Tracing cool molecular gas and star formation on $\sim 100$pc scales within a $z=2.3$ galaxy

Abstract

We present new, high-angular resolution interferometric observations with the Karl G. Jansky Very Large Array of ${}^{12}$CO $J=1-0$ line emission and 4-8 GHz continuum emission in the strongly lensed, $z=2.3$ submillimetre galaxy, SMM J21352-0102. Using these data, we identify and probe the conditions in $\sim 100$pc clumps within this galaxy, which we consider to be potential giant molecular cloud complexes, containing up to half of the total molecular gas in this system. In combination with far-infrared and submillimetre data, we investigate the far-infrared/radio correlation, measuring $q_{IR}=2.39\pm 0.17$ across SMM J21352. We search for variations in the properties of the interstellar medium throughout the galaxy by measuring the spatially-resolved $q_{IR}$ and radio spectral index, ${\alpha }_{\mathrm{radio}}$, finding ranges $q_{IR}=[2.1,2.6]$ and ${\alpha }_{\mathrm{radio}}=[-1.5,-0.7]$. We argue that these ranges in ${\alpha }_{\mathrm{radio}}$ and $q_{IR}$ may reflect variations in the age of the ISM material. Using multi-$J$ ${}^{12}$CO data, we quantitatively test a recent theoretical model relating the star-formation rate surface density to the excitation of ${}^{12}$CO, finding good agreement between the model and the data. Lastly, we study the Schmidt-Kennicutt relation, both integrated across the system and within the individual clumps. We find small offsets between SMM J21352 and its clumps relative to other star-forming galaxy populations on the Schmidt-Kennicutt plot - such offsets have previously been interpreted as evidence for a bi-modal star-formation law, but we argue that they can be equally-well explained as arising due to a combination of observational uncertainties and systematic biases in the choice of model used to interpret the data.

15 pages, 5 colour figures; accepted for publication in MNRAS

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