The faintness of the 158 micron [C II] transition in the z=6.42 quasar SDSS J1148+5251

We report the nondetection of the [C II] (P-2(3/2) --> P-2(1/2)) 157.74 mum transition in the z = 6.42 quasar SDSS J1148 + 5251 after 37.5 hr of integration with the James Clerk Maxwell Telescope. This transition is the main cooling line of the star-forming interstellar medium and usually the brightest far-infrared (FIR) line in galaxies. Our observed 1 sigma rms = 1.3 mK in the T-A* scale translates to L-[C II] < 2.6 x 10(9) L-.. Using a recent estimate of the FIR continuum of this quasar, we derive for SDSS J1148 + 5251 L-[C II]/L-FIR < 5 x 10(-4), a ratio similar to that observed in local ultraluminous infrared galaxies but considerably smaller than what is typical in nearby normal and starburst galaxies. This indicates that the small L-[C II]/ L-FIR ratio observed locally in luminous FIR objects also persists at the highest redshifts.