An imperfectly passive nature: Bright sub-millimeter emission from
  dust-obscured star formation in the z=3.717 "passive" system, ZF20115
  The identification of high-redshift massive galaxies with old stellar
populations may pose challenges to some models of galaxy formation. However, to
securely classify a galaxy as quiescent, it is necessary to exclude significant
ongoing star formation, something that can be challenging to achieve at high
redshift. In this letter, we analyse deep ALMA/870um and SCUBA-2/450um imaging
of the claimed "post-starburst" galaxy ZF-20115 at z=3.717 that exhibits a
strong Balmer break and absorption lines. The far-infrared imaging reveals a
luminous starburst located 0.4+/-0.1 arcsec (~3kpc in projection) from the
position of the rest-frame ultra-violet/optical emission, with an obscured
star-formation rate of 100 Mo/yr. This star-forming component is undetected in
the rest-frame ultraviolet but contributes significantly to the lower angular
resolution photometry at restframe wavelengths >3500A, significantly
complicating the determination of a reliable stellar mass. Importantly, in the
presence of dust obscuration, strong Balmer features are not a unique signature
of a post-starburst galaxy and are indeed frequently observed in
infrared-luminous galaxies. We conclude that the ZF20015 system does not pose a
challenge to current models of galaxy formation and that deep sub-/millimeter
observations are a prerequisite for any claims of quiescence. The
multi-wavelength observations of ZF20115 unveil a complex system with an
intricate and spatially-varying star-formation history. ZF20115 demonstrates
that understanding high-redshift obscured starbursts will only be possible with
multi-wavelength studies that include high-resolution observations, available
with the JWST, at mid-infrared wavelengths.
