Surface photometry of early–type galaxies in the Hubble deep field ^*

¹ Astronomical Observatory of Padova, Vicolo dell' Osservatorio 5, 35122 Padova, Italy
² Department of Astronomy of the Padova University, Vicolo dell' Osservatorio 5, 35122 Padova, Italy e-mail: gfasano@leda.pd.astro.it

Send offprint request to: G. Fasano

Received: 16 September 1997
Accepted: 5 November 1997

Abstract

The detailed surface photometry of a sample of early–type galaxies in the Hubble Deep Field is presented as part of a long–term project aimed to settle strong observational constraints to the theories modelling the evolution of elliptical galaxies from the early stages. The sample has been extracted, in the V₆₀₆ band, from the database provided by the ESO-STECF-HDF Group (Couch 1996). The selection criteria involve the total magnitude, the number of pixels detected above the background level and an automatic star/galaxy classifier. Moreover, form visual inspection of the frames, we excluded the galaxies showing unambiguous late–type morphology. The analysis of the luminosity and geometrical profiles, carried out on the 162 candidates obeying our selection criteria, resulted in a list of 99 "bona fide” early–type galaxies, for which accurate total magnitudes and effective radii were computed on the basis of the equivalent luminosity profiles. The comparison with the magnitudes given by Williams et al. (1996) indicates that the automated photometry tends to underestimate the total luminosity of the ellipticals. The luminosity profiles of most of galaxies in our sample follow fairly well the de Vaucouleurs law ("Normal” profiles). However, a relevant fraction of galaxies, even following the law in the main body light distribution, exhibit in the inner region a flattening of the luminosity profile not attributable to the PSF ("Flat” profiles) or, in some cases, a complex (multi–nucleus) structure ("Merger” profiles). A statistically significant correlation is found between the shapes of the luminosity profiles and the ellipticity distribution. In particular, the average ellipticity of galaxies belonging to the "Flat” and "Merger” classes is significantly higher than that of the "Normal” galaxies. Finally, even taken into account the relevant uncertainty of the outer position angle profiles, the amount of isophotal twisting of HDF ellipticals turns out to be significantly larger with respect to that of the local samples.