Starburst activity in the host galaxy of the

We report the detection of CO emission from the optically luminous, radio-quiet quasar J140955.5+562827 (hereafter J1409+5628), at a redshift $z_{\rm CO} =2.583$. We also present VLA continuum maps and VLBA high spatial resolution observations at 1.4 GHz. Both the CO(3$\rightarrow$2) and CO(7$\rightarrow$6) emission lines are detected using the IRAM Plateau de Bure interferometer. The 3$\rightarrow$2/7$\rightarrow$6 line luminosity ratio is about 1/3, indicating the presence of warm and dense molecular gas with an estimated mass of 6 $\times$ 1010 $M_\odot$. The infrared-to-CO luminosity ratio $L_{\rm FIR}/L^\prime_{\rm CO(1\rightarrow0)} \approx 500$ $L_\odot$ (K km s-1 pc2)-1, comparable to values found for other high-z sources where CO line emission is seen. J1409+5628 is detected using the VLA with a 1.4 GHz rest-frame luminosity density of 4.0 $\times$ $10^{25}~\rm W~Hz^{-1}$. The rest-frame radio to far-infrared ratio, q, has a value of 2.0 which is similar to the values found in star forming galaxies. At the 30 mas resolution of the VLBA, J1409+5628 is not detected with a 4σ upper limit to the surface brightness of 0.29 mJy beam-1. This implies a limit to the intrinsic brightness temperature of 2 $\times$ $10^5~{\rm K}$ at 8 GHz, typical for nuclear starbursts and more than two orders of magnitude fainter than typical radio-loud active galactic nuclei. Both the properties of the CO line emission and the radio emission from J1409+5628 are therefore consistent with those expected for a star forming galaxy. In J1409+5628 young massive stars are the dominant source of dust heating, accounting for most of the infrared luminosity. The massive reservoir of molecular gas can sustain the star formation rate of a few $1000~M_\odot~{\rm yr}^{-1}$ implied by the far-infrared luminosity for about 10 million years