The Evolutionary Map of the Universe Pilot Survey (2108.00569v1)

Abstract: We present the data and initial results from the first Pilot Survey of the Evolutionary Map of the Universe (EMU), observed at 944 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The survey covers 270 \sqdeg of an area covered by the Dark Energy Survey, reaching a depth of 25--30 \ujybm\ rms at a spatial resolution of $\sim$ 11--18 arcsec, resulting in a catalogue of $\sim$ 220,000 sources, of which $\sim$ 180,000 are single-component sources. Here we present the catalogue of single-component sources, together with (where available) optical and infrared cross-identifications, classifications, and redshifts. This survey explores a new region of parameter space compared to previous surveys. Specifically, the EMU Pilot Survey has a high density of sources, and also a high sensitivity to low surface-brightness emission. These properties result in the detection of types of sources that were rarely seen in or absent from previous surveys. We present some of these new results here.

• The paper demonstrates the novel use of ASKAP’s phased-array feed to achieve a 25–30 μJy rms sensitivity over 270 square degrees.
• It applies advanced multi-scale deconvolution techniques to catalogue approximately 220,000 radio sources with robust statistical validation.
• The survey’s findings, including the detection of enigmatic Odd Radio Circles, set the stage for deeper cosmological investigations and refined radio mapping.

An Overview of the Evolutionary Map of the Universe Pilot Survey

Introduction

The Evolutionary Map of the Universe (EMU) Pilot Survey is a preliminary exploration undertaken at 944 MHz using the Australian Square Kilometre Array Pathfinder (ASKAP). This survey is positioned as a precursor to the larger EMU project and serves to test and refine both the survey methodology and data processing paradigms. With the utilization of innovative radio astronomy technology, the EMU Pilot Survey (EMU-PS) covers 270 square degrees of the sky, coinciding with areas mapped by the Dark Energy Survey (DES). The research aims to investigate a new region of parameter space, contributing to the understanding of large-scale cosmological structures and radio galaxy evolution.

Methodology and Observational Strategy

ASKAP, as the instrument providing the data for EMU-PS, employs a phased-array feed to accomplish a substantial field of view exceeding prior synthesis arrays. The phased-array feed enables a survey speed capable of creating large-scale radio maps efficiently. This setup is crucial for the survey's objective of increasing known extragalactic radio sources significantly beyond the current catalog of approximately 2.5 million sources. Such an expansion is pivotal for advancing cosmological studies and understanding rare galaxy evolutionary states.

The survey was designed to meet several observational criteria, such as focusing on regions below declination -30° to ensure effective $u,v$ coverage and targeting areas at high galactic latitudes to minimize galactic interference. Additionally, the selection of a field with robust optical counterparts in the DES maximizes the potential for cross-identification with optical/infrared datasets, yielding insights into the multi-wavelength characteristics of the detected sources.

Data Processing and Results

The processing chain for EMU-PS involves advanced data reduction techniques facilitated by ASKAPsoft. This system incorporates multi-scale, multi-frequency deconvolution algorithms to achieve precise imaging and spectral index estimation across the survey's bandwidth. Initial results of the survey catalogued approximately 220,000 radio sources, of which about 180,000 were deemed single-component sources. The data processing focused on creating a value-added release, including source counts corrected for common biases, and assessing cross-matches with optical and infrared surveys.

Numerical Outcomes and Discoveries

In the field of numerical data, the survey claims a 25–30 μJy rms sensitivity level, setting a new benchmark for wide-field radio surveys at comparable depths. The source counts align well with predictions from extragalactic simulations, showcasing the EMU-PS's capability to offer unparalleled statistical robustness spanning flux densities from sub-mJy to Jy levels.

Several peculiar and unexpected radio sources have been detected, including objects characterized by diffuse and structured radio emissions. Noteworthy among these are the Odd Radio Circles (ORCs), an enigmatic classification of distant, circular radio emissions without direct optical counterparts yet observable through supplementary methods.

Implications and Future Directions

The EMU-PS not only reinforces the observatory's technical proficiency but also provides critical insights into low-surface-brightness phenomena. As ASKAP's technology matures, expectations are high for further surveys to refine the quality and fidelity of data, potentially enabling deeper and broader examinations into the cosmic origins and developments of radio sources.

From theoretical and practical perspectives, this survey's framework paves the path for developing large-scale cosmological models connecting radio emissions with galaxy evolution and cosmic web structures. The anticipated full EMU survey bears the potential to revolutionize radio astronomy's capacity to reveal the universe's evolutionary history on a grand scale.

The EMU Pilot Survey, therefore, marks a significant advance in the execution of extensive radio continuum mappings, setting a precedent for future efforts with ASKAP and upcoming arrays such as the Square Kilometre Array (SKA). The knowledge and techniques gleaned from this pilot paper are poised to enrich the strategies employed in upcoming surveys and to refine our global understanding of astrophysical processes.