2015A&A...576A.127S

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) 325MHz and 3GHz Jansky Very Large Array (VLA) to probe the rest-frame 1.4GHz emission at z=4, and to estimate the sizes of the star formation regions of these sources, respectively. We find that only one SMG is clearly resolved at a resolution of 0.6"x0.7" at 3GHz, 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")x(0.35"±0.05" or 4.1x2.3kpc² (majorxminor 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 ∼280Myr, 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_*∼1.4x10¹¹M_☉, T_dust∼43K, and L_IR∼1.3x10¹³L_☉, 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.