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Deep sub-arcsecond wide-field imaging of the Lockman Hole field at 144 MHz

Nature Astronomy volume 6, pages 350–356 (2022)Cite this article

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Abstract

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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Fig. 1: An overview of the field illustrating both the wide-field and high-resolution aspects.
Fig. 2: Illustration of the direction-dependent self-calibration process for different sources across the field and various degrees of extendedness.

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Data availability

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.

Code availability

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.

References

  1. Shimwell, T. W. et al. The LOFAR Two-metre Sky Survey. II. First data release. Astron. Astrophys. 622, A1 (2019).

    Article  Google Scholar 

  2. de Gasperin, F., Mevius, M., Rafferty, D. A., Intema, H. T. & Fallows, R. A. The effect of the ionosphere on ultra-low-frequency radio-interferometric observations. Astron. Astrophys. 615, A179 (2018).

    Article  Google Scholar 

  3. Lenc, E., Garrett, M. A., Wucknitz, O., Anderson, J. M. & Tingay, S. J. A deep, high-resolution survey of the low-frequency radio sky. Astron. J. 673, 78–95 (2008).

    Article  Google Scholar 

  4. Morgan, J. S. et al. VLBI imaging throughout the primary beam using accurate UV shifting. Astron. Astrophys. 526, A140 (2011).

    Article  Google Scholar 

  5. Chi, S., Barthel, P. D. & Garrett, M. A. Deep, wide-field, global VLBI observations of the Hubble deep field north (HDF-N) and flanking fields (HFF). Astron. Astrophys. 550, A68 (2013).

    Article  ADS  Google Scholar 

  6. Middelberg, E. et al. Mosaiced wide-field VLBI observations of the Lockman Hole/XMM. Astron. Astrophys. 551, A97 (2013).

    Article  Google Scholar 

  7. Radcliffe, J. F. et al. Nowhere to hide: radio-faint AGN in GOODS-N field. I. Initial catalogue and radio properties (Corrigendum). Astron. Astrophys. 625, C1 (2019).

    Article  Google Scholar 

  8. Muxlow, T. W. B. et al. The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description. Mon. Not. R. Astron. Soc. 495, 1188–1208 (2020).

    Article  ADS  Google Scholar 

  9. van Haarlem, M. P. et al. LOFAR: The LOw-Frequency ARray. Astronomy Astrophys. 556, A2 (2013).

  10. de Gasperin, F. et al. Systematic effects in LOFAR data: a unified calibration strategy. Astron. Astrophys. 622, A5 (2019).

    Article  Google Scholar 

  11. Morabito, L. et al. Sub-arcsecond imaging with the International LOFAR Telescope. I. Foundational calibration strategy and pipeline. Preprint at https://arxiv.org/abs/2108.07283 (2021).

  12. Jackson, N. J. et al. Sub-arcsecond imaging with the International LOFAR Telescope. II. Completion of the LOFAR Long-Baseline Calibrator Survey. Preprint at https://arxiv.org/abs/2108.07284 (2021).

  13. White, R. L., Becker, R. H., Helfand, D. J. & Gregg, M. D. A catalog of 1.4 GHz radio sources from the FIRST survey. Astrophys. J. 475, 479–493 (1997).

    Article  ADS  Google Scholar 

  14. Offringa, A. R. et al. WSCLEAN: an implementation of a fast, generic wide-field imager for radio astronomy. Mon. Not. R. Astron. Soc. 444, 606–619 (2014).

    Article  ADS  Google Scholar 

  15. van der Tol, S., Veenboer, B. & Offringa, A. R. Image Domain Gridding: a fast method for convolutional resampling of visibilities. Astron. Astrophys. 616, A27 (2018).

    Article  Google Scholar 

  16. Veenboer, B., Petschow, M., van der Tol, S. & Offringa, A. R. IDG: Image Domain Gridding. Astrophysics Source Code Library ascl: 1911.011 (2019).

  17. Ivison, R. J. et al. Deep radio imaging of the SCUBA 8-mJy survey fields: submillimetre source identifications and redshift distribution. Mon. Not. R. Astron. Soc. 337, 1–25 (2002).

    Article  ADS  Google Scholar 

  18. Kondapally, R. et al. The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1. III. Host-galaxy identifications and value added catalogues. Astron. Astrophys. 648, A3 (2021).

    Article  Google Scholar 

  19. Tasse, C. et al. The LOFAR Two-meter Sky Survey: Deep Fields Data Release 1. I. Direction-dependent calibration and imaging. Astron. Astrophys. 648, A1 (2021).

    Article  Google Scholar 

  20. Intema, H. T., Jagannathan, P., Mooley, K. P. & Frail, D. A. The GMRT 150 MHz all-sky radio survey. First alternative data release TGSS ADR1. Astron. Astrophys. 598, A78 (2017).

    Article  ADS  Google Scholar 

  21. Tasse, C. et al. Faceting for direction-dependent spectral deconvolution. Astron. Astrophys. 611, A87 (2018).

    Article  Google Scholar 

  22. van Weeren, R. J. et al. LOFAR observations of galaxy clusters in HETDEX. Extraction and self-calibration of individual LOFAR targets. Astron. Astrophys. 651, A115 (2021).

    Article  Google Scholar 

  23. van Diepen, G., Dijkema, T. J. & Offringa, A. DPPP: Default Pre-Processing Pipeline. Astrophysics Source Code Library ascl: 1804:003 (2018).

  24. van Weeren, R. J. et al. LOFAR facet calibration. Astrophys. J. Suppl. Ser. 223, 2 (2016).

    Article  ADS  Google Scholar 

  25. Veenboer, B. & Romein, J. W. Radio-astronomical imaging on graphics processors. Astron. Comput. 32, 100386 (2020).

    Article  Google Scholar 

  26. Mohan, N. & Rafferty, D. PyBDSF: Python Blob Detection and Source Finder. Astrophysics Source Code Library ascl: 1502.007 (2015).

  27. Bridle, A. H. & Schwab, F. R. in Synthesis Imaging in Radio Astronomy II Vol. 180 (eds Taylor, G. B. et al.) 371–381 (Astronomical Society of the Pacific, 1999).

  28. Rickett, B. J. Radio propagation through the turbulent interstellar plasma. Annu. Rev. Astron. Astrophys. 28, 561–605 (1990).

    Article  ADS  Google Scholar 

  29. Cohen, M. H. & Cronyn, W. M. Scintillation and apparent angular diameter. Astrophys. J. 192, 193–197 (1974).

    Article  ADS  Google Scholar 

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Acknowledgements

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.

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Authors and Affiliations

  1. Sterrewacht Leiden, University of Leiden, Leiden, The Netherlands

    F. Sweijen, R. J. van Weeren, H. J. A. Röttgering, J. B. R. Oonk & T. W. Shimwell

  2. Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham, UK

    L. K. Morabito

  3. Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester, UK

    N. Jackson

  4. Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo, The Netherlands, Postbus 2, 7990 AA

    A. R. Offringa, S. van der Tol, B. Veenboer, J. B. R. Oonk & T. W. Shimwell

  5. SURF Cooperative, Amsterdam, The Netherlands

    J. B. R. Oonk

  6. Institute for Astronomy, Royal Observatory, University of Edinburgh, Edinburgh, UK

    P. N. Best

  7. Istituto di Radioastronomia, INAF, Bologna, Italy, Via Gobetti 101, 40129

    M. Bondi

  8. GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Meudon, France, 5 place Jules Janssen, 92190

    C. Tasse

  9. Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown, South Africa, 6140

    C. Tasse

  10. USN, Observatoire de Paris, CNRS, PSL, UO, Nançay, France

    C. Tasse

  11. Jodrell Bank Centre for Astrophysics, The University of Manchester, Manchester, UK

    A. P. Thomson

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Contributions

F.S. led the paper, reduced the data and produced the images. R.J.v.W. developed the self-calibration routine and helped expand it to international baselines. H.J.A.R. helped scope and write the paper. L.K.M. and N.J. conducted the LBCS survey and developed the LOFAR-VLBI pipeline, which served as the foundation for this work. A.R.O., S.v.d.T. and B.V. maintain WSClean, IDG and were of great help in fixing problems and adding features to the software. J.B.R.O. helped secure resources on SURFsara and provide support for our data reduction on their Spider platform. P.B. led the proposal that obtained the data and contributed to editing of the paper. M.B. is a member of the long-baseline working group and contributed to editing of the paper. T.W.S. contributed towards some of the various data processing pipelines used in this work and produced the deep Lockman Hole 6″ image. C.T. developed the DDFacet software used and produced the deep Lockman Hole 6″ image. A.P.T. is a member of the long-baseline working group and helped determine the flux scale.

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Correspondence to F. Sweijen.

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The authors declare no competing interests.

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Nature Astronomy thanks Preshanth Jagannathan, Sphesihle Makhathini and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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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

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