Recent observations of the radio sky show that the vast majority of sources detected at 144 MHz are unresolved at the typical resolution of a few arcseconds1, demonstrating the need for sub-arcsecond-resolution surveys to make detailed studies. At low radio frequencies, high spatial resolution is challenged by the ionosphere and by the propagation delay of radio waves that it induces2. If not adequately corrected for, this blurs the images to arcsecond or even arcminute scales. In addition, the required image size to map the degree-scale field of view of low-frequency radio telescopes at sub-arcsecond resolution is far greater than what typical software and hardware are currently capable of handling. Here we present deep degree-scale sub-arcsecond imaging at low radio frequencies. We derive ionospheric corrections in 44 directions on individual sources with compact sub-arcsecond structures. This has yielded a sensitive 6.6 deg2 144 MHz map with a resolution of 0.38″ × 0.30″ and a sensitivity of 25 μJy per beam, near the phase centre. This will allow mapping of the entire northern low-frequency sky at sub-arcsecond resolution.
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A source catalogue and the full-resolution images are accessible through the LOFAR Surveys Key Science Project webpage at https://www.lofar-surveys.org/hdfields.html.
The various software and pipelines used in this work are publicly available at https://github.com/lofar-astron/prefactor (PREFACTOR), https://github.com/saopicc/DDFacet (DDFACET), https://github.com/lmorabit/lofar-vlbi (LOFAR-VLBI), https://github.com/mhardcastle/ddf-pipeline (DDF-PIPELINE), https://gitlab.com/aroffringa/wsclean (WSClean) and https://gitlab.com/astron-idg/idg (IDG). Code from other parts not using these pipelines is not directly available because it is not part of a complete pipeline, but is available upon reasonable request to the corresponding author.
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F.S. would like to thank SURFsara for beta access to the Spider platform. This paper is based (in part) on data obtained with the ILT under project code LT10_012. LOFAR (van Haarlem et al.9) is the Low Frequency Array designed and constructed by ASTRON. It has observing, data processing and data storage facilities in several countries; these facilities are owned by various parties (each with their own funding sources) and are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d’Orléans, France; BMBF, MIWF-NRW and MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; and The Science and Technology Facilities Council, UK. R.J.v.W. acknowledges support from the ERC Starting Grant ClusterWeb 804208. This work was supported by the Medical Research Council (grant no. MR/T042842/1). This work made use of the Dutch national e-infrastructure with the support of the SURF Cooperative using grant no. EINF-251. P.B. is grateful for support from the UK STFC via grants ST/R000972/1 and ST/V000594/1. This work was performed using the computing resources from the Academic Leiden Interdisciplinary Cluster Environment (ALICE) provided by Leiden University. This work also made use of SciPy, NumPy, Matplotlib and TOPCAT.
The authors declare no competing interests.
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Sweijen, F., van Weeren, R.J., Röttgering, H.J.A. et al. Deep sub-arcsecond wide-field imaging of the Lockman Hole field at 144 MHz. Nat Astron 6, 350–356 (2022). https://doi.org/10.1038/s41550-021-01573-z