Ionized nitrogen at high redshift

Abstract

We present secure [N II]205 μm detections in two millimeter-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 [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)Jykms-1 and (7.4 ± 0.5)Jykms-1, respectively, yielding line luminosities of L([N II]205 μm) = (1.8 ± 0.3) × 109 μ-1L for APM08279+5255 and L([N II]205 μm) = (2.8 ± 0.2) × 109 μ-1L for MM18423+5938. Our high-resolution map of the [N II]205 μm and 1mm 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 (Σ FIRΣNCO, 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 APM08279+5255 and MM18423+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. © 2012. The American Astronomical Society. All rights reserved..