Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: varenius@chalmers.se
² Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, M13 9 PL Manchester, UK
³ The Netherlands Institute for Radio Astronomy (ASTRON), PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁵ Instituto de Astrofísica de Andalucía, Glorieta de las Astronomía, s/n, 18008 Granada, Spain
⁶ Centro de Estudios de la Física del Cosmos de Aragón, 44001 Teruel, Spain
⁷ Departamento de Física Teorica, Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain
⁸ Obserwatorium Astronomiczne Uniwersytetu, Jagiellońskiego, ul. Orla 171, 30-244 Kraków, Poland
⁹ SUPA, Institute for Astronomy, Royal Observatory Edinburgh, Blackford Hill, EH9 3 HJ Edinburgh, UK
¹⁰ Ruhr-Universität Bochum, Astronomisches Institut, 44780 Bochum, Germany
¹¹ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹² INAF-Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹³ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁴ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁶ Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
¹⁷ Astrophysics, Department of Physics, Keble Road, OX1 3 RH Oxford, UK
¹⁸ Department of Physics, University of the Western Cape, Private Bag X17, 7535 Bellville, South Africa
¹⁹ Department of Physical Sciences, The Open University, MK7 6 AA Milton Keynes, UK
²⁰ RALSpace, The Rutherford Appletion Laboratory, Chilton, Didcot, OX11 0 NL Oxfordshire, UK

Received: 1 October 2014
Accepted: 26 November 2014

Abstract

Context. The nuclear starburst in the nearby galaxy M82 provides an excellent laboratory for understanding the physics of star formation. This galaxy has been extensively observed in the past, revealing tens of radio-bright compact objects embedded in a diffuse free-free absorbing medium. Our understanding of the structure and physics of this medium in M82 can be greatly improved by high-resolution images at low frequencies where the effects of free-free absorption are most prominent.

Aims. The aims of this study are, firstly, to demonstrate imaging using international baselines of the Low Frequency Array (LOFAR), and secondly, to constrain low-frequency spectra of compact and diffuse emission in the central starburst region of M82 via high-resolution radio imaging at low frequencies.

Methods. The international LOFAR telescope was used to observe M82 at 110−126 MHz and 146−162 MHz. Images were obtained using standard techniques from very long baseline interferometry. images were obtained at each frequency range: one only using international baselines, and one only using the longest Dutch (remote) baselines.

Results. The 154 MHz image obtained using international baselines is a new imaging record in terms of combined image resolution (0.3′′) and sensitivity (σ = 0.15 mJy/beam) at low frequencies (<327 MHz). We detected 16 objects at 154 MHz, six of these also at 118 MHz. Seven objects detected at 154 MHz have not been catalogued previously. For the nine objects previously detected, we obtained spectral indices and emission measures by fitting models to spectra (combining LOFAR with literature data). Four weaker but resolved features are also found: a linear (50 pc) filament and three other resolved objects, of which two show a clear shell structure. We do not detect any emission from either supernova 2008iz or from the radio transient source 43.78+59.3. The images obtained using remote baselines show diffuse emission, associated with the outflow in M82, with reduced brightness in the region of the edge-on star-forming disk.