[Paper Review] GALFACTS: The G-ALFA Continuum Transit Survey

Russ Taylor, C. J. Salter|arXiv (Cornell University)|Aug 29, 2010

Astronomy and Astrophysical Research1 references4 citations

TL;DR

GALFACTS is a high-resolution, full-Stokes, spectro-polarimetric survey of 12,734 square degrees of the northern sky using the Arecibo telescope’s ALFA receiver, achieving 3.5′ angular resolution and 90 µJy sensitivity at 1225–1525 MHz. It enables detailed mapping of Galactic polarized synchrotron emission, Faraday rotation measures, and a catalog of ~100,000 extragalactic radio sources, significantly advancing studies of the Milky Way’s magnetic field and the magneto-ionic interstellar medium.

ABSTRACT

The GALFACTS project is using the L-band seven feed array receiver system on the Arecibo telescope to carry out an imaging spectro-polarimetric survey of the 30% of the sky visible from Arecibo. GALFACTS observations will create full-Stokes image cubes at an angular resolution of 3.5', with several thousand spectral channels covering 1225 - 1525 MHz, and band-averaged sensitivity of 90 uJy, allowing sensitive imaging of polarized radiation and Faraday Rotation Measure from both diffuse emission and against a high density grid of extragalactic sources. GALFACTS will be a major observational advance in imaging of the polarized radiation from the Milky Way and will provide a rich new database for exploration of the magnetic field of the Galaxy and the properties of the magneto-ionic medium.

Motivation & Objective

• To map the polarized radio emission of the Milky Way with unprecedented angular resolution and sensitivity to probe the structure and dynamics of the Galactic magnetic field.
• To measure Faraday Rotation Measures (RMs) of extragalactic sources across a dense grid to trace the magneto-ionic medium in the Galactic disk and halo.
• To detect and characterize diffuse polarized emission structures arising from differential Faraday rotation in the interstellar medium, particularly at arcminute scales.
• To identify and study discrete Galactic radio sources such as supernova remnants and HII regions through their polarized emission and Faraday depolarization signatures.
• To enable cross-identification with multi-wavelength surveys (e.g., IRIS, CMB, RRL) to disentangle thermal and non-thermal components of Galactic continuum emission.

Proposed method

• Observations are conducted using the Arecibo telescope’s L-band seven-feed ALFA receiver, which provides seven spatial beams with orthogonal linear polarization states.
• The telescope performs meridian scans at 1.53° per sidereal minute, creating a zig-zag track pattern with beam spacing of 1.83′, achieving near-Nyquist sampling for 3.5′ FWHM beams.
• Data are acquired in full-Stokes mode (I, Q, U, V) across 1225–1525 MHz with several thousand spectral channels, enabling high spectral resolution for Faraday rotation analysis.
• The survey covers three regions: a southern zone (−0.8° < δ < 16.8°), a northern zone (19.8° < δ < 37.8°), and a zenith zone (16.3° < δ < 20.3°), totaling 12,734 square degrees.
• Calibration is performed daily using strong compact sources to measure the full-Stokes response of the ALFA system and correct for instrumental effects.
• Nighttime observations minimize solar interference, and data are processed to produce full-Stokes image cubes with 3.5′ resolution and 90 µJy sensitivity.

Experimental results

Research questions

• RQ1 What is the spatial structure of the Galactic magnetic field on arcminute to kpc scales, as revealed by polarized synchrotron emission?
• RQ2 How do Faraday Rotation Measures of extragalactic sources trace the distribution and properties of thermal and relativistic plasmas in the interstellar medium?
• RQ3 What is the polarization structure and depolarization behavior of diffuse Galactic emission, and how do they relate to differential Faraday rotation?
• RQ4 How do magnetic fields in HII regions and supernova remnants influence the polarization of background emission and what do they reveal about local field geometry?
• RQ5 Can GALFACTS detect transient or circularly polarized radio sources, such as those from low-mass stars or brown dwarfs?

Key findings

• GALFACTS achieves 3.5′ angular resolution and 90 µJy sensitivity, offering an order of magnitude improvement in brightness sensitivity over previous single-dish surveys.
• The survey will produce full-Stokes image cubes over 12,734 square degrees of the sky, enabling detailed study of polarized emission and Faraday rotation across the Galactic plane and high latitudes.
• An estimated 100,000 extragalactic radio sources will be detected down to polarized flux densities of a few 100 µJy, with polarization position angles and rotation measures measured with high accuracy.
• The survey will resolve fine-scale polarized structures in the diffuse Galactic emission, interpreted as signatures of differential Faraday rotation in the magneto-ionic medium.
• GALFACTS data will allow the separation of thermal and non-thermal components of Galactic continuum emission when combined with IRIS and RRL surveys.
• The survey is sensitive enough to probe magnetic fields in the Galactic halo and the disk-halo interface, potentially revealing turbulence and field structure in these poorly understood regions.