We study the evolution of the relation between half-light (effective) radius, r e , and mean surface brightness, < µ > e , known as Kormendy relation, out to redshift z = 0.64 in V-band restframe on the basis of a large sample of spheroidal galaxies (N = 228) belonging to three clusters of galaxies. The present sample constitutes the largest data set for which the Kormendy relation is investigated up to a lookback time of ∼ 6 Gyr (H 0 = 70 Kms −1 Mpc −1 , Ω M = 0.3, Ω Λ = 0.7). A new fitting procedure, which suitably accounts for selection criteria effects, allows for the first time to study the trend of the slope (β) and of the intrinsic dispersion (σ (i) <µ>e ) of the Kormendy relation, and the properties of the whole distribution in the r e -< µ > e plane as a function of look-back time. The slope β of the relation does not change from z = 0.64 to the present epoch: β = 2.92 ± 0.08, implying a tight constraint of 18-28% on the variation of the stellar formation epoch along the sequence of spheroidal galaxies per decade of radius. The intrinsic dispersion of the relation, σ (i) <µ>e = 0.40 ± 0.03, does not vary with redshift and the distribution of galaxy sizes as well as the distribution in the plane of the effective parameters do not vary among the clusters, as proven by the Kolmogorov-Smirnov tests. We conclude that, whatever the mechanism driving galaxy evolution is, it does not affect significantly the properties of bright galaxies in the log r e -< µ > e plane at least since z = 0.64. The evolution of the zeropoint of the Kormendy relation is fully explained by the cosmological dimming in an expanding universe plus the passive luminosity evolution of stellar populations with high formation redshift (z f > 2).

Abstract. In this paper we describe the U-band imaging of the F02 deep field, one of the fields in the VIRMOS Deep Imaging Survey. The observations were done at the ESO/MPG 2.2 m telescope at La Silla (Chile) using the 8k × 8k Wide-Field Imager (WFI). The field is centered at α(J2000) = 02 h 26 m 00 s and δ(J2000) = −04• 30 00 , the total covered area is 0.9 deg 2 and the limiting magnitude (50% completeness) is U AB ∼ 25.4 mag. Reduction steps, including astrometry, photometry and catalogue extraction, are first discussed. The achieved astrometric accuracy (rms) is ∼0.2 with reference to the I-band catalog and ∼0.07 internally (estimated from overlapping sources in different exposures). The photometric accuracy including uncertainties from photometric calibration, is <0.1 mag. Various tests are then performed as a quality assessment of the data. They include: (i) the color distribution of stars and galaxies in the field, done together with the BVRI data available from the VIMOS survey; (ii) the comparison with previous published results of U-band magnitude-number counts of galaxies.

Abstract. We study the structure and the internal colour gradients of cluster galaxies from UV to NIR restframe, in the redshift range z = 0.21−0.64. Structural parameters (half light radius r e , mean surface brightness <µ> e and Sersic index n) are derived for galaxies in the clusters A 209 at z = 0.21 and EIS 0048 at z = 0.64. This data set, together with previous data for the cluster AC 118 at z = 0.31, constitutes the first large (N ∼ 270) sample of cluster galaxies whose internal structure in UV, optical (OPT) and NIR (U-, V-and H-band restframe) can be investigated up to a look-back time of ∼6 Gyr (Ω m = 0.3, Ω Λ = 0.7 and H 0 = 70 km s −1 Mpc −1 ). Galaxies are classified as spheroids or disks according to the shape of the light profile, and the evolution of the two populations are investigated separately. On average, both spheroids and disks are more concentrated at longer wavelengths: the galaxy sizes become smaller from UV to NIR, while Sersic indices increase. This trend shows an evolution in disks, where the mean ratio of optical to NIR Sersic indices decreases from z = 0.31 to z = 0.64. Colour gradients are on average negative at all redshifts and are stronger in disks than in spheroids. But while for spheroids both grad (UV−OPT) and grad (OPT−NIR) are only weakly dependent on redshift, the optical-NIR gradients of disks become significantly smaller at z = 0.64. Colour gradients and central colours are compared with models of metallicity, age, and dust extinction gradients. Metallicity turns out to be the primary driver of colour gradients in spheroids, the age gradient being constrained to be smaller than ∼25%. For disks, two kinds of models fit the present data: (i) age gradients (in the range [30, 50]%) with significant dust extinction, and (ii) "pure" dust models, in which the gradients of colour excess are a factor of two higher in EIS 0048 than in the other clusters. Since colour gradients of disks seem not to correlate significantly with inclination, we argue that age gradient models could represent a more likely explanation of the present data, in agreement with what expected on the basis of hierarchical merging scenarios.

We study the evolution of the galaxy UV luminosity density as a function of redshift in the Hubble Deep Field North (HDF-N). We estimate the amount of energy absorbed by dust and hidden from optical observations by analyzing the HDF-N photometric data with the spectral energy distribution fitting method. According to our results, at redshifts 1 ≤ z ≤ 4.5, the global energy observed in the UV rest-frame at λ = 1500Å corresponds to only 7-11 % of the stellar energy output, the rest of it being absorbed by dust and re-emitted in the far-IR. Our estimates of the comoving star formation rate density in the universe from the extinction-corrected UV emission are consistent with the recent results obtained with Submillimeter Common-User Bolometer Array (SCUBA) at faint sub-millimeter flux levels.