IONIZED NITROGEN AT HIGH REDSHIFT

We present secure [N II](205 mu m) detections in two millimeter-bright, strongly lensed objects at high redshift, APM 08279+5255 (z = 3.911) and MM 18423+5938 (z = 3.930), using the IRAM Plateau de Bure Interferometer. Due to its ionization energy [N II](205 mu m) is a good tracer of the ionized gas phase in the interstellar medium. The measured fluxes are S([N II](205 mu m)) = (4.8 +/- 0.8) Jy km s(-1) and (7.4 +/- 0.5) Jy km s(-1), respectively, yielding line luminosities of L([N II](205 mu m)) = (1.8 +/- 0.3) x 10(9) mu(-1) L-circle dot for APM 08279+5255 and L([N II](205 mu m)) = (2.8 +/- 0.2) x 10(9) mu(-1) L-circle dot for MM 18423+5938. Our high-resolution map of the [N II](205 mu m) and 1 mm continuum emission in MM 18423+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) proportional to Sigma(N)(CO), 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 [N II](205 mu m)/FIR luminosity ratio in APM 08279+5255 and MM 18423+5938 of 9.0 x 10(-6) and 5.8 x 10(-6), respectively. This is in agreement with the decrease of the [N II](205 mu m)/FIR ratio at high FIR luminosities observed in local galaxies.