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Stellar Population Studies in Galaxy Nuclei

其他書名	A Practical Approach to the Population Synthesis Problem
出版	University of Sussex, 1993
URL	http://books.google.com.hk/books?id=x3EZHQAACAAJ&hl=&source=gbs_api

註釋With the objective of better understanding the spectra of galaxies at high redshifts, the issue of the stellar population content in galaxy nuclei has been addressed by an optimizing population synthesis approach. The formulation adopted for the synthesis, based on data from a star cluster spectra library, presents several advantages like: a wide span in age anti metallicity of the star cluster data set; the implicit inclusion of the initial mass function dependence and stellar evolution and consequent reduction in the number of free parameters; a constraint-free problem formulation with explicit observational error dependence. The internal consistency of the star cluster data base has been examined by means of a Principal Component Analysis technique. Results show that the data distribution is essentially bi-parametric with a ~10% dispersion due to observational errors, and that considerable degenerative effects on the solutions may rise from strong internal correlation between components corresponding to close evolutionary stages and/or similar inetallicities. Special attention has been devoted to the existence of non-unique solutions to the problem. In particular, a new optimization procedure has been devised to address the multiplicity of solutions issue from a statistical point of view. Rather than finding an "optimal solution" to the problem, the new algorithm determines a "representative solution" from the set of all possible solutions that can be found by sweeping the parameter space of solutions at fixed steps and estimating local minima. From a series of pre-determined population synthesis tests it was possible to evaluate the efficiency of the method. Results show that errors are. in general, smaller than 5% (in terms of component proportions) and that less accurate results, up to a factor of 3. should be expected for systems with strong blue composite populations for which the current data base can not be stringent enough. Nevertheless, characteristics like the maximum chemical enrichment and the presence of young star formation bursts in composite systems are always possible to be determined. Population synthesis has been performed for galaxies NGC278 and NGC404 and also for some eminent galaxies of the Loral Group: M31, M 32. M 33 and NGC205. Synthesis results give models consistent with evolutionary scenarios where the stellar populations are basically described as a composite in age and metallicity. Parameters like the maximum metallicity attained by each nucleus, the relative contribution of young populations to the total flux and their corresponding proportions in mass are presented for all nuclei and show good agreement with independent data about these objects. In the case of NGC278 and NGC 404 the stellar-free emission component spectrum has been isolated and analysed. In the last part of this work simulations of star formation events superimposed on old populations are presented, obtained by combining star cluster spectra of different ages with those of red strong-lined galaxy nuclei and following the spectral evolution of the composite system over 3 x 10° years. Interesting burst-plus-galaxy configurations are discussed, whenever the burst ami galaxy contributions are of comparable importance, depending on the burst age and strength. The effect of the starburst on DVRI magnitudes and colours, as well as on the equivalent widths for a set of metallic and Balmer lines are also shown.