FIGS: Spectral fitting constraints on the star formation history of
  massive galaxies since Cosmic Noon
  We constrain the stellar population properties of a sample of 52 massive
galaxies, with stellar mass log Ms>10.5, over the redshift range 0.5<z<2 by use
of observer-frame optical and near-infrared slitless spectra from HST's ACS and
WFC3 grisms. The deep exposures (~100 ks) allow us to target individual spectra
of massive galaxies to F160W=22.5AB. Our spectral fitting approach uses a set
of six base models adapted to the redshift and spectral resolution of each
observation, and fits the weights of the base models, including potential dust
attenuation, via an MCMC method. Our sample comprises a mixed distribution of
quiescent (19) and star-forming galaxies (33). We quantify the width of the age
distribution (Dt) that is found to dominate the variance of the retrieved
parameters according to Principal Component Analysis. The population parameters
follow the expected trend towards older ages with increasing mass, and Dt
appears to weakly anti-correlate with stellar mass, suggesting a more efficient
star formation at the massive end. As expected, the redshift dependence of the
relative stellar age (measured in units of the age of the Universe at the
source) in the quiescent sample rejects the hypothesis of a single burst (aka
monolithic collapse). Radial colour gradients within each galaxy are also
explored, finding a wider scatter in the star-forming subsample, but no
conclusive trend with respect to the population parameters.
