Astron. Astrophys. 323, 337-348 (1997)

1. Introduction

Many observational efforts over a long period of time have been devoted to measuring the evolution of the properties of the Hubble types. This has mainly been done by comparing the properties of the types at large distances (z ) with a nearby reference sample (see as exceptions, e.g. Courteau, de Jong & Broeils 1996, Zepf 1996).

When comparing the properties of the Hubble types, a first problem came from the difficulty, and in part the subjectivity, in estimating Hubble types. Expert morphologists are able to classify galaxies in morphological types with a reproducibility in the range 50 to 80 % (Andreon & Davoust 1997) when galaxies are classified in three types. The reproducibility is obviously lower when each morphologist uses his personal definitions for the types. Unfortunately, many recent studies of distant galaxies (e.g. Casertano et al. 1995, Glazebrook et al. 1995, Driver, Windhorst & Griffits 1995), but not all (Dressler et al. 1994a, b, Abraham et al. 1996, van den Bergh et al. 1996) use definitions for the types which are not identical to those used by morphologists for nearby galaxies. Morphologists classify (nearby) galaxies by their resemblance to standards (Hubble 1936, de Vaucouleurs 1959, Sandage 1961), whereas distant galaxies are often classified without any reference to standards and in classes that are not easy to link to the traditional ones (for example Glazebrook et al. 1995 classify usual edge-on lenticulars as spirals, and put together in one class compact galaxies and ellipticals).

Second, the comparison of types at different redshifts has been hampered for a long time by the lack of angular resolution of groundbased telescopes that do not allow imaging distant galaxies with the same restframe spatial resolution as nearby ones, and therefore do not allow one to classify distant galaxies, in spite of great efforts to observe distant galaxies in sub-arcsecond seeing conditions (e.g. Thompson 1986, Lavery, Pierce & McClure 1992). The Hubble Space Telescope (HST) has now opened a new era and the situation has been partly reversed, since images of distant galaxies are now available with better restframe resolution than for nearby galaxies (see Sect. 3.1 for details).

The goal of this paper is to study the galaxies in Cl 0939+4713, a distant cluster observed by the HST, to compare their properties to the ones of galaxies in the Coma and Perseus clusters, presented in previous papers of this series (Andreon 1994, 1996a), in order to gain insight into the epoch dependence of the properties of the Hubble types. This is done using the same method for classifying all the galaxies from images of similar restframe resolution and for samples selected in the same restframe passband down to the same absolute magnitude.

The paper is organized as follows. Section 2 outlines our present understanding of the properties of the galaxy populations in Cl 0939+4713. Section 3 deals with the problems that could bias the comparison of morphological types of galaxies at different redshifts. The morphological types of galaxies in Cl 0939+4713 are presented and discussed in Section 4. In Section 5 we study the properties of the types, among themselves and with respect to local counterparts, and we devote Section 6 to a discussion and a summary of the results. We adopt km s^-1 and , but, when appropriate, we discuss the consequences of the adopted cosmological model.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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