GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue

Abstract: Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array (SKA1 LOW) precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) survey, and present the resulting extragalactic catalogue, utilising the first year of observations. The catalogue covers 24,831 square degrees, over declinations south of $+30^\circ$ and Galactic latitudes outside $10^\circ$ of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307,455 radio sources with 20 separate flux density measurements across 72--231MHz, selected from a time- and frequency- integrated image centred at 200MHz, with a resolution of $\approx 2$'. Over the catalogued region, we estimate that the catalogue is 90% complete at 170mJy, and 50% complete at 55mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97% above the detection threshold of $5\sigma$, which itself is typically 50mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low frequency flux density scale of the southern sky to better than 10%. This paper presents details of the flagging, imaging, mosaicking, and source extraction/characterisation, as well as estimates of the completeness and reliability. All source measurements and images are available online (http://www.mwatelescope.org/science/gleam-survey). This is the first in a series of publications describing the GLEAM survey results.

• The paper demonstrates a comprehensive survey of low-frequency radio observations, cataloguing 307,455 extragalactic sources using advanced imaging and calibration techniques.
• It applies a drift scan imaging strategy with rigorous data processing to achieve 90% completeness for sources over 170 mJy and 99.97% reliability for detections above 50 mJy.
• The survey establishes a robust flux density scale for the southern sky and paves the way for future research with the Square Kilometre Array (SKA).

Overview of the GLEAM Survey: A Low-Frequency Extragalactic Catalogue

The paper "GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey I: A low-frequency extragalactic catalogue" presents the results of a comprehensive radio sky survey utilizing the Murchison Widefield Array (MWA). This study meticulously documents the low-frequency (72-231 MHz) observations and the subsequent construction of a detailed extragalactic catalogue, encapsulating a wide area of the southern sky. With 307,455 unique radio sources cataloged, the GLEAM survey offers a seminal contribution to the field, particularly in the context of low-frequency radio astronomy and preparation for the Square Kilometre Array (SKA).

Key Aspects of the Survey

1. Extensive Sky Coverage and Selection:
   • The survey encompasses 24,831 square degrees, focusing on declinations south of +30° and excluding areas for specific reasons such as the Galactic Plane or proximity to the Magellanic Clouds.
   • A drift scan imaging strategy was employed, with observations organized over the southern celestial hemisphere, optimizing systematics and minimizing the calibration complexities associated with wide fields.
2. Catalogue Completeness and Reliability:
   • Estimates place the catalogue's completeness at 90% for sources with flux densities above 170 mJy, and 50% for sources more than 55 mJy.
   • The reliability of detections is remarkable at 99.97% for sources above the typical detection threshold of 50 mJy.
3. Scientific and Technical Contributions:
   • The paper details the observational methodologies, data processing techniques (including flagging, calibration, and imaging), and strategies for mosaicking and source extraction.
   • The GLEAM survey establishes a flux density scale for the southern sky, achieving calibration to better than 10%, providing a robust foundation for subsequent low-frequency radio astronomical research.
4. Source Characterization and Spectral Analysis:
   • The survey includes 20 separate flux density measurements for each detected source across 72-231 MHz, offering significant spectral resolution capabilities.
   • The synthesis of these data allows for accurate modeling of source spectral energy distributions and aids in exploring phenomena such as synchrotron emission mechanisms in extragalactic objects.

Implications and Future Directions

The GLEAM survey exemplifies a significant step forward in radio astronomy, demonstrating the instrumental capabilities of the MWA and setting a new standard for low-frequency, wide-field surveys. It serves not only as a foundational dataset for extragalactic studies but also as a pathfinder project for the development of the SKA. The insights gained from the intricacies of ionospheric calibration, imaging under wide-field conditions, and data handling at large scales contribute to the broader field of Big Data in radio astronomy.

Looking to the future, the GLEAM catalogue will undoubtedly support a myriad of research endeavors, from the characterization of radio galaxy populations to the investigation of the cosmic web. Moreover, the survey's approach to integrating extensive observational data offers a blueprint for future surveys aiming to map the cosmos at low frequencies. As data from the continued operation of the MWA are processed, the depth and breadth of the catalogue will increase, further enhancing its value to the community. This work is anticipated to stimulate new discoveries and foster advancements in our understanding of the Universe's radio-emitting components.