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Understanding the process of quenching is one of the major open questions ingalaxy evolution, and crucial insights may be obtained by studying quenchedgalaxies at high redshifts, at epochs when the Universe and the galaxies wereyounger and simpler to model. However, establishing the degree of quiescence inhigh redshift galaxies is a challenging task. One notable example is Hyde, arecently discovered galaxy at z=3.709. As compact (r~0.5 kpc) and massive(M*~1e11 Msun) as its quenched neighbor Jekyll, it is also extremely obscuredyet only moderately luminous in the sub-millimeter. Panchromatic modelingsuggested it could be the first galaxy found in transition to quenching at z>3,however the data were also consistent with a broad range of star-formationactivity, including moderate SFR in the lower scatter of the galaxymain-sequence (MS). Here, we describe ALMA observations of the [CII] 157um and[NII] 205um far-infrared emission lines. The [CII] emission within thehalf-light radius is dominated by ionized gas, while the outskirts aredominated by PDRs or neutral gas. This suggests that the ionization in thecenter is not primarily powered by on-going star formation, and could comeinstead from remnant stellar populations formed in an older burst, or from amoderate AGN. Accounting for this information in the multi-wavelength modelingprovides a tighter constraint on the star formation rate ofSFR=$50^{+24}_{-18}$ Msun/yr. This rules out fully quenched solutions, andfavors SFRs more than factor of two lower than expected for a galaxy on the MS,confirming the nature of Hyde as a transition galaxy. Theses results suggestthat quenching happens from inside-out, and starts before the galaxy expels orconsumes all its gas reservoirs. Similar observations of a larger sample woulddetermine whether this is an isolated case or the norm for quenching athigh-redshift. [abriged]

A low [CII]/[NII] ratio in the center of a massive galaxy at z=3.7: witnessing the transition to quiescence at high-redshift?