We detect HCN J=5-4 emission from the ultraluminous quasar APM08279+5255 at z=3.911 using the IRAM Plateau de Bure interferometer .
This object is strongly gravitationally lensed , yet still thought to be one of the most intrinsically luminous objects in the Universe .
The new data imply a line luminosity L ^ { \prime } _ { HCN ( J = 5 - 4 ) } = 4.0 \pm 0.5 \times 10 ^ { 10 } K km s ^ { -1 } pc ^ { 2 } .
The \sim 440 km s ^ { -1 } full width half maximum of the HCN J=5-4 line matches that of the previously observed high-J CO lines in this object and suggests that the emission from both species emerges from the same region : a warm , dense circumnuclear disk .
Simple radiative transfer models suggest an enhanced abundance of HCN relative to CO in the nuclear region of APM08279+5255 , perhaps due to increased ionization , or possibly the selective depletion of oxygen .
The ratio of far-infrared luminosity to HCN luminosity is at the high end of the range found for nearby star forming galaxies , but comparable to that observed in the few high redshift objects detected in the HCN J=1-0 line .
This is the first clear detection of high-J HCN emission redshifted into the 3-millimeter atmospheric window .