The link between the star formation history and [α/Fe ]


  • Ignacio G. de la Rosa,

    Corresponding author
    1. Instituto de Astrofísica de Canarias, C/Vía Láctea s/n, E-38200 La Laguna, Tenerife, Spain
    2. Departamento de Astrofísica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain
    3. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT
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  • Francesco La Barbera,

    1. INAF – Osservatorio Astronomico di Capodimonte 80131, Napoli, Italy
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  • Ignacio Ferreras,

    1. Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT
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  • Reinaldo R. de Carvalho

    1. Instituto Nacional de Pesquisas Espaciais/MCT, S. J. dos Campos SP12227-010, Brazil
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The abundance ratios between key elements such as iron and α-process elements carry a wealth of information on the star formation history (SFH) of galaxies. So far, simple chemical evolution models have linked [α/Fe ] with the SFH time-scale, correlating large abundance ratios with short-lived SFH. The incorporation of full spectral fitting to the analysis of stellar populations allows for a more quantitative constraint between [α/Fe ] and the SFH. In this letter, we provide, for the first time, an empirical correlation between [α/Fe ] (measured from spectral indices) and the SFH (determined via a non-parametric spectral-fitting method). We offer an empirical version of the iconic outline of Thomas et al., relating star formation time-scale with galaxy mass, although our results suggest, in contrast, a significant population of old (≳10 Gyr) stars even for the lowest mass ellipticals (M/dyn ∼ 3 × 1010 M). In addition, the abundance ratio is found to be strongly correlated with the time to build up the stellar component, showing that the highest [α/Fe ] (≳+0.2) are attained by galaxies with the shortest half-mass formation time (≲2 Gyr), or equivalently, with the smallest (≲40 per cent) fraction of populations younger than 10 Gyr. These observational results support the standard hypothesis that star formation incorporates the Fe-enriched interstellar medium into stars, lowering the high abundance ratio of the old populations.