On the Kennicutt-Schmidt scaling law of submillimetre galaxies

Abstract

The star formation rate per unit area correlates well with the gas surface density for different types of galaxies. However, this Kennicutt-Schmidt law has not yet been examined for a large, homogeneously selected sample of SMGs, which could provide useful SF implementation information for models of massive galaxy formation and evolution. We aim at determining the K-S law parameters for the first time for a well-selected, statistical sample of SMGs. We used ALMA to conduct a 0.2" resolution, 870 $μ$m imaging survey of 40 SMGs in COSMOS, which were initially selected at 1.1 mm. We analysed a sample of 32/40 target SMGs, for which our new ALMA 870 $μ$m 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. We divided our sample into equally large subsamples of main-sequence objects and starbursts, and found their K-S relations to be of the form $Σ_{\rm SFR} \propto Σ_{\rm gas}^{0.81\pm0.01}$ and $Σ_{\rm SFR} \propto Σ_{\rm gas}^{0.84\pm0.39}$, respectively. The slightly sub-linear K-S slopes we derived suggest that the SF efficiency is nearly constant across the $Σ_{\rm gas}$ range probed. Under the assumption of a Galactic CO-to-H$_2$ conversion factor 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%. The corresponding gas depletion times are $\sim480$ Myr and 370 Myr. On average, our SMGs have $Σ_{\rm gas}\gtrsim10^{3.9}$ M$_{\odot}$ 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 $α_{\rm CO}$ values of SMGs are needed to further constrain their $Σ_{\rm gas}$ values.

Accepted for publication in A&A Letters; 4 pages, 1 figure, plus 3 appendices with 3 figures and 2 tables