IONIZED NITROGEN AT HIGH REDSHIFT

Abstract

We present secure [NII[ detections in two mm-bright, strongly lensed objects\nat high redshift, APM08279+5255 (z=3.911) and MM18423+5938 (z=3.930), using the\nIRAM Plateau de Bure Interferometer. Due to its ionization energy [NII] is a\ngood tracer of the ionized gas phase in the interstellar medium. The measured\nfluxes are S([NII])=(4.8+/-0.8) Jy km/s and (7.4+/-0.5) Jy km/s respectively,\nyielding line luminosities of L([NII]) =(1.8+/-0.3) x 10^9 \\mu^{-1} Lsun for\nAPM08279+5255 and L([NII]) =(2.8+/-0.2) x 10^9 \\mu^{-1} Lsun for MM18423+5938.\nOur high-resolution map of the [NII] and 1 mm continuum emission in\nMM18423+5938 clearly resolves an Einstein ring in this source, and reveals a\nvelocity gradient in the dynamics of the ionized gas. A comparison of these\nmaps with high-resolution EVLA CO observations enables us to perform the first\nspatially-resolved study of the dust continuum-to-molecular gas surface\nbrightness (Sigma_{FIR} Sigma_{CO}^N, which can be interpreted as the star\nformation law) in a high-redshift object. We find a steep relation\n(N=1.4+/-0.2), consistent with a starbursting environment. We measure a\n[NII]/FIR luminosity ratio in APM0828+5255 and MM18423+5938 of 9.0 x 10^{-6}\nand 5.8 x 10^{-6}, respectively. This is in agreement with the decrease of the\n[NII]/FIR ratio at high FIR luminosities observed in local galaxies.\n