The Faintness of the 158 um [CII] Transition in the z=6.42 Quasar SDSS J1148+5251

We report the non-detection of the [CII] 157.74 um transition in the z=6.42 quasar SDSS J1148+5251 after 37.5 hours 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 FIR line in galaxies. Our observed RMS of 1.3 mK in the Ta* scale translates to L([CII])<2.6 x 10^9 Lsun. Using a recent estimate of the far-infrared continuum of this quasar, we derive for SDSS J1148+5251 L([CII])/L(FIR)<5 x 10^-4, a ratio similar to that observed in local ultra-luminous infrared galaxies but considerably smaller than what is typical in nearby normal and starburst galaxies. This indicates that the small L([CII])/L(FIR) ratio observed locally in luminous far-infrared objects also persists at the highest redshifts.