High-resolution CO and radio imaging of z~2 ULIRGs: extended CO structures, and implications for the universal star formation law
Abstract
We present high spatial resolution (0.4", ~3.5 kpc) PdBI interferometric data on three ultra-luminous infrared galaxies (ULIRGs) at z~2: two sub-millimetre galaxies and one sub-millimetre faint star forming radio galaxy. The three galaxies have been detected in CO rotational transitions, either 12CO(J=4-3) or 12CO(J=3-2). All galaxies are robustly detected, allowing their sizes and gas masses to be accurately constrained. The galaxies appear highly extended, having a mean radius of 3.7 kpc. High-resolution (0.3") combined MERLIN-VLA observations of their radio continua allow an analysis of the star formation behaviour of these galaxies, on comparable spatial scales to that of the CO observations. This 'matched beam' approach sheds light on the spatial distribution of both molecular gas and star formation, and we can therefore calculate accurate star formation rates and gas surface densities: this allows us to place the three systems in the context of a Kennicutt-Schmidt (KS)-style star formation law. We find a difference in size between the CO and radio emission regions, and as such we suggest that using the spatial extent of the CO emission region to estimate the surface density of star formation may lead to error. This size difference also causes the star formation efficiencies within systems to vary by up to a factor of 5. We also find, with our new accurate sizes, that SMGs lie significantly above the KS relation, indicating that stars are formed more efficiently in these extreme systems than in other high-z star forming galaxies.Keywords
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