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Astron. Astrophys. 319, 747-756 (1997)

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8. Redshift dependence of the classification

Up to now, we have compared our morphological types with those of traditional morphologists for present day galaxies. But how does the quality of our type estimates depend on redshift? Since the redshift does no enter at all in the classification skill, our expectation is that the quality of our Hubble estimates is as good for distant as for nearby galaxies, provided, of course, that the images of distant galaxies are as good as the nearby ones in terms of rest-frame resolution, sampling, depth, etc.

Distant ([FORMULA]) clusters are now currently observed by HST and their galaxies are beginning to be classified by traditional morphologists. Dressler & Oemler classified the galaxies of the distant cluster Cl0939+4713 ([FORMULA]) (Dressler et al. 1994b). For 31 galaxies, their type is listed in Stanford, Eisenhardt & Dickinson (1995). We re-classified the same galaxies from the same images (Andreon, Davoust & Heim, 1996). For these galaxies, we found the same rate of agreement (23 %) between our and Dressler & Oemler's classifications as for nearby galaxies. This result implies that the agreement between our classes and the traditional ones is not strongly redshift-dependent and that Dressler & Oemler classify distant galaxies in the same way as they classify nearby ones.

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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998