Resolving the ISM at the peak of cosmic star formation with ALMA - The distribution of CO and dust continuum in z~2.5 sub-millimetre galaxies

Abstract

We use ALMA observations of four sub-millimetre galaxies (SMGs) at $z\sim 2-3$ to investigate the spatially resolved properties of the inter-stellar medium (ISM) at scales of 1--5 kpc (0.1--0.6${}^{\prime \prime }$). The velocity fields of our sources, traced by the ${}^{12}$CO($J$=3-2) emission, are consistent with disk rotation to first order, implying average dynamical masses of $\sim$3$\times 10^{11}$M${}_{\odot }$ within two half-light radii. Through a Bayesian approach we investigate the uncertainties inherent to dynamically constraining total gas masses. We explore the covariance between the stellar mass-to-light ratio and CO-to-H${}_2$ conversion factor, ${\alpha }_{\mathrm{CO}}$, finding values of ${\alpha }_{\mathrm{CO}}=1.1_{-0.7}^{+0.8}$ for dark matter fractions of 15 \%. We show that the resolved spatial distribution of the gas and dust continuum can be uncorrelated to the stellar emission, challenging energy balance assumptions in global SED fitting. Through a stacking analysis of the resolved radial profiles of the CO(3-2), stellar and dust continuum emission in SMG samples, we find that the cool molecular gas emission in these sources (radii $\sim$5--14 kpc) is clearly more extended than the rest-frame $\sim$250 $\mu$m dust continuum by a factor $>2$. We propose that assuming a constant dust-to-gas ratio, this apparent difference in sizes can be explained by temperature and optical-depth gradients alone. Our results suggest that caution must be exercised when extrapolating morphological properties of dust continuum observations to conclusions about the molecular gas phase of the ISM.

Submitted to ApJ, 16 pages, 6 figures

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