Galaxy properties from integrated light spectroscopy versus resolved stellar population studies

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.

In this project 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. The integrated spectra are obtained by scanning carefully chosen regions within Local Group galaxies for which high-quality CMDs (reaching the oMSTO) are available (see Figure 1 for an example in the LMC case).

Figure 1: Left panel: Image of the LMC bar and its surroundings (Credit: John Gleason). The location in the centre of the bar of the field studied in this paper is indicated. Right panel: Positions of the two WFPC2 fields (red shaded areas), and the 2.5'x5' area covered by sweeping the slit in the east-west direction (blue shaded area), superimposed on a VIMOS B-band image.

As a first attempt we explore the SFH of a region within the bar of the LMC recovered via STECKMAP and the analysis of a deep CMD (see Figure 2). The good agreement between both approaches in this particular example encouraged us to expand such comparison to other systems displaying different SFHs such as LeoA, LeoI, or the Andromeda companions (work in progress).

Figure 2: Comparison between the LMC bar SFH from the CMD and the integrated spectrum using full spectrum fitting techniques (STECKMAP). The three panels show, from top to bottom, the normalized SFR(t), the age–metallicity relation, and the cumulative mass fraction with a zoom at young ages for the normalized SFR(t) and the AMR.