  (Abridged) In the last decade several massive molecular gas reservoirs were
found <100 kpc distance from active galactic nuclei (AGNs), residing in
gas-rich companion galaxies. The study of AGN-gas-rich companion systems opens
the opportunity to determine whether the stellar mass of massive local galaxies
was formed in their host after a merger event or outside of their host galaxy
in a close starbursting companion and later incorporated via mergers. We study
the quasar-companion galaxy system of SMM J04135+10277 (z=2.84) and investigate
the expected frequency of quasar-starburst galaxy pairs at high redshift using
a cosmological galaxy formation model. We use archive data and new APEX ArTeMiS
data to construct and model the spectral energy distribution of SMM J04135. We
also carry out a comprehensive analysis of the cosmological galaxy formation
model GALFORM with the aim of characterising how typical the system of SMM
J04135 is and whether quasar-star-forming galaxy pairs may constitute an
important stage in galaxy evolution. The companion galaxy of SMM J04135 is a
heavily dust-obscured starburst galaxy with a median star formation rate (SFR)
of $700\,\rm{M_{\odot}\,yr^{-1}}$, median dust mass of $5.1\times
10^9\,\rm{M_{\odot}}$ and median dust luminosity of $\textrm 9.3 \times
10^{12}\,\rm{L_{\odot}}$. Our simulations, performed at z=2.8, suggest that SMM
J04135 is not unique. In fact, at a distance of <100 kpc, 22% of our simulated
quasar sample have at least one companion galaxy of a stellar mass $>10^8\,
\rm{M_{\odot}}$, and 0.3% have at least one highly star-forming companion
($\rm{SFR}>100\,\rm{M_{\odot}\,yr^{-1}}$). Our results suggest that
quasar-gas-rich companion galaxy systems are common phenomena in the early
Universe and the high incidence of companions makes the study of such systems
crucial to understand the growth and hierarchical build-up of galaxies and
black holes.
