[Paper Review] Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr

J. F. C. Santos, Joan Clària+4 more|ArXiv.org|Nov 25, 2005

Stellar, planetary, and galactic studies27 references18 citations

TL;DR

This study presents integrated spectroscopy of 17 blue concentrated star clusters in the Large Magellanic Cloud (LMC) with ages between 40 and 300 Myr, using two methods—template matching and equivalent width (EW) calibration of Balmer, Ca II, CH, Mg I, and Fe lines. It confirms a radial-age gradient in the LMC, with younger clusters concentrated in the inner regions, and establishes a reliable spectral library for cluster age determination in the intermediate-age range.

ABSTRACT

Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic Cloud (LMC) clusters is presented and its spectral evolution studied. The spectra span the range ~3600-6800A with a resolution of ~14A FWHM, being used to determine cluster ages and, in connection with their spatial distribution, to explore the LMC structure and cluster formation history. Cluster reddening values were estimated by interpolation, using the available extinction maps. We used two methods to derive cluster ages: (i) template matching, in which line strengths and continuum distribution of the cluster spectra were compared and matched to those of template clusters with known astrophysical properties, and (ii) equivalent width (EW) method, in which new age/metallicity calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta). The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr (NGC1932 and SL709), a good agreement between the results of the two methods being obtained. Combining the present sample with additional ones indicates that cluster deprojected distances from the LMC center are related to age in the sense that inner clusters tend to be younger. Spectral libraries of star clusters are useful datasets for spectral classifications and extraction of parameter information for target star clusters and galaxies. The present cluster sample complements previous ones, in an effort to gather a spectral library with several clusters per age bin.

Motivation & Objective

• To determine accurate ages for 17 blue concentrated star clusters in the Large Magellanic Cloud (LMC) using integrated spectroscopy.
• To investigate the spatial distribution of these clusters in relation to LMC structure and cluster formation history.
• To validate and apply new age/metallicity calibrations based on equivalent widths (EWs) of key spectral lines (Hβ, Hγ, Hδ, K Ca II, G band, Mg I).
• To test the reliability of template matching and EW-based methods for age determination in unresolved stellar systems.
• To contribute to the development of a comprehensive spectral library with multiple clusters per age bin for future studies of galaxy evolution.

Proposed method

• Obtained high-resolution (FWHM ≈ 14 Å) integrated spectra over 3600–6800 Å for 17 LMC clusters using the 2.15 m telescope at CASLEO.
• Estimated cluster reddening via interpolation from available extinction maps, using E(B-V) values derived from the maps.
• Applied two age-determination methods: (1) template matching comparing continuum and line strengths to known cluster templates; (2) EW-based calibration using diagnostic diagrams of summed line strengths.
• Used spectral features including K Ca II (3934 Å), Hδ (4102 Å), G band (CH, 4300 Å), Hγ (4340 Å), Hβ (4861 Å), and Mg I (5173 Å) for age estimation.
• Cross-validated results between the two methods and compared with literature ages where available, especially for NGC 1839.
• Combined the new sample with prior data to analyze the radial dependence of cluster age in the LMC, using deprojected distances from the LMC center.

Experimental results

Research questions

• RQ1 What are the ages of 17 blue concentrated star clusters in the LMC with ages between 40 and 300 Myr, as derived from integrated spectroscopy?
• RQ2 How do the results from template matching compare with those from equivalent width-based age calibration for these clusters?
• RQ3 Is there a radial gradient in cluster age distribution in the LMC, with younger clusters located closer to the center?
• RQ4 Can the new spectral library of intermediate-age clusters improve the accuracy of age and metallicity estimates for unresolved stellar systems?
• RQ5 How do the observed spectral features (e.g., Hβ, Mg I, K Ca II) correlate with cluster age in the 40–300 Myr range?

Key findings

• Cluster ages derived via both template matching and EW-based methods show good agreement, with ages ranging from 40 Myr (NGC 2130 and SL 237) to 300 Myr (NGC 1932 and SL 709).
• The cluster NGC 1839, previously studied via Washington photometry, yielded consistent age estimates (0.09 Gyr), validating the method's reliability.
• A radial-age gradient is observed: younger clusters (40–100 Myr) are preferentially located closer to the LMC center, while older clusters (200–300 Myr) are found at larger deprojected distances.
• The sample complements existing spectral libraries by providing multiple clusters per age bin, enhancing the utility of integrated spectra for stellar population studies.
• The equivalent width of Mg I lines shows strong age dependence, with higher values in older clusters (e.g., SL 709: 7.0 Å at 5156 Å), supporting its use as a diagnostic tool.
• The method using summed equivalent widths (S_h and S_m) provides a robust age indicator, with S_h (sum of Hδ, Hγ, Hβ) and S_m (sum of Mg I, K Ca II, G band) showing clear trends with age across the sample.