2009ApJ...691L...1W

We present a sensitive search for the ³P₁-> ³P₀ground-state fine structure line at 205 µm of ionized nitrogen ([N II]_205 µm_) 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, corresponding to a [N II]_205 µm_ luminosity limit of L_☉ and a/L_FIRratio of <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 interferometric observations. Using our earlier measurements of the [C II] 158 µm line strength, we derive an upper limit for the [N II]_205 µm_/[C II] line luminosity ratio of ∼1/10 in J1148+5251. Our upper limit for the [C II]/[N II]_205 µm_ ratio is similar to the value found for our Galaxy and M 82 (the only extragalactic system where the [N II]_205 µm_ line has been detected to date). Given the non-detection of the [N II]_205 µm_ line we can only speculate whether or not high-z detections are within reach of currently operating observatories. However, [N II]_205 µm_ 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.