The stellar initial mass function at 0.9<z<1.5
  We explore the stellar initial mass function (IMF) of a sample of 49 massive
quiescent galaxies (MQGs) at 0.9$<$z$<$1.5. We base our analysis on
intermediate resolution spectro-photometric data in the GOODS-N field taken in
the near-infrared and optical with the HST/WFC3 G141 grism and the Survey for
High-z Absorption Red and Dead Sources (SHARDS). To constrain the slope of the
IMF, we have measured the TiO$_2$ spectral feature, whose strength depends
strongly on the content of low-mass stars, as well as on stellar age. Using
ultraviolet to near-infrared individual and stacked spectral energy
distributions, we have independently estimated the stellar ages of our
galaxies. Knowing the age of the stellar population, we interpret the strong
differences in the TiO$_2$ feature as an IMF variation. In particular, for the
heaviest z$\sim$1 MQGs (M$>$10$^{11}$Msun) we find an average age of
1.7$\pm$0.3 Gyr and a bottom-heavy IMF ($\Gamma_b$=3.2$\pm$0.2). Lighter MQGs
(2$\times$10$^{10}$$<$M$<$10$^{11}$ Msun) at the same redshift are younger on
average (1.0$\pm$0.2 Gyr) and present a shallower IMF slope
($\Gamma_b=2.7^{+0.3}_{-0.4}$). Our results are in good agreement with the
findings about the IMF slope in early-type galaxies of similar mass in the
present-day Universe. This suggests that the IMF, a key characteristic of the
stellar populations in galaxies, is bottom-heavier for more massive galaxies
and has remained unchanged in the last $\sim$8 Gyr.
