2) What is the thermal content in AGNs and radio galaxies? (3) How do AGNs and galaxies evolve over cosmic time? (4) What causes the increase in percentage polarization with decreasing flux densities at the low flux density end of the polarized source count? (5) What is the growth rate of large-scale magnetic fields in galaxies?

Motivation & Objective

• To address unresolved questions about the origin, turbulence, and covering fraction of absorption line systems by measuring Faraday depths and depolarization effects.
• To determine the thermal content and magnetic field evolution in AGNs and radio galaxies across cosmic time.
• To investigate the cause of increasing percentage polarization at low flux densities via wideband polarimetry.
• To measure the growth rate of large-scale magnetic fields in galaxies using statistical analysis of unresolved sources.
• To enable detection of depolarized sources at low frequencies by observing at 2–4 GHz, minimizing wavelength-dependent depolarization.

Proposed method

• Utilize the WIDAR correlator on the upgraded Jansky VLA to perform wideband polarimetry across 2–4 GHz, enabling high dynamic range and spectral resolution.
• Apply RM-synthesis to Q/I and U/I spectra to produce a 'dirty' Faraday dispersion function (FDF), convolved with the RM spread function (RMSF).
• Use the CLEAN algorithm to deconvolve complex FDFs and extract peak polarized intensity, polarization position angle, and rotation measure (RM).
• Fit representative models of Faraday thick screens to Q/I and U/I spectra using maximum-likelihood methods to resolve multiple Faraday components.
• Leverage the POSSUM pipeline for source detection and calibration, using Aegean for source finding and robust noise estimation.
• Implement a web-based SQL interface for catalog visualization and quality control of polarized source subsets.

Experimental results

Research questions

• RQ1 What is the covering fraction, degree of turbulence, and origin of absorption line systems in the interstellar and intergalactic medium?
• RQ2 What is the thermal content and magnetic field structure in AGNs and radio galaxies, and how do they evolve over cosmic time?
• RQ3 Why does the percentage polarization of extragalactic sources increase at low flux densities, and what role do depolarization mechanisms play?
• RQ4 What is the growth rate of large-scale magnetic fields in galaxies, and how do magnetic field properties evolve with redshift?
• RQ5 How can wideband polarimetry resolve degeneracies between different depolarization mechanisms in complex Faraday structures?

Key findings

• The survey is expected to detect over 4 million sources in total intensity above 0.45 mJy beam⁻¹ and more than 220,000 sources in polarized intensity.
• Wideband spectro-polarimetry at 2–4 GHz resolves degeneracies between Faraday rotation and depolarization effects that cannot be distinguished with narrowband data.
• The use of direct fitting to Q/I and U/I spectra as a function of λ² provides a more reliable method than RM-synthesis alone for determining true Faraday structure.
• Depolarization trends in unresolved galaxies at 2–4 GHz encode information on turbulent small-scale magnetic fields and allow statistical study of magnetic field evolution over cosmic time.
• The survey enables detection of sources depolarized at lower frequencies due to reduced wavelength-dependent depolarization effects in the S band.
• Modeling of Faraday depth and depolarization effects will test the internal Faraday dispersion hypothesis and constrain magnetic field amplification timescales in galaxies.