CN as a tracer of galaxy assembly timescales

Abstract

We present the values of CN and Mg overabundances with respect to Fe, for a large sample of elliptical galaxies in different environments. Abundances were derived by confronting observed absorption line indices with stellar population model spectra. We obtained significant differences between the [CN/Fe] and [Mg/Fe] abundance ratios as a functions of: i) the environment, and ii) the galaxy mass. This is interpreted as implying varying formation timescales for CN, Mg and Fe, combined with different star formation histories in elliptical galaxies depending on their mass and environment. Our principal conclusions are: 1) CN is sensitive to the characteristic assembly timescales of elliptical galaxies, 2) more massive elliptical galaxies are assembled on shorter timescales than less massive ones, 3) elliptical galaxies in denser environments are assembled on shorter timescales than those in lower density environments, and 4) our results strongly suggest an upper limit for the assembly timescale of ∼1 Gy, in all cases.