SHARDS: stellar populations and star formation histories of a mass-selected sample of 0.65 < z < 1.1 galaxies

Abstract

We report on results from the analysis of a stellar mass-selected (log (M_*/M_⊙) ≥ 9.0) sample of 1644 galaxies at 0.65 < z < 1.1 with ultradeep (m_AB < 26.5) optical medium-band (R ∼ 50) photometry from the Survey for High-z Absorption Red and Dead Sources (SHARDS). The spectral resolution of SHARDS allows us to consistently measure the strength of the 4000 Å spectral break [D_n(4000), an excellent age indicator for the stellar populations of quiescent galaxies] for all galaxies at z ∼ 0.9 down to log (M_*/M_⊙) ∼ 9. The D_n(4000) index cannot be resolved from broad-band photometry, and measurements from optical spectroscopic surveys are typically limited to galaxies at least 10 times more massive. When combined with the rest-frame U - V colour, (U - V)_r, D_n(4000) provides a powerful diagnostic of the extinction affecting the stellar population that is relatively insensitive to degeneracies with age, metallicity or star formation history. We use this novel approach to estimate de-reddened colours and light-weighted stellar ages for individual sources. We explore the relationships linking stellar mass, (U - V)_r, and D_n(4000) for the sources in the sample, and compare them to those found in local galaxies. The main results are: (a) both D_n(4000) and (U - V)_r correlate with M_*. The dispersion in D_n(4000) values at a given M_* increases with M_*, while the dispersion for (U - V)_r decreases due to the higher average extinction prevalent in massive star-forming galaxies. (b) For massive galaxies, we find a smooth transition between the blue cloud and red sequence in the intrinsic U - V colour, in contrast with other recent results. (c) At a fixed stellar age, we find a positive correlation between extinction and stellar mass. (d) The fraction of sources with declining or halted star formation increases steeply with the stellar mass, from ∼5 per cent at log (M_*/M_⊙) = 9.0-9.5 to ∼80 per cent at log (M_*/M_⊙) > 11, in agreement with downsizing scenarios.