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We report new single dish CO J=6--5 line observations for the archetypalUltra Luminous Infrared Galaxy (ULIRG) Arp 220 with the James Clerk MaxwellTelescope atop Mauna Kea in Hawaii. The J=6--5 line is found to be faint, withbrightness temperature ratios (6--5)/(1--0), (6--5)/(3--2) of R_{65/10} =0.080+/-0.017 and R_{65/32} = 0.082+/-0.019, suggesting very low excitationconditions that cannot be reconciled with the warm and very dense molecular gaspresent in one of the most extreme starbursts in the local Universe. We findthat an optically thick dust continuum, with tau(f>350 GHz)>=1 for the bulk ofthe warm dust and gas in Arp 220, submerges this line to an almost black bodycurve, reducing its flux, and {\it affecting its CO Spectral Line EnergyDistribution (SLED) at high frequencies}. This also resolves the CII linedeficiency in this object, first observed by {\it ISO}: the near absence ofthat line is a dust optical depth effect, not a dense Photodissociation Region(PDR) phenomenon. Finally we briefly comment on the possibility of such extremeISM states in other ULIRGs in the distant Universe, and its consequences forthe diagnostic utility of high frequency molecular and atomic ISM lines in suchsystems. In the case of Arp 220 we anticipate that the now spaceborne HerschelSpace Observatory will find faint high-J CO lines at f>=690 GHz that wouldappear as sub-thermally excited with respect to the low-J ones as a result ofthe effects of dust absorption.

CO J=6--5 in Arp 220: strong effects of dust on high-J CO lines