Dissecting galactic bulges in space and time I: The importance of early formation scenarios vs. secular evolution

This article has been accepted for publication in Monthly Notices Royal Astronomical Society ©: 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reservedThe details of bulge formation via collapse, mergers, secular processes or their interplay remain unresolved. To start answering this question and quantify the importance of distinct mechanisms, we mapped a sample of three galactic bulges using data from the integral field spectrograph WiFeS on the ANU's 2.3-m telescope in Siding Spring Observatory. Its highresolution gratings (R ~ 7000) allow us to present a detailed kinematic and stellar population analysis of their inner structures with classical and novel techniques. The comparison of those techniques calls for the necessity of inversion algorithms in order to understand complex substructures and separate populations. We use line-strength indices to derive single stellar population equivalent ages and metallicities. Additionally, we use full spectral fitting methods, here the code STECKMAP, to extract their star formation histories. The high quality of our data allows us to study the 2D distribution of different stellar populations (i.e. young, intermediate and old). We can identify their dominant populations based on these age-discriminated 2D light and mass contribution. In all galactic bulges studied, at least 50 per cent of the stellar mass already existed 12 Gyr ago, more than currently predicted by simulations. A younger component (age between ~1 and ~8 Gyr) is also prominent and its present day distribution seems to be affected much more strongly by morphological structures, especially bars, than the older one. This in-depth analysis of the three bulges supports the notion of increasing complexity in their evolution, likely to be found in numerous bulge structures if studied at this level of detail, which cannot be achieved by mergers alone and require a non-negligible contribution of secular evolutionMKS acknowledges the support of the Instituto de Astrofísica de Canarias via an Astrophysicist Resident fellowship and Ignacio Martín-Navarro, Andra Stroe and Stéphane Courteau for useful discussions. RC acknowledges the Ministerio de Ciencia e Innovación by means of their FPI program (grant AYA- 2010-21322-C03-03 and AYA-2013-48226-C3-3-P). JFB acknowledges support from the Ramón y Cajal Program and from the FP7 Marie Curie Actions of the European Commission, via the Initial Training Network DAGAL under REA grant agreement number 289313. TRL thanks the support of the Spanish Ministerio de Educación, Cultura y Deporte by means of the FPU fellowship. This research has been supported by the Spanish Ministry of Economy and Competitiveness (MINECO; grants AYA2010-21322-C03-02 and AYA2009-11137) and by the Spanish Ministry of Science and Innovation (MICINN; grants AYA2011-24728 and Consolider-Ingenio CSD2010-00064) and by the Junta de Andalucía (FQM-108