Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz

A kpc-scale star forming disk surrounding nuclei with shocked outflows^⋆

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: varenius@chalmers.se
² Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA
³ The Netherlands Institute for Radio Astronomy (ASTRON), PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁴ Jodrell Bank Centre for Astrophysics, Alan Turing Building, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁵ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain
⁶ Departamento de Física Teorica, Facultad de Ciencias, Universidad de Zaragoza, Spain
⁷ Department of Astronomy, University of Wisconsin-Madison, WI 53706, USA
⁸ Department of Physics, University of Wisconsin-Madison, WI 53706, USA
⁹ Center for Magnetic Self-Organization in Laboratory and Astrophysical Plasmas, University of Wisconsin-Madison, WI 53706, USA
¹⁰ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 12 April 2016
Accepted: 5 July 2016

Abstract

Context. Arp 220 is the prototypical ultra luminous infrared galaxy (ULIRG). Despite extensive studies, the structure at MHz-frequencies has remained unknown because of limits in spatial resolution.

Aims. This work aims to constrain the flux and shape of radio emission from Arp 220 at MHz frequencies.

Methods. We analyse new observations with the International Low Frequency Array (LOFAR) telescope, and archival data from the Multi-Element Radio Linked Interferometer Network (MERLIN) and the Karl G. Jansky Very Large Array (VLA). We model the spatially resolved radio spectrum of Arp 220 from 150 MHz to 33 GHz.

Results. We present an image of Arp 220 at 150 MHz with resolution 0.̋65 × 0.̋35, sensitivity 0.15 mJy beam^-1, and integrated flux density 394 ± 59 mJy. More than 80% of the detected flux comes from extended (6′′≈ 2.2 kpc) steep spectrum (α = −0.7) emission, likely from star formation in the molecular disk surrounding the two nuclei. We find elongated features extending 0.3′′ (110 pc) and 0.9′′ (330 pc) from the eastern and western nucleus respectively, which we interpret as evidence for outflows. The extent of radio emission requires acceleration of cosmic rays far outside the nuclei. We find that a simple three component model can explain most of the observed radio spectrum of the galaxy. When accounting for absorption at 1.4 GHz, Arp 220 follows the FIR/radio correlation with q = 2.36, and we estimate a star formation rate of 220 M_⊙ yr^-1. We derive thermal fractions at 1 GHz of less than 1% for the nuclei, which indicates that a major part of the UV-photons are absorbed by dust.

Conclusions. International LOFAR observations shows great promise to detect steep spectrum outflows and probe regions of thermal absorption. However, in LIRGs the emission detected at 150 MHz does not necessarily come from the main regions of star formation. This implies that high spatial resolution is crucial for accurate estimates of star formation rates for such galaxies at 150 MHz.