Arp220: extinction and merger-induced star formation
  We analyse new spatially resolved integral field spectroscopic H- and K-band
data at a resolution of 0.3arcsec (100pc) and re-analyse interferometric
CO(2-1) line observations of the prototypical merging system Arp220. We find
that the majority of the K-band luminosity is due to a 10Myr old starburst,
with a significant contribution from an underlying ca 1Gyr old stellar
population, and a small contribution from stars less than 8Myr old. The
Calzetti (2000) reddening law provides the best fit to photometric datapoints
spanning 0.45um -- 2.12um. Furthermore, estimates of the bolometric luminosity
from IRAS fluxes in conjunction with our stellar population analysis indicate
that we observe less than 10% of the emitted K-band light. The stellar and
CO(2-1) kinematic centre of the western nucleus coincides with the compact hot
dust emission, indicating that the latter marks the centre of the gravitational
potential. In the eastern nucleus, the CO(2-1) data are well matched by a model
in which the gas orbits around the peak of the dust emission. This, and the
similarity of the K-band tracer kinematics, shows that despite the irregular
morphology, the eastern nucleus is also a kinematically coherent structure.
Comparison of the extinction map and EW_CO and EW_Br-gamma maps indicates that
the lower half of the eastern nucleus is significantly more extincted than the
upper half, suggesting that the lower half is buried in the larger-scale gas
disk.
