SIMBAD references

We investigate the photometric properties of six super stellar clusters (SSCs) seen in both the optical and near-infrared Hubble Space Telescope images of the local ultraluminous starburst galaxy Arp 220. Three of the SSCs are located in the central 0.5-kpc region. The remaining three are in the circumnuclear region between 0.5 and 2.5kpc from the centre. Comparing the observed spectral energy distributions (SEDs) of the SSCs with the Starburst99 models of Leitherer et al., we confirm that all three nuclear SSCs are heavily obscured {formmu1} Considering the results from this comparison in conjunction with measurements of the near-infrared CO absorption index and of millimetre CO linewidths and luminosities, we estimate the ages of the nuclear SSCs to be 10⁷-10⁸yr. The bolometric luminosity of the three nuclear SSCs is at most one-fifth of the total bolometric luminosity of Arp 220. On the other hand, the circumnuclear SSCs have little internal extinction {formmu2} These contribute negligibly to the total bolometric luminosity.