Ionized Nitrogen at High Redshift

Abstract

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^(–1) and (7.4 ± 0.5) Jy km s^(–1), respectively, yielding line luminosities of L([N II]_(205 μm)) = (1.8 ± 0.3) × 10^9 μ^(–1) L_⊙ for APM 08279+5255 and L([N II]_(205 μm)) = (2.8 ± 0.2) × 10(^9) 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)α 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 9.0 × 10^(–6) and 5.8 × 10^(–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.

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