A low [CII]/[NII] ratio in the center of a massive galaxy at z=3.7:
  witnessing the transition to quiescence at high-redshift?
  Understanding the process of quenching is one of the major open questions in
galaxy evolution, and crucial insights may be obtained by studying quenched
galaxies at high redshifts, at epochs when the Universe and the galaxies were
younger and simpler to model. However, establishing the degree of quiescence in
high redshift galaxies is a challenging task. One notable example is Hyde, a
recently 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 obscured
yet only moderately luminous in the sub-millimeter. Panchromatic modeling
suggested 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-formation
activity, including moderate SFR in the lower scatter of the galaxy
main-sequence (MS). Here, we describe ALMA observations of the [CII] 157um and
[NII] 205um far-infrared emission lines. The [CII] emission within the
half-light radius is dominated by ionized gas, while the outskirts are
dominated by PDRs or neutral gas. This suggests that the ionization in the
center is not primarily powered by on-going star formation, and could come
instead from remnant stellar populations formed in an older burst, or from a
moderate AGN. Accounting for this information in the multi-wavelength modeling
provides a tighter constraint on the star formation rate of
SFR=$50^{+24}_{-18}$ Msun/yr. This rules out fully quenched solutions, and
favors 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 suggest
that quenching happens from inside-out, and starts before the galaxy expels or
consumes all its gas reservoirs. Similar observations of a larger sample would
determine whether this is an isolated case or the norm for quenching at
high-redshift. [abriged]
