Abstract
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 TiO2 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. For the heaviest z~1 MQGs (M > 10^11.0 Msun) we find an average age
of 1.7$\pm$0.3 Gyr and a bottom-heavy IMF (\Gamma=3.2$\pm$0.2). Lighter MQGs
(10^10.5 < M < 10^11.0 Msun) at the same redshift are younger on average
(1.0$\pm$0.2 Gyr) and present a shallower IMF slope (\Gamma=2.7$\pm$0.3). 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
~8 Gyr.
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