2012ApJ...752....2D

We present secure [N II]_205 µm_ 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 µm_ is a good tracer of the ionized gas phase in the interstellar medium. The measured fluxes are S([N II]_205 µm_) = (4.8±0.8) Jy km/s and (7.4±0.5) Jy km/s, respectively, yielding line luminosities of L([N II]_205 µm_) = (1.8±0.3)x10⁹ µ^–1 L_☉ for APM 08279+5255 and L([N II]_205 µm_) = (2.8 ±0.2)x10⁹ µ^–1 L_☉ for MM 18423+5938. Our high-resolution map of the [N II]_205 µm_ 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 (Σ_FIR∝Σ^ 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 µm_/FIR luminosity ratio in APM 08279+5255 and MM 18423+5938 of 9.0x10^–6 and 5.8x10^–6, respectively. This is in agreement with the decrease of the [N II]_205 µm_/FIR ratio at high FIR luminosities observed in local galaxies.