The gas content of galaxies is a key factor for their growth , starting from star formation and black hole accretion to galaxy mergers .
Thus , characterising its properties through observations of tracers like the CO emission line is of big importance in order to understand the bigger picture of galaxy evolution .
We present Atacama Large Millimeter/submillimeter Array ( ALMA ) observations of dust continuum , CO ( 5-4 ) and CO ( 8-7 ) line emission in the quasar–star-forming companion system SMM J04135+10277 ( z = 2.84 ) .
Earlier low- J CO studies of this system found a huge molecular gas reservoir associated to the companion galaxy , while the quasar appeared gas-poor .
Our CO observations revealed that the host galaxy of the quasar is also gas-rich , with an estimated molecular gas mass of \sim ( 0.7 - 2.3 ) \times 10 ^ { 10 } M _ { \sun } .
The CO line profiles of the companion galaxy are very broad ( \sim 1000 km s ^ { -1 } ) , and show signs of rotation of a compact , massive system .
In contrast to previous far-infrared observations , we resolve the continuum emission and detect both sources , with the companion galaxy dominating the dust continuum and the quasar having a \sim 25 \% contribution to the total dust emission .
By fitting the infrared spectral energy distribution of the sources with MR-MOOSE and empirical templates , the infrared luminosities of the quasar and the companion are in the range of L _ { IR,QSO } \sim ( 2.1 - 9.6 ) \times 10 ^ { 12 } L _ { \sun } and L _ { IR,Comp . } \sim ( 2.4 - 24 ) \times 10 ^ { 12 } L _ { \sun } , while the estimated star formation rates are \sim 210 - 960 M _ { \sun } yr ^ { -1 } and \sim 240 - 2400 M _ { \sun } yr ^ { -1 } , respectively .
Our results demonstrate that non-detection of low- J CO transition lines in similar sources does not necessarily imply the absence of massive molecular gas reservoir but that the excitation conditions favour the excitation of high- J transitions .