An ALMA survey of submillimetre galaxies in the COSMOS field: The extent of the radio-emitting region revealed by 3 GHz imaging with the Very Large Array^⋆

¹ Department of Physics, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia
e-mail: oskari@phy.hr
² Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
³ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ AIM Unité Mixte de Recherche CEA CNRS, Université Paris VII UMR n158, 75013 Paris, France
⁷ Infrared Processing and Analysis Center, California Institute of Technology, MC 100-22, 770 South Wilson Ave., Pasadena, CA 91125, USA
⁸ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Department of Astronomy, The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
¹⁰ Yale Center for Astronomy and Astrophysics, 260 Whitney Avenue, New Haven, CT 06520, USA
¹¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹² Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
¹³ Astronomisches Institut, Ruhr-Universität Bochum, Universitätstrasse 150, 44801 Bochum, Germany
¹⁴ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
¹⁵ CAS Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Nandan Road 80, 200030 Shanghai, PR China
¹⁶ Chinese Academy of Sciences South America Center for Astronomy, 7591245 Santiago, Chile
¹⁷ Sorbonne Universités, UPMC Université Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis Boulevard Arago, 75014 Paris, France
¹⁸ Dipartimento di Fisica e Astronomia, Università di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
¹⁹ Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
²⁰ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, Santiago, Chile
²¹ Centro de Astro-Ingeniería, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
²² Astronomy Department, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
²³ Max-Planck-Institut für extraterrestrische Physik, Garching bei München, 85741 Garching bei München, Germany
²⁴ North American ALMA Science Center, NRAO, 520 Edgemont Rd, Charlottesville, VA 22903, USA
²⁵ NASA Headquarters, 300 E. St.SW, Washington, DC 20546, USA
²⁶ Osservatorio di Radioastronomia – INAF, Bologna, via P. Gobetti 101, 40129 Bologna, Italy
²⁷ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy

Received: 16 January 2017
Accepted: 16 February 2017

Abstract

Context. The observed spatial scale of the radio continuum emission from star-forming galaxies can be used to investigate the spatial extent of active star formation, constrain the importance of cosmic-ray transport, and examine the effects of galaxy interactions.

Aims. We determine the radio size distribution of a large sample of 152 submillimetre galaxies (SMGs) in the COSMOS field that were pre-selected at 1.1 mm, and later detected with the Atacama Large Millimetre/submillimetre Array (ALMA) in the observed-frame 1.3 mm dust continuum emission at a signal-to-noise ratio (S/N) of ≥5.

Methods. We used the deep, subarcsecond-resolution (1σ = 2.3μJy beam^-1; .̋ 75) centimetre radio continuum observations taken by the Karl G. Jansky Very Large Array (VLA)-COSMOS 3 GHz Large Project.

Results. One hundred and fifteen of the 152 target SMGs (76% ± 7%) were found to have a 3 GHz counterpart (≥ 4.2σ), which renders the radio detection rate notably high. The median value of the deconvolved major axis full width at half maximum (FWHM) size at 3 GHz is derived to be 0.̋59 ± 0.̋05 , or 4.6 ± 0.4 kpc in physical units, where the median redshift of the sources is z = 2.23 ± 0.13 (23% are spectroscopic and 77% are photometric values). The radio sizes are roughly log-normally distributed, and they show no evolutionary trend with redshift, or difference between different galaxy morphologies. We also derived the spectral indices between 1.4 and 3 GHz, and 3 GHz brightness temperatures for the sources, and the median values were found to be α_{1.4 GHz}^{3 GHz} = -0.67 (S_ν ∝ ν^α) and T_B = 12.6 ± 2 K. Three of the target SMGs, which are also detected with the Very Long Baseline Array (VLBA) at 1.4 GHz (AzTEC/C24b, 61, and 77a), show clearly higher brightness temperatures than the typical values, reaching T_B(3 GHz) > 10^4.03 K for AzTEC/C61.

Conclusions. The derived median radio spectral index agrees with a value expected for optically thin non-thermal synchrotron radiation, and the low median 3 GHz brightness temperature shows that the observed radio emission is predominantly powered by star formation and supernova activity. However, our results provide a strong indication of the presence of an active galactic nucleus in the VLBA and X-ray-detected SMG AzTEC/C61 (high T_B and an inverted radio spectrum). The median radio-emitting size we have derived is ~ 1.5–3 times larger than the typical far-infrared dust-emitting sizes of SMGs, but similar to that of the SMGs’ molecular gas component traced through mid-J line emission of carbon monoxide. The physical conditions of SMGs probably render the diffusion of cosmic-ray electrons inefficient, and hence an unlikely process to lead to the observed extended radio sizes. Instead, our results point towards a scenario where SMGs are driven by galaxy interactions and mergers. Besides triggering vigorous starbursts, galaxy collisions can also pull out the magnetised fluids from the interacting disks, and give rise to a taffy-like synchrotron-emitting bridge. This provides an explanation for the spatially extended radio emission of SMGs, and can also cause a deviation from the well-known infrared-radio correlation owing to an excess radio emission. Nevertheless, further high-resolution observations are required to examine the other potential reasons for the very compact dust-emitting sizes of SMGs, such as the radial dust temperature and metallicity gradients.