Recovering star formation histories: Integrated-light analyses vs. stellar colour-magnitude diagrams

Astronomy and Astrophysics 583 (2015): A60 reproduced with permission from Astronomy & Astrophysics, © ESOContext. Accurate star formation histories (SFHs) of galaxies are fundamental for understanding the build-up of their stellar content. However, the most accurate SFHs - those obtained from colour-magnitude diagrams (CMDs) of resolved stars reaching the oldest main-sequence turnoffs (oMSTO) - are presently limited to a few systems in the Local Group. It is therefore crucial to determine the reliability and range of applicability of SFHs derived from integrated light spectroscopy, as this affects our understanding of unresolved galaxies from low to high redshift. Aims. We evaluate the reliability of current full spectral fitting techniques in deriving SFHs from integrated light spectroscopy by comparing SFHs from integrated spectra to those obtained from deep CMDs of resolved stars. Methods. We have obtained a high signal-to-noise (S/N 36.3 per Å) integrated spectrum of a field in the bar of the Large Magellanic Cloud (LMC) using EFOSC2 at the 3.6-metre telescope at La Silla Observatory. For this same field, resolved stellar data reaching the oMSTO are available. We have compared the star formation rate (SFR) as a function of time and the age-metallicity relation (AMR) obtained from the integrated spectrum using STECKMAP, and the CMD using the IAC-star/MinnIAC/IAC-pop set of routines. For the sake of completeness we also use and discuss other synthesis codes (STARLIGHT and ULySS) to derive the SFR and AMR from the integrated LMC spectrum. Results. We find very good agreement (average differences 4.1%) between the SFR (t) and the AMR obtained using STECKMAP on the integrated light spectrum, and the CMD analysis. STECKMAP minimizes the impact of the age-metallicity degeneracy and has the advantage of preferring smooth solutions to recover complex SFHs by means of a penalized χ2. We find that the use of single stellar populations (SSPs) to recover the stellar content, using for instance STARLIGHT or ULySS codes, hampers the reconstruction of the SFR (t) and AMR shapes, yielding larger discrepancies with respect to the CMD results. These discrepancies can be reduced if spectral templates based on known and complex SFHs are employed rather than SSPsThis research has been partly supported by the Spanish Ministry of Science and Innovation
(MICINN) under grants AYA2011-24728 and Consolider-Ingenio CSD2010-00064; and by the Junta de Andalucía (FQM-108). We also acknowledge financial support from the research projects under grants AYA2010-21887-C04-03, AYA2010-21322-C03-03. TRL thanks the support of the Spanish Ministerio de Educación, Cultura y Deporte by means of the FPU fellowship. D.A. acknowledgesthe support of the Haute Provence Observatory as well as the support of the IAC and ESO for visits during which this study was finalized. M.K. is a postdoctoral fellow of the Fund for Scientific Research- Flanders, Belgium (FWO 65052 / 12E4115N LV). P.S.-B. acknowledges support from the Ramón y Cajal program, grant ATA2010-21322-C03-02 from the Spanish Ministry of Economy and Competitiveness (MINECO