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
  We present deep ALMA CO(5-4) observations of a main sequence, clumpy galaxy
at z=1.5 in the HUDF. Thanks to the ~0.5" resolution of the ALMA data, we can
link stellar population properties to the CO(5-4) emission on scales of a few
kpc. 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) gas
component appears more concentrated than other star formation tracers or the
dust distribution in this galaxy. We discuss possible implications of this
difference in terms of star formation efficiency and mass build-up at the
galaxy centre. Conversely, we do not detect any CO(5-4) emission from the
UV-bright clumps. This might imply that clumps have a high star formation
efficiency (although they do not display unusually high specific star formation
rates) and are not entirely gas dominated, with gas fractions no larger than
that of their host galaxy (~50%). Stellar feedback and disk instability torques
funnelling gas towards the galaxy centre could contribute to the relatively low
gas content. Alternatively, clumps could fall in a more standard star formation
efficiency regime if their actual star-formation rates are lower than generally
assumed. We find that clump star-formation rates derived with several
different, plausible methods can vary by up to an order of magnitude. The
lowest estimates would be compatible with a CO(5-4) non-detection even for
main-sequence like values of star formation efficiency and gas content.
