Multiwavelength dissection of a massive heavily dust-obscured galaxy and
  its blue companion at z $\sim $2
  In this work we study a system of two galaxies, Astarte and Adonis, at z
$\sim $2 when the Universe was undergoing its peak of star formation activity.
Astarte is a dusty star-forming galaxy at the massive-end of the main sequence
(MS) and Adonis is a less-massive, bright in ultraviolet (UV), companion galaxy
with an optical spectroscopic redshift. We analyse the physical properties of
this system, and probe the gas mass of Astarte with its ALMA CO emission, to
investigate whether this ultra-massive galaxy is quenching or not. We use
CIGALE - a spectral energy distribution modeling code - to derive the key
physical properties of Astarte and Adonis, mainly their star formation rates
(SFRs), stellar masses, and dust luminosities. We inspect the variation of the
physical parameters depending on the assumed dust attenuation law. We also
estimate the molecular gas mass of Astarte from its CO emission, using
different $\alpha_{CO}$ and transition ratios ($r_{31}$) and discuss the
implication of the various assumptions on the gas mass derivation. We find that
Astarte exhibits a MS-like star formation activity, while Adonis is undergoing
a strong starburst (SB) phase. The molecular gas mass of Astarte is far below
the gas fraction of typical star-forming galaxies at z=2. This low gas content
and high SFR, result in a depletion time of $0.22\pm0.07$ Gyrs, slightly
shorter than what is expected for a MS galaxy at this redshift. The CO
luminosity versus the total IR luminosity suggests a MS-like activity assuming
a galactic conversion factor and a low transition ratio. The SFR of Astarte is
of the same order using different attenuation laws, unlike its stellar mass
that increases using shallow attenuation laws. We discuss these properties and
suggest that Astarte might be experiencing a recent decrease of star formation
activity and is quenching through the MS following a SB epoch.
