Physical properties of z > 4 submillimeter galaxies in the COSMOS field

¹ Department of Physics, University of Zagreb, Bijenička cesta 32, 10000 Zagreb, Croatia
e-mail: vs@phy.hr
² Argelander Institute for Astronomy, Auf dem Hügel 71, 53121 Bonn, Germany
³ Department of Astronomy, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
⁴ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Department of Astronomy, California Institute of Technology, MC 249-17, 1200 East California Blvd, Pasadena, CA 91125, USA
⁶ Spitzer Science Center, 314-6 Caltech, Pasadena, CA 91125, USA
⁷ INAF–Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁸ INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹⁰ CSIRO Australia Telescope National Facility, PO Box 76, Epping, 1710 NSW, Australia
¹¹ Research School of Astronomy and Astrophysics, Australian National University, Weston Creek, 2611 ACT, Australia
¹² National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
¹³ Yale Center for Astronomy and Astrophysics, Physics Department, Yale University, PO Box 208120, New Haven, CT 06520-8120, USA
¹⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
¹⁶ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
¹⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁹ Sorbonne Universités, UPMC Univ Paris 06, UMR 7095, Institut d’Astrophysique de Paris, 75005 Paris, France
²⁰ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre et Marie Curie, 98bis boulevard Arago, 75014 Paris, France
²¹ Department of Physics and Astronomy, University of California, Riverside, CA 92521, USA
²² Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching bei München, Germany
²³ Astronomy Centre, Department of Physics and Astronomy, University of Sussex, Falmer, Brighton BN1 9QH, UK
²⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
²⁵ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Mariesvej 30, 2100 Copenhagen, Denmark

Received: 16 September 2014
Accepted: 3 December 2014

Abstract

We investigate the physical properties of a sample of six submillimeter galaxies (SMGs) in the COSMOS field, spectroscopically confirmed to lie at redshifts z> 4. While the redshifts for four of these SMGs were previously known, we present here two newly discovered z_spec> 4 SMGs. For our analysis we employ the rich (X-ray to radio) COSMOS multi-wavelength datasets. In particular, we use new data from the Giant Meterwave Radio Telescope (GMRT) 325 MHz and 3 GHz Jansky Very Large Array (VLA) to probe the rest-frame 1.4 GHz emission at z = 4, and to estimate the sizes of the star formation regions of these sources, respectively. We find that only oneSMG is clearly resolved at a resolution of 0''̣6 × 0''̣7 at 3 GHz, two may be marginally resolved, while the remaining three SMGs are unresolved at this resolution. Combining this with sizes from high-resolution (sub-)mm observations available in the literature for AzTEC 1 and AzTEC 3 we infer a median radio-emitting size for our z> 4 SMGs of (0''̣63 ± 0''̣12) × (0''̣35 ± 0''̣05) or 4.1 × 2.3 kpc² (major × minor axis; assuming z = 4.5) or lower if we take the two marginally resolved SMGs as unresolved. This is consistent with the sizes of star formation regions in lower-redshift SMGs, and local normal galaxies, yet higher than the sizes of star formation regions of local ultraluminous infrared galaxies (ULIRGs). Our SMG sample consists of a fair mix of compact and more clumpy systems with multiple, perhaps merging, components. With an average formation time of ~280 Myr, as derived through modeling of the UV IR spectral energy distributions, the studied SMGs are young systems. The average stellar mass, dust temperature, and IR luminosity we derive are M_⋆ ~ 1.4 × 10¹¹ M_⊙, T_dust ~ 43 K, and L_IR ~ 1.3 × 10¹³L_⊙, respectively. The average L_IR is up to an order of magnitude higher than for SMGs at lower redshifts. Our SMGs follow the correlation between dust temperature and IR luminosity as derived for Herschel-selected 0.1 <z< 2 galaxies. We study the IR-radio correlation for our sources and find a deviation from that derived for z< 3 ULIRGs (⟨ q_IR ⟩ = 1.95 ± 0.26 for our sample, compared to q ≈ 2.6 for IR luminous galaxies at z< 2). In summary, we find that the physical properties derived for our z> 4 SMGs put them at the high end of the L_IR–T_dust distribution of SMGs, and that our SMGs form a morphologically heterogeneous sample. Thus, additional in-depth analyses of large, statistical samples of high-redshift SMGs are needed to fully understand their role in galaxy formation and evolution.