Resolving the age bimodality of galaxy stellar populations on kpc scales

Zibetti, S. et. al.Galaxies in the local Universe are known to follow bimodal distributions in the global stellar population properties. We analyse the distribution of the local average stellar population ages of 654 053 sub-galactic regions resolved on similar to 1 kpc scales in a volume-corrected sample of 394 galaxies, drawn from the Calar Alto Legacy Integral Field Area (CALIFA) DR3 integral-field- spectroscopy survey and complemented by Sloan Digital Sky Survey (SDSS) imaging. We find a bimodal local-age distribution, with an old and a young peak primarily due to regions in early-type galaxies and star-forming regions of spirals, respectively. Within spiral galaxies, the older ages of bulges and interarm regions relative to spiral arms support an internal age bimodality. Although regions of higher stellar mass surface density, mu(*), are typically older, mu(*) alone does not determine the stellar population age and a bimodal distribution is found at any fixed mu(*). We identify an 'old ridge' of regions of age similar to 9 Gyr, independent of mu(*), and a 'young sequence' of regions with age increasing with mu(*) from 1-1.5 to 4-5 Gyr. We interpret the former as regions containing only old stars, and the latter as regions where the relative contamination of old stellar populations by young stars decreases as mu(*) increases. The reason why this bimodal age distribution is not inconsistent with the unimodal shape of the cosmic-averaged star formation history is that (i) the dominating contribution by young stars biases the age low with respect to the average epoch of star formation, and (ii) the use of a single average age per region is unable to represent the full time extent of the star formation history of ` young sequence' regions.SZ and ARG have been supported by the EU Marie Curie Career Integration Grant 'SteMaGE' Nr. PCIG12-GA-2012-326466 (Call Identifier: FP7-PEOPLE-2012 CIG). YA is financially supported by the Ramon y Cajal programme (contract RyC-2011-09461) and project AYA2016-79724-C4-1-P from the Spanish Mineco, as well as the exchange programme SELGIFS FP7-PEOPLE-2013-IRSES-612701 funded by the EU. SC acknowledges support from the European Research Council via an Advanced Grant under grant agreement no. 321323 (NEOGAL). LG was supported in part by the US National Science Foundation under Grant AST-1311862. RGB acknowledges support from grants P12-FQM-2828 of Junta de Andalucia and AYA2014-57490P from MINECO. AdLC acknowledges support from the CONACyT-125180, DGAPA-IA100815 and DGAPA-IA101217 projects. RAM acknowledges support by the Swiss National Science Foundation. IM acknowledges financial support from the grant AYA2013-42227-P from the Spanish Ministerio de Ciencia e Innovacion. SFS thanks the CONACYT-125180 and DGAPA-IA100815 projects. GvdV acknowledges partial support from Sonderforschungsbereich SFB 881 'The Milky Way System' (subproject A7 and A8) funded by the German Research Foundation.Peer reviewe