A Sensitive Search For [N Ii](205 Mu M) Emission In A Z=6.4 Quasar Host Galaxy

We present a sensitive search for the P-3(1) -> P-3(0) ground-state fine structure line at 205 mu m of ionized nitrogen ([N II](205 mu m)) in one of the highest-redshift quasars (J1148+5251 at z = 6.42) using the IRAM 30 m telescope. The line is not detected at a (3 sigma) depth of 0.47 Jy km s(-1), corresponding to a [N II](205 mu m) luminosity limit of L-[N II] < 4.0 x 10(8) L-circle dot and a L-[N II]/L-FIR ratio of < 2 x 10(-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 interferometric observations. Using our earlier measurements of the [C II](158 mu m) line strength, we derive an upper limit for the [N II](205 mu m)/[C II] line luminosity ratio of similar to 1/10 in J1148+5251. Our upper limit for the [C II]/[N II](205 mu m) ratio is similar to the value found for our Galaxy and M 82 (the only extragalactic system where the [N II](205 mu m) line has been detected to date). Given the non-detection of the [N II](205 mu m) line we can only speculate whether or not high-z detections are within reach of currently operating observatories. However, [N II](205 mu m) and other fine-structure lines will play a critical role in characterizing the interstellar medium at the highest redshifts (z > 7) using the Atacama Large Millimeter/submillimeter Array, for which the highly excited rotational transitions of CO will be shifted outside the accessible (sub-)millimeter bands.