Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994ap%26ss.217..279j&link_type=abstract
Astrophysics and Space Science (ISSN 0004-640X), vol. 217, no. 1-2, p. 279-281
Astronomy and Astrophysics
Astrophysics
Galactic Evolution, Spiral Galaxies, Star Clusters, Star Formation, Stellar Spectra, Calibrating, Classifications, Identifying, Morphology, Populations, Red Shift, Stellar Spectrophotometry
Scientific paper
The infrared domain is appealing for all those who are interested in distant galaxies, at large red shifts. Since the pioneer work by Butcher and Oemler, one knows that the fraction of blue galaxies in clusters increases with increasing redshift. A lot of work has since then been dedicated to trying to identify and quantify the cosmological evolution revealed by this 'bluering'. Many questions, if not all, are still open. Do these blue colors trace an enhancement of mergers with increasing lookback time? Were the spiral galaxies more numerous then? Are we looking at still forming galaxies? One major but difficult point is the identification of the morphological type of a galaxy, once its spectrum has been confused with the strong flux of new generation of stars. For example, how well can we distinguish between different spiral Hubble types? To solve this problem, one has to calibrate on nearby galaxies, whose Hubble types are known, the relation between stellar content and morphology, and have a clear measure of the dispersion of this relation. Some results of this methodology are presented here. A method of population synthesis has been developed, which does avoid any a-priori in the analyses. It makes use of a base of integrated spectrophotometric properties of star clusters. The spectra of galaxies are reproduced by linear combinations of star clusters, each of them representing a generation of stars, and thus allowing a direct analysis of the star formation history of the observed galaxy. The population syntheses results are displayed in tables of contributions (%) of each generation of stars to the flux of the galaxy. The results of population syntheses for two kinds of spiral galaxies, respectively NGC 2903 and NGC 4775 are shown in tabular form. They correspond to the extremes in the shapes of the spirals spectra, the latter being much bluer than the former. Dust is responsible for their difference, masking very strongly the disc's stellar population in NGC 2903. The same dispersion should be expected at high redshift.
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