The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release (1611.02700v1)

Abstract: The LOFAR Two-metre Sky Survey (LoTSS) is a deep 120-168 MHz imaging survey that will eventually cover the entire Northern sky. Each of the 3170 pointings will be observed for 8 hrs, which, at most declinations, is sufficient to produce ~5arcsec resolution images with a sensitivity of ~0.1mJy/beam and accomplish the main scientific aims of the survey which are to explore the formation and evolution of massive black holes, galaxies, clusters of galaxies and large-scale structure. Due to the compact core and long baselines of LOFAR, the images provide excellent sensitivity to both highly extended and compact emission. For legacy value, the data are archived at high spectral and time resolution to facilitate subarcsecond imaging and spectral line studies. In this paper we provide an overview of the LoTSS. We outline the survey strategy, the observational status, the current calibration techniques, a preliminary data release, and the anticipated scientific impact. The preliminary images that we have released were created using a fully-automated but direction-independent calibration strategy and are significantly more sensitive than those produced by any existing large-area low-frequency survey. In excess of 44,000 sources are detected in the images that have a resolution of 25arcsec, typical noise levels of less than 0.5 mJy/beam, and cover an area of over 350 square degrees in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45d00m00s to 57d00m00s).

• The paper introduces a novel survey strategy using 3170 pointings with 8-hour integrations to deliver 5-arcsecond resolution and 100 µJy/beam sensitivity.
• It reports preliminary data from 54 calibrated pointings covering over 350 square degrees and detecting more than 44,000 radio sources.
• Robust calibration addressing ionospheric effects promises enhanced imaging fidelity and lays the groundwork for future direction-dependent improvements.

An Overview of the LOFAR Two-metre Sky Survey (LoTSS)

The LOFAR Two-metre Sky Survey (LoTSS) represents a significant endeavour to map the entire Northern sky at 120-168 MHz, leveraging the capabilities of the International LOFAR Telescope. The survey is designed to provide deep, high-resolution images with a target resolution of 5 arcseconds and a sensitivity of approximately 100 µJy/beam. This extensive survey is directed primarily at probing the evolution and structure of massive black holes, galaxies, galaxy clusters, and large-scale cosmic structures, embodying a substantial leap in low-frequency radio astronomy.

Survey Design and Execution

The LoTSS strategy encompasses 3170 discrete pointings, each observed for 8 hours. This strategy effectively balances survey depth, sky coverage, and operational efficiency. The observations are conducted using LOFAR's HBA configuration, focusing on frequencies with low radio frequency interference to optimize sensitivity. Data from these observations are archived with high spectral and temporal resolution, facilitating future subarcsecond imaging and spectral line studies. This approach is meant to maximize the surveys' legacy and scientific utility, notwithstanding the considerable data volumes it generates.

Preliminary Data Release and Results

In the initial phase, the survey has focused on a region known as the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00'00" to 57°00'00"), resulting in 54 calibrated pointings from a total of 63. Despite the challenges of ionospheric distortion and instrumental calibration, this preliminary release covers over 350 square degrees and identifies more than 44,000 radio sources. The median noise level within the region is approximately 380 µJy/beam, showcasing a sensitivity unmatched by prior large-area surveys at these frequencies. This significant sensitivity allows for novel studies of both faint and extended sources.

Calibration and Imaging Challenges

Critical to the survey's success is the robust calibration of LOFAR's complex dataset, particularly addressing direction-dependent effects, such as ionospheric variations and beam model errors, to achieve the survey's imaging goals. The current direction-independent approach provides reliable data, but full direction-dependent calibration holds promise for achieving the desired imaging fidelity and resolution. Examples of successful direction-dependent calibrations underline potential improvements, showing reductions in noise levels to around 100 µJy/beam and resolutions below 5 arcseconds.

Scientific Potential

The LoTSS has immense scientific potential across various astrophysical domains. It enhances our understanding of cosmic magnetism, the role of AGN in galaxy evolution, and the characteristics of high redshift radio galaxies. Furthermore, it will facilitate studies on cosmic rays in galaxies, the diffuse radio emission in galaxy clusters, and the detection of radio-loud quasars at extreme redshifts. The survey's breadth and depth will augment our ability to examine rare and faint cosmic phenomena, contributing expansively to the fields of galaxy cluster dynamics and cosmic evolution.

Future Developments

Completing the direction-dependent calibration techniques is critical for future data releases, where achieving uniform high-resolution images could enable comprehensive studies of cosmic phenomena. As these methods are refined, the survey is expected to achieve its overall scientific goals and potentially redefine the landscape of low-frequency radio astronomy.

In summary, LoTSS represents a remarkable step in radio surveys, offering unprecedented depth and coverage at low frequencies, thus setting the stage for significant astronomical discoveries and fostering a deeper understanding of the universe's structure and formation processes.