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Lyubenova, Mariya (2009): Towards calibrating stellar population models in the near-IR: clues from globular clusters and early-type galaxies. Dissertation, LMU München: Faculty of Physics
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Abstract

This thesis presents a study of the near-IR spectral properties of six globular clusters (GCs) and 21 early-type galaxies in the local Universe, 10 of which are in low density environments and 11 are in the Fornax cluster of galaxies. Our observations were acquired using integral field unit (IFU) and long-slit spectrographs. The common goal of the various projects was to verify the predictions of current stellar population models in the near-IR using GCs and early-type galaxies data. We provide the first spectral library of the integrated near-IR light of globular clusters with ages > 1 Gyr. Using VLT/SINFONI we obtained integrated luminosity weighted spectra of six GCs in the Large Magellanic Cloud (LMC). Three of them are old > 10 Gyr) and metal poor ([Z/H]~-1.4), the other three have intermediate ages (1< age < 3 Gyr) and higher metallicity ([Z/H]~-0.4). We have measured the strength of the near-IR K-band features Na I, Ca I and 12CO(2-0) and compared the Dco index, used to measure the strength of 12CO(2-0), with the predictions of the stellar population models of Maraston (2005). We find reasonable agreement between the data and the model predictions for old and metal poor GCs. For intermediate age and half solar metallicity GCs we find a marked disagreement with the models. While the models predict an increase of the index with younger ages, we find a strong drop of index strength at ~1 Gyr. We consider that this is due to the different spectral properties of galactic carbon rich AGB stars, used to empirically calibrate the models, and the ones present in LMC globular clusters. This conclusion is based on the different Dco index strength that we measure for the carbon stars in our LMC sample and the galactic carbon stars for a given (J-K) colour. Our study of early-type galaxies in low density environments and in the Fornax cluster reveals that the near-IR Na I and Dco indices follow similar index-sigma scaling relations as optical metallicity indices. The Na I index shows a good positive correlation with the metallicity at old ages and additionally increases for younger ages in contrast to the behaviour of optical metal indices. We derive an empirical calibration for the Na I index as function of metallicity and suggest that a diagnostic diagram, based on the optical hydrogen Balmer index and the near-IR Na I index, will be a good age-metallicity estimator once more detailed SSP models in the near-IR become available. The Dco index exhibits similar trends with metallicity, albeit with a shallower slope and more complex behaviour at young ages. We find evidence of saturation of the Dco index above twice solar metallicity. Utilising the superb spatial resolution of adaptive optics assisted IFU observations in the near-IR we observed the centre of the Fornax cD galaxy NGC 1399. We confirm the presence of a central velocity dispersion drop within r<0.2" and reveal drops in the Na I and Dco index with the same extent and location. We suggest that the centre of NGC 1399 harbours a cold subsystem with a distinct stellar population, which is either more metal poor, or younger and more metal poor than the main body of the galaxy. We detect a negative gradient of the Na I index within r<1.4". Using our calibration of Na I vs. metallicity we derive a metallicity gradient, which appears to be shallower than the typical metallicity gradients measured at larger radii in other early-type galaxies.