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We present a sensitive search for the ^3P_1->^3P_0 ground state finestructure line at 205 microns of ionized nitrogen ([NII]) in one of the highestredshift quasars (J1148+5251 at z=6.42) using the IRAM 30m telescope. The lineis not detected at a (3 sigma) depth of 0.47 Jy km s^-1, corresponding to a[NII] luminosity limit of L_[NII] < 4.0x10^8 L_sun and a L_[NII]/L$_FIR ratioof <2x10^-5. In parallel, we have observed the CO(J=6-5) line in J1148+5251,which is detected at a flux level consistent with earlier interferometricobservations. Using our earlier measurements of the [CII] 158 micron linestrength, we derive an upper limit for the [NII]/[CII] line luminosity ratio of~1/10 in J1148+5251. Our upper limit for the [CII]/[NII] ratio is similar tothe value found for our Galaxy and M82 (the only extragalactic system where the[NII] line has been detected to date). Given the non-detection of the [NII]line we can only speculate whether or not high-z detections are within reach ofcurrently operating observatories. However, [NII] and other fine strucure lineswill play a critical role in characterizing the interstellar medium at thehighest redshifts (z>7) using the Atacama Large Millimeter/submillimeter Array(ALMA), for which the highly excited rotational transitions of CO will beshifted outside the accessible (sub-)millimeter bands.

A Sensitive Search For [NII] Emission in a z=6.4 Quasar Host Galaxy