The road to the red sequence: A detailed view of the formation of a
  massive galaxy at z~2
  (Abridged) We present here a detailed analysis of the star formation history
(SFH) of FW4871, a massive galaxy at z=1.893+-0.002. We compare rest-frame
optical and NUV slitless grism spectra from the Hubble Space Telescope with a
large set of composite stellar populations to constrain the underlying star
formation history. Even though the morphology features prominent tidal tails,
indicative of a recent merger, there is no sign of on-going star formation
within an aperture encircling one effective radius, which corresponds to a
physical extent of 2.6 kpc. A model assuming truncation of an otherwise
constant SFH gives a formation epoch zF~10, with a truncation after 2.7 Gyr,
giving a mass-weighted age of 1.5 Gyr and a stellar mass of 0.8-3E11Msun,
implying star formation rates of 30-110 Msun/yr. A more complex model including
a recent burst of star formation places the age of the youngest component at
145 Myr, with a mass contribution lower than 20%, and a maximum amount of dust
reddening of E(B-V)<0.4 mag (95% confidence levels). This low level of dust
reddening is consistent with the low emission observed at 24 micron,
corresponding to rest-frame 8 micron, where PAH emission should contribute
significantly if a strong formation episode were present. The colour profile of
FW4871 does not suggest a significant radial trend in the properties of the
stellar populations out to 3Re. We suggest that the recent merger that formed
FW4871 is responsible for the quenching of its star formation.
