High-resolution CO and radio imaging of 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\non three ultra-luminous infrared galaxies (ULIRGs) at z~2: two submillimetre\ngalaxies and one submillimetre faint star forming radio galaxy. The three\ngalaxies have been ro- bustly detected in CO rotational transitions, either\n12CO(J=4-3) or 12CO(J=3-2), allowing their sizes and gas masses to be\naccurately constrained. These are the highest spatial resolution observations\nobserved to date (by a factor of ~2) for intermediate-excitation CO emission in\nz~2 ULIRGs. The galaxies appear extended over several resolution elements,\nhaving a mean radius of 3.7 kpc. High-resolution (0.3") combined MERLIN-VLA\nobservations of their radio continua allow an analysis of the star formation\nbehaviour of these galaxies, on comparable spatial scales to that of the CO\nobservations. This 'matched beam' approach sheds light on the spatial\ndistribution of both molecular gas and star formation, and we can therefore\ncalculate accurate star formation rates and gas surface densities: this allows\nus to place the three systems in the context of a Kennicutt-Schmidt (KS)-style\nstar formation law. We find a difference in size between the CO and radio\nemission regions, and as such we suggest that using the spatial extent of the\nCO emission region to estimate the surface density of star formation may lead\nto error. This size difference also causes the star formation efficiencies\nwithin systems to vary by up to a factor of 5. We also find, with our new\naccurate sizes, that SMGs lie significantly above the KS relation, indicating\nthat stars are formed more efficiently in these extreme systems than in other\nhigh-z star forming galaxies.\n