Ages of LMC Star Clusters from Their Integrated Properties PDF Download

Author: Randa Asa'd
Publisher:
ISBN:
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Languages : en
Pages : 163

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Book Description
Star Clusters are the building blocks of galaxies. Determining their ages gives us information about the formation history of their hosting galaxies. For far-away galaxies, star clusters are not resolved. Only their integrated properties can be observed. Both integrated photometry and integrated spectra have been used as age indicators of stellar clusters. The Large Magellanic Cloud (LMC) is a perfect galaxy to test these methods of age determination, because its clusters are close enough to see their individual stars, but also far enough away so that each cluster can be observed as a whole. This work first shows that the traditional methods of using the integrated broad-band photometry for age determination are highly inaccurate. This is attributed primarily to two things. First, the UBV integrated broad-band aging methods require matching a cluster with an expected model prediction of the cluster colors as a function of age. The biggest problem we find is the stellar clusters in our sample do not typically lie on the model line based on their known age and extinction. That is to say, real cluster colors often do not match the model colors and can be found some distance from expected model values. A second issue, which has been previously documented in numerous studies, is the strong degeneracy between age and extinction in the UBV color-color plane. Certainly, providing more photometric bands will reduce degeneracy between age and reddening. Better yet, if extinction can be independently determined, we show that ages from methods based on integrated broad-band colors will more closely match those obtained from more accurate methods based on stellar photometry. But the underlying issue remains. Simple stellar population models often do not accurately represent the colors of real stellar clusters due to the incomplete and stochastic sampling of the stellar mass function in low and moderate mass stellar clusters. On the other hand, integrated spectra provide better age predictions than broad-band photometry in the wavelength range 3626' 6248 A when compared with high resolution computational models. I obtained the integrated spectra of 20 clusters that didn't have integrated spectra in the optical range, or they have never been observed before. Using ths sample and 7 other clusters from the literature I show that the statistical Kolmogorov-Smirnov (KS) test can better find the closest match between the observed spectrum and theoretical model than the traditional Chi squared. Finally, I present a new software routine that efficiently predicts the age of a star cluster given its optical integrated spectrum compared to spectra generated by computational models.