Subarcsecond international LOFAR radio images of Arp 220 at 150 MHz: A kpc-scale star forming disk surrounding nuclei with shocked outflows

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, 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. 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.A. A. and M.A.P.T. acknowledge support from the Spanish MINECO through grants AYA2012-38491-C02-02 and AYA2015-63939-C2-1-P, partially funded by FEDER funds.The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement No. 283393 (RadioNet3).Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts.Peer Reviewe

The far-infrared-radio correlation in galaxies

The tightness and universality of the far-infrared (FIR) to radio continuum (RC) correlation is still not completely understood. This correlation is followed by all star-forming galaxies not dominated by an Active Galactic Nucleus, both globally as well as locally within the disks. There is a general consensus that star formation (SF) is the ultimate driver of the relation, in the sense that the bulk of dust emission in the FIR is powered by young stars ending their lives as supernovae which are the main sites of Cosmic Ray (CR) acceleration. Although this simplistic view is correct, it neglects many of the additional parameters that affect the correlation. Thus, a detailed understanding is still missing which is crucial in order to correctly use the RC emission as a tracer of recent SF with the important advantage not to be affected by extinction. Furthermore, a detailed understanding of the correlation will lead to a deeper understanding of dust heating, the interstellar medium (ISM) and propagation of CRs. The capabilities of the SKA are needed to make progress in our understanding of the correlation. In particular, they will allow us to (i) extend the study of the correlation to low-luminosity dwarf galaxies which are expected not to follow the correlation so well, (ii) extend the correlation to high-z objects and test whether the correlation is still fulfilled, and (iii) study the properties of CR propagation in galactic halos via changes in the spectral index in order to be able to compare the relative relevance of propagation, escape and energy losses.UL acknowledges support by the research projects AYA2011-24728 from the Spanish Ministerio de Ciencia y Educación and the Junta de Andalucía (Spain) grants FQM108. MAPT and AA acknowledge support from the Spanish Ministerio de Economía y Competitividad (MINECO) through grant AYA2012-38491-C02-02. AAH acknowledges support from the Spanish Plan Nacional de Astronomia y Astrofisica under grant AYA2012-31447, and LC from grant LC from grant AYA2012-32295.Peer Reviewe

Nearby normal and luminous infrared galaxies with the SKA

The SKA will routinely provide µJy sensitivity and sub-arcsecond angular resolutions at radio wavelengths. Planned SKA surveys will image vast numbers of nearby galaxies, which are expected to provide a cornerstone in our understanding of star-formation and accretion activity in the local Universe. Here, we outline some of the key continuum and molecular line studies of local galaxies, where the SKA will have a significant scientific impact and where the Spanish astrophysical community is particularly active.MAPT, AA, and RHI acknowledge support from the Spanish Ministerio de Economía y Competitividad (MINECO) through grant AYA2012-38491-C02-02. S.G.B. acknowledges support from the Spanish MINECO through grants AYA2010-15169, AYA2012-32295, and program CONSOLIDER INGENIO 2010, under grant Molecular Astrophysics: The Herschel and ALMA EraASTROMOL (ref CSD2009-00038). S.G.B also acknowledges support from the Junta de Andalucia through TIC-114 and the Excellence Project P08-TIC-03531. A.A.H. and LC acknowledge support from the Spanish MINECO through grants AYA-2012-31447 and AYA-2012-32295. JHK acknowledges financial support to the DAGAL network from the People Programme (Marie Curie Actions) of the European Unions Seventh Framework Programme FP7/2007-2013/ under REA grant agreement number PITNGA-2011-289313, and from the Spanish MINECO under grant number AYA2013-41243-P.Peer Reviewe

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