The road to the red sequence: A detailed view of the formation of a massive galaxy at z ∼ 2

Over half of the census of massive galaxies at z 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 ± 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 SFH. Even though the morphology features prominent tidal tails, indicative of a recent merger, there is no sign of ongoing star formation within an aperture encircling one effective radius, which corresponds to a physical extent of 2.6kpc. A model assuming truncation of an otherwise constant SFH gives a formation epoch z _F 10 with a truncation after 2.7Gyr, giving a mass-weighted age of 1.5Gyr and a stellar mass of (0.8-3) × 10 ¹¹ M _· (the intervals representing the output from different population synthesis models), implying star formation rates of 30-110M _· yr ^-1. A more complex model including a recent burst of star formation places the age of the youngest component at 145 ⁺⁴⁵⁰ _{- 70}Myr, with a mass contribution lower than 20%, and a maximum amount of dust reddening of E(B - V) < 0.4mag (95% confidence levels). This low level of dust reddening is consistent with the low emission observed at 24 μm, corresponding to rest-frame 8 μm, where polycyclic aromatic hydrocarbon emission should contribute significantly if a strong formation episode were present. The color profile of FW4871 does not suggest a significant radial trend in the properties of the stellar populations out to 3 R _e. We suggest that the recent merger that formed FW4871 is responsible for the quenching of its star formation.