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

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 χ 2 . 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.

Survival of molecular gas in a stellar feedback-driven outflow witnessed with the MUSE TIMER project and ALMA

“…To recover the recent SFH in different regions of the central ring in NGC 3351 we use the full spectral fitting routine STECKMAP (STEllar Content and Kinematics via Maximum a Posteriori; Ocvirk et al 2006b,a). The justification and testing of this methodology is outlined in e.g., Sánchez-Blázquez et al (2011);Ruiz-Lara et al (2015), and our adopted methodology is described in detail in Sánchez-Blázquez et al (2014). In short, the integrated spectrum within a given spaxel is compared to a grid of composite stellar population models in order to recover the light weighted mass fractions in different age bins.…”

Star formation quenching imprinted on the internal structure of naked red nuggets

“…This extreme chemistry also makes the comparison with resolved stellar population studies less direct as the latter have generally studied less metal-rich systems (see e.g. Ruiz-Lara et al 2015Ocvirk 2010). Moreover, as described above, we allowed in our analysis for stellar populations older than the age of the Universe to account for model systematics (e.g.…”

Unresolved versus resolved: testing the validity of young simple stellar population models with VLT/MUSE observations of NGC 3603

“…Other efforts along the same line of study have also been performed by Koleva et al (2008) and Riffel et al (2011), who concluded that SSP models quite satisfactorily reproduce the observed parameters, but that there were minor caveats. In the context of studying more complex objects with non-instantaneous star formation history, more recently Ruiz-Lara et al (2015) presented a comparison of resolved and unresolved stellar populations within a field in the Large Magellanic Cloud in a study of the star formation history in the region.…”