The physical scale of the far-infrared emission in the most luminous submillimetre galaxies II: evidence for merger-driven star formation

TL;DR

This study uses high-resolution interferometric observations to measure the physical size of far-infrared emission in extremely luminous submillimetre galaxies, providing evidence that their star formation is merger-driven and occurs on compact scales of 5-8 kpc.

Contribution

It offers the first direct measurement of the far-infrared emission scale in luminous SMGs, supporting merger-driven starburst models over extended disk scenarios.

Findings

Sizes of 5-8 kpc for the far-infrared emission regions.

Evidence that star formation occurs near the Eddington limit.

Supports merger-driven starburst scenario for luminous SMGs.

Abstract

We present high-resolution 345 GHz interferometric observations of two extreme luminous (L_{IR}>10^{13} L_sun), submillimetre-selected galaxies (SMGs) in the COSMOS field with the Submillimeter Array (SMA). Both targets were previously detected as unresolved point-sources by the SMA in its compact configuration, also at 345 GHz. These new data, which provide a factor of ~3 improvement in resolution, allow us to measure the physical scale of the far-infrared in the submillimetre directly. The visibility functions of both targets show significant evidence for structure on 0.5-1 arcsec scales, which at z=1.5 translates into a physical scale of 5-8 kpc. Our results are consistent with the angular and physical scales of two comparably luminous objects with high-resolution SMA followup, as well as radio continuum and CO sizes. These relatively compact sizes (<5-10 kpc) argue strongly for merger-driven starbursts, rather than extended gas-rich disks, as the preferred channel for forming SMGs. For the most luminous objects, the derived sizes may also have important physical consequences; under a series of simplifying assumptions, we find that these two objects in particular are forming stars close to or at the Eddington limit for a starburst.