Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows
Journal article, 2016
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.
galaxies: starburst
galaxies: individual: Arp 220
ISM: structure
techniques: high angular resolution
Author
Eskil Varenius
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
John Conway
Chalmers, Earth and Space Sciences, Onsala Space Observatory
Ivan Marti-Vidal
Chalmers, Earth and Space Sciences, Onsala Space Observatory
Susanne Aalto
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
Loreto Barcos-Munoz
University of Virginia
Sabine König
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
Miguel Angel Perez-Torres
University of Zaragoza
Institute of Astrophysics of Andalusia (IAA)
A. Deller
Netherlands Institute for Radio Astronomy (ASTRON)
J. Moldon
University of Manchester
J. S. Gallagher III
University of Wisconsin Madison
Tova M. Yoast-Hull
University of Wisconsin Madison
Cathy Horellou
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
L. K. Morabito
Leiden University
Antxon Alberdi
Institute of Astrophysics of Andalusia (IAA)
N. Jackson
University of Manchester
R. J. Beswick
University of Manchester
Tobia Carozzi
Chalmers, Earth and Space Sciences, Onsala Space Observatory
O. Wucknitz
Max Planck Society
Naim Ramirez-Olivencia
Institute of Astrophysics of Andalusia (IAA)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 593 A86- A86Advanced Radio Astronomy in Europe (RADIONET3)
European Commission (EC) (EC/FP7/283393), 2012-01-01 -- 2015-12-31.
Subject Categories
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.1051/0004-6361/201628702