A Sensitive Search for [N II]205 μm Emission in a z = 6.4 Quasar Host Galaxy

We present a sensitive search for the 3P1 → 3P0 ground-state fine structure line at 205 μm of ionized nitrogen ([N II]205μm) 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 (3σ) depth of 0.47 Jy km s^−1, corresponding to a [N II]205μm luminosity limit of L[N II] < 4.0×10^8 L☉ and a L[N II]/LFIR ratio of < 2×10^−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 μm line strength, we derive an upper limit for the [N II]205μm/[C II] line luminosity ratio of ~1/10 in J1148+5251. Our upper limit for the [C II]/[N II]205μm ratio is similar to the value found for our Galaxy and M82 (the only extragalactic system where the [N II]205μm line has been detected to date). Given the nondetection of the [N II]205μm line we can only speculate whether or not high-z detections are within reach of currently operating observatories. However, [N II]205μm 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.