Clarifying the nature of the brightest submillimetre sources: interferometric imaging of LH 850.02
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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.
