Ionized nitrogen at high redshift
  We present secure [NII[ detections in two mm-bright, strongly lensed objects
at high redshift, APM08279+5255 (z=3.911) and MM18423+5938 (z=3.930), using the
IRAM Plateau de Bure Interferometer. Due to its ionization energy [NII] is a
good tracer of the ionized gas phase in the interstellar medium. The measured
fluxes are S([NII])=(4.8+/-0.8) Jy km/s and (7.4+/-0.5) Jy km/s respectively,
yielding line luminosities of L([NII]) =(1.8+/-0.3) x 10^9 \mu^{-1} Lsun for
APM08279+5255 and L([NII]) =(2.8+/-0.2) x 10^9 \mu^{-1} Lsun for MM18423+5938.
Our high-resolution map of the [NII] and 1 mm continuum emission in
MM18423+5938 clearly resolves an Einstein ring in this source, and reveals a
velocity gradient in the dynamics of the ionized gas. A comparison of these
maps with high-resolution EVLA CO observations enables us to perform the first
spatially-resolved study of the dust continuum-to-molecular gas surface
brightness (Sigma_{FIR} Sigma_{CO}^N, which can be interpreted as the star
formation law) in a high-redshift object. We find a steep relation
(N=1.4+/-0.2), consistent with a starbursting environment. We measure a
[NII]/FIR luminosity ratio in APM0828+5255 and MM18423+5938 of 9.0 x 10^{-6}
and 5.8 x 10^{-6}, respectively. This is in agreement with the decrease of the
[NII]/FIR ratio at high FIR luminosities observed in local galaxies.
