Clarifying the nature of the brightest submillimetre sources: interferometric imaging of LH850.02

TL;DR

High-resolution interferometric imaging of the brightest submillimetre galaxy LH850.02 reveals it as a compact source with complex starburst and AGN activity, emphasizing the importance of precise astrometry for understanding high-redshift ULIRGs.

Contribution

This study provides the first high-resolution submm imaging of LH850.02, clarifying its true emission source and revealing its multi-component nature at high redshift.

Findings

LH850.02 is a single compact submm source with <1 arcsec size.

The submm emission originates from LH850.02N, not LH850.02S.

LH850.02N shows starburst-dominated SED, while LH850.02S has AGN features.

Abstract

We present high-resolution interferometric imaging of LH850.02, the brightest 850- and 1200-micron submillimetre (submm) galaxy in the Lockman Hole. Our observations were made at 890 micron with the Submillimetre Array (SMA). Our high-resolution submm imaging detects LH850.02 at >6-sigma as a single compact (size < 1 arcsec or < 8 kpc) point source and yields its absolute position to ~0.2-arcsec accuracy. LH850.02 has two alternative radio counterparts within the SCUBA beam (LH850.02N & S), both of which are statistically very unlikely to be so close to the SCUBA source position by chance. However, the precise astrometry from the SMA shows that the submm emission arises entirely from LH850.02N, and is not associated with LH850.02S (by far the brighter of the two alternative identifications at 24-micron). Fits to the optical-infrared multi-colour photometry of LH850.02N & S indicate that both lie at z~3.3, and are therefore likely to be physically associated. At these redshifts, the 24 micron--to--submm flux density ratios suggest that LH850.02N has an Arp220-type starburst-dominated far-IR SED, while LH850.02S is more similar to Mrk231, with less dust-enshrouded star-formation activity, but a significant contribution at 24-micron (rest-frame ~5-6 micron) from an active nucleus. This complex mix of star-formation and AGN activity in multi-component sources may be common in the high redshift ultraluminous galaxy population, and highlights the need for precise astrometry from high resolution interferometric imaging for a more complete understanding.