An ALMA survey of submillimetre galaxies in the COSMOS field: Physical properties derived from energy balance spectral energy distribution modelling

¹ Department of Physics, Faculty of Science, 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 Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ AIM Unité Mixte de Recherche CEA CNRS, Université Paris VII UMR 158, 75013 Paris, France
⁶ Infrared Processing and Analysis Center, California Institute of Technology, MC 100-22, 770 South Wilson Ave., Pasadena, CA 91125, USA
⁷ Spitzer Science Center, California Institute of Technology, Pasadena, CA 91125, USA
⁸ Department of Astronomy, The University of Texas at Austin, 2515 Speedway Blvd Stop C1400, Austin, TX 78712, USA
⁹ Center for Computational Astrophysics, Flatiron Institute, 162 Fifth Avenue, New York, NY 10010, USA
¹⁰ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹¹ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
¹² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹³ CAS Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Nandan Road 80, 200030 Shanghai, PR China
¹⁴ Chinese Academy of Sciences South America Center for Astronomy, 7591245 Santiago, Chile
¹⁵ Sorbonne Universités, UPMC Université Paris 6 et CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
¹⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Av. Gran Bretaña 1111, Valparaíso, Chile
¹⁷ Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna, 4860 Santiago, Chile
¹⁸ Centro de Astro-Ingeniería, Pontificia Universidad Católica de Chile, Avda. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
¹⁹ Astronomy Department, Cornell University, 220 Space Sciences Building, Ithaca, NY 14853, USA
²⁰ Max-Planck-Institut für extraterrestrische Physik, Garching bei München, 85741 Garching bei München, Germany
²¹ North American ALMA Science Center, NRAO, 520 Edgemont Rd, Charlottesville, VA, USA 22903
²² NASA Headquarters, 300 E. St. SW, Washington, DC 20546, USA

Received: 10 March 2017
Accepted: 27 June 2017

Abstract

Context. Submillimetre galaxies (SMGs) represent an important source population in the origin and cosmic evolution of the most massive galaxies. Hence, it is imperative to place firm constraints on the fundamental physical properties of large samples of SMGs.

Aims. We determine the physical properties of a sample of SMGs in the COSMOS field that were pre-selected at the observed-frame wavelength of λ_obs = 1.1 mm, and followed up at λ_obs = 1.3 mm with the Atacama Large Millimetre/submillimetre Array (ALMA).

Methods. We used the MAGPHYS model package to fit the panchromatic (ultraviolet to radio) spectral energy distributions (SEDs) of 124 of the target SMGs, which lie at a median redshift of z = 2.30 (19.4% are spectroscopically confirmed). The SED analysis was complemented by estimating the gas masses of the SMGs by using the λ_obs = 1.3 mm dust emission as a tracer of the molecular gas component.

Results. The sample median and 16th–84th percentile ranges of the stellar masses, obscured star formation rates, dust temperatures, and dust and gas masses were derived to be log(M_⋆/M_⊙) = 11.09^+0.41_-0.53, SFR = 402⁺⁶⁶¹_-233 M_⊙ yr^-1, T_dust = 39.7^+9.7_-7.4 K, log(M_dust/M_⊙) = 9.01^+0.20_-0.31, and log(M_gas/M_⊙ = 11.34^+0.20_-0.23, respectively. The M_dust/M_⋆ ratio was found to decrease as a function of redshift, while the M_gas/M_dust ratio shows the opposite, positive correlation with redshift. The derived median gas-to-dust ratio of 120⁺⁷³_-30 agrees well with the canonical expectation. The gas fraction (M_gas/ (M_gas + M_⋆)) was found to range from 0.10 to 0.98 with a median of 0.62^+0.27_-0.23. We found that 57.3% of our SMGs populate the main sequence (MS) of star-forming galaxies, while 41.9% of the sources lie above the MS by a factor of greater than three (one source lies below the MS). These super-MS objects, or starbursts, are preferentially found at z ≳ 3, which likely reflects the sensitivity limit of our source selection. We estimated that the median gas consumption timescale for our SMGs is ~535 Myr, and the super-MS sources appear to consume their gas reservoir faster than their MS counterparts. We found no obvious stellar mass–size correlations for our SMGs, where the sizes were measured in the observed-frame 3 GHz radio emission and rest-frame UV. However, the largest 3 GHz radio sizes are found among the MS sources. Those SMGs that appear irregular in the rest-frame UV are predominantly starbursts, while the MS SMGs are mostly disk-like.

Conclusions. The physical parameter distributions of our SMGs and those of the equally bright, 870 μm selected SMGs in the ECDFS field (the so-called ALESS SMGs) are unlikely to be drawn from common parent distributions. This might reflect the difference in the pre-selection wavelength. Albeit being partly a selection bias, the abrupt jump in specific SFR and the offset from the MS of our SMGs at z ≳ 3 might also reflect a more efficient accretion from the cosmic gas streams, higher incidence of gas-rich major mergers, or higher star formation efficiency at z ≳ 3. We found a rather flat average trend between the SFR and dust mass, but a positive SFR−M_gas correlation. However, to address the questions of which star formation law(s) our SMGs follow, and how they compare with the Kennicutt-Schmidt law, the dust-emitting sizes of our sources need to be measured. Nonetheless, the larger radio-emitting sizes of the MS SMGs compared to starbursts is a likely indication of their more widespread, less intense star formation activity. The irregular rest-frame UV morphologies of the starburst SMGs are likely to echo their merger nature. The current stellar mass content of the studied SMGs is very high, so they must quench to form the so-called red-and-dead massive ellipticals. Our results suggest that the transition from high-z SMGs to local ellipticals via compact, quiescent galaxies (cQGs) at z ~ 2 might not be universal, and the latter population might also descend from the so-called blue nuggets. However, z ≳ 4 SMGs could be the progenitors of higher redshift, z ≳ 3 cQGs, while our results are also consistent with the possibility that ultra-massive early-type galaxies found at 1.2 ≲ z ≲ 2 experienced an SMG phase at z ≤ 3.