The ESO nearby Abell cluster survey

VIII. Morphological and spectral classification of galaxies

Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: katgert@strw.leidenuniv.nl

Received: 20 June 2005
Accepted: 14 September 2005

Abstract

We determine the morphological types of 2295 galaxies from the ESO Nearby Abell Cluster Survey (ENACS) from CCD images obtained with the Dutch telescope on La Silla. A comparison with morphological types from the literature for 450 of our galaxies shows that the reliability of our classification is quite comparable to that of other classifiers. We recalibrate the ENACS spectral classification with the new morphological types, and find that early- and late-type galaxies can be distinguished from their spectra with 83% reliability. Ellipticals and S0 galaxies can hardly be distinguished on the basis of their spectra, but late spirals can be classified from the spectrum alone with more than 70% reliability. We derive pseudo-colors and linestrengths from the ENACS spectra for the galaxies of different morphological types. We consider the bright () and faint () subsets of the galaxies without emission lines (non-ELG) separately. We find a strong and significant correlation between the average color and the average strength of the metal absorption lines. The average metallicity decreases and the average color gets bluer towards later Hubble type. Also, the faint galaxies in each morphological class are bluer and less metal-rich than their brighter counterparts, which extends the well-established color-magnitude relation of early-type galaxies to (late) spirals. In view of these very strong global trends, the colors and metallicities of faint S0 galaxies and bright early spirals are remarkably similar. The bright early spirals may, on average, have somewhat stronger Hδ absorption than the other galaxies, which could be due to recent starformation. The galaxies with emission lines (ELG) have a bluer spectral continuum than the non-ELG, and the amount of blueing hardly depends on morphological type. The fraction of ELG depends strongly on morphological type (varying from % for ellipticals to % for late spirals), but for each of the morphological types it varies very little with projected distance from the cluster center.