On the Kennicutt-Schmidt scaling law of submillimetre galaxies

¹ Department of PhysicsFaculty 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

Received: 11 May 2017
Accepted: 30 May 2017

Abstract

Context. The star formation rate (SFR) per unit area correlates well with the gas surface density for different types of galaxies. However, this Kennicutt-Schmidt (K-S) law has not yet been examined for a large, homogeneously selected sample of submillimetre galaxies (SMGs), which could provide useful SF implementation information for models of massive galaxy formation and evolution.

Aims. We aim at determining the K-S law parameters for the first time for a well-selected, statistical sample of SMGs.

Methods. We used the Atacama Large Millimetre/submillimetre Array (ALMA) to conduct a high resolution (0.̋2), 870 μm continuum imaging survey of 40 SMGs, which were initially selected at 1.1 mm in the COSMOS field. We analysed a sample of 32 out of the 40 target SMGs, for which our new ALMA 870 μm data provide information about the spatial extent of dust emission, and all of which have dust-obscured SFR and dust-based gas mass estimates available from our previous study.

Results. We divided our sample into equally large subsamples of main-sequence (MS) objects and starbursts (factor of > 3 above the MS), and found their K-S relations to be of the form Σ_SFR ∝Σ_gas^0.81±0.01 and Σ_SFR ∝Σ_gas^0.84±0.39, respectively.

Conclusions. The slightly sub-linear K-S slopes we derived suggest that the SF efficiency (SFE) is nearly constant across the Σ_gas range probed. Under the assumption of a Galactic CO-to-H₂ conversion factor (α_CO) for the whole sample, the MS SMGs obey a constant global SFE of about 21% per 100 Myr, while that of starburst SMGs is about 27% per 100 Myr. The corresponding gas depletion times are ~ 480 Myr and 370 Myr. On average, our SMGs have Σ_gas ≳ 10^3.9 M_⊙ pc^-2, which suggests that they are Eddington-limited. This is consistent with the theoretical expectation of a linear K-S relation for such systems. However, size measurements of the CO-emitting regions of SMGs, and the α_CO values of SMGs are needed to further constrain their Σ_gas values.