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We present deep ALMA CO(5-4) observations of a main sequence, clumpy galaxyat z=1.5 in the HUDF. Thanks to the ~0.5" resolution of the ALMA data, we canlink stellar population properties to the CO(5-4) emission on scales of a fewkpc. We detect strong CO(5-4) emission from the nuclear region of the galaxy,consistent with the observed $L_{\rm IR}$-$L^{\prime}_{\rm CO(5-4)}$correlation and indicating on-going nuclear star formation. The CO(5-4) gascomponent appears more concentrated than other star formation tracers or thedust distribution in this galaxy. We discuss possible implications of thisdifference in terms of star formation efficiency and mass build-up at thegalaxy centre. Conversely, we do not detect any CO(5-4) emission from theUV-bright clumps. This might imply that clumps have a high star formationefficiency (although they do not display unusually high specific star formationrates) and are not entirely gas dominated, with gas fractions no larger thanthat of their host galaxy (~50%). Stellar feedback and disk instability torquesfunnelling gas towards the galaxy centre could contribute to the relatively lowgas content. Alternatively, clumps could fall in a more standard star formationefficiency regime if their actual star-formation rates are lower than generallyassumed. We find that clump star-formation rates derived with severaldifferent, plausible methods can vary by up to an order of magnitude. Thelowest estimates would be compatible with a CO(5-4) non-detection even formain-sequence like values of star formation efficiency and gas content.

ALMA constraints on star-forming gas in a prototypical z=1.5 clumpy galaxy: the dearth of CO(5-4) emission from UV-bright clumps