Over half of the census of massive galaxies at z \sim 2 are dominated by quiescent stellar populations .
The formation mechanism for these galaxies is still under debate , with models relying either on massive and early mergers , or cold accretion .
It is therefore imperative to understand in detail the properties of these galaxies .
We present here a detailed analysis of the star formation history ( SFH ) of FW4871 , a massive galaxy at z= 1.893 \pm 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 z _ { F } \sim 10 with a truncation after 2.7 Gyr , giving a mass-weighted age of 1.5 Gyr and a stellar mass of 0.8–3 \times 10 ^ { 11 } M _ { \odot } ( the intervals representing the output from different population synthesis models ) , implying star formation rates of 30–110 M _ { \odot } /yr .
A more complex model including a recent burst of star formation places the age of the youngest component at 145 _ { -70 } ^ { +450 } 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 \mu m , corresponding to rest-frame 8 \mu m , 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 3R _ { e } .
We suggest that the recent merger that formed FW4871 is responsible for the quenching of its star formation .