Recovering star formation histories: Integrated-light analyses vs. stellar colour–magnitude diagrams^⋆,⋆⋆

¹ Departamento de Física Teórica y del Cosmos, Universidad de Granada, Campus de Fuentenueva, 18071 Granada, Spain
e-mail: ruizlara@ugr.es
² Instituto Carlos I de Física Teórica y computacional, Universidad de Granada, 18071 Granada, Spain
³ Instituto de Astrofísica de Canarias, Calle vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁴ Departamento de Astrofísica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
⁵ Observatoire de Haute Provence, Institut Pytheas CNRS-Université Aix-Marseille, 04870 Saint Michel l’ Observatoire, France
⁶ Sterrenkundig Observatorium, Krijgslaan 281, 9000 Gent, Ghent University, Belgium
⁷ European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Vitacura, Santiago 19, Chile
⁸ Departamento de Física Teórica, Universidad Autónoma de Madrid, 28049 Cantoblanco, Spain
⁹ Cours Charlemagne Nancy, 11 avenue Saint-Michel 54220 Malzeville, France
¹⁰ National Radio Astronomy Observatory Avenida Nueva Costanera 4091, 7631097 Vitacura, Santiago, Chile
¹¹ Departamento de Astronomía, Universidad de Chile Casilla 36-D, 7630197 Santiago, Chile
¹² Universidade do Estado de Santa Catarina, Rua Beloni Trombeta Zanin 680E, Bairro Santo Antônio, 89815-630 Chapecó, SC, Brazil

Received: 15 June 2015
Accepted: 6 September 2015

Abstract

Context. 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 SSPs.