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

• Published in: arXiv.org 2018-04
• Main Authors: Rivera, Gabriela Calistro, Hodge, J A, Smail, Ian, Swinbank, A M, Weiß, A, Wardlow, J L, Walter, F, Rybak, M, Chian-Chou, Chen, Brandt, W N, Coppin, K, da Cunha, E, Dannerbauer, H, Greve, T R, Karim, A, Knudsen, K K, Schinnerer, E, Simpson, J M, Venemans, B, P P van der Werf
• Format: Article
• Language: English
• Subjects: Bayesian analysis; Continuum radiation; Cosmic dust; Covariance; Dark matter; Dust; Emissions; Galaxies; Galaxy distribution; Molecular gases; Rotating disks; Spatial distribution; Star & galaxy formation; Star formation; Stellar mass; Vapor phases; Velocity distribution
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1804.06852

We use ALMA observations of four sub-millimetre galaxies (SMGs) at \(z\sim2-3\) to investigate the spatially resolved properties of the inter-stellar medium (ISM) at scales of 1--5 kpc (0.1--0.6\(''\)). 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\(\times10^{11}\)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, \(\alpha_{\rm CO}\), finding values of \(\alpha_{\rm CO}=1.1^{+0.8}_{-0.7}\) 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.