We use Hubble Space Telescope (HST) near-infrared imaging to explore the shapes of the surface brightness profiles of bulges of S0-Sbc galaxies at high resolution. Modeling extends to the outer bulge via bulge-disk decompositions of combined HST-ground-based profiles. Compact, central unresolved components similar to those reported by others are found in ∼84% of the sample. We also detect a moderate frequency (∼34%) of nuclear components with exponential profiles that may be disks or bars. Adopting the Sérsic functional form 1/n r for the bulge, none of the bulges have an behavior; derived Sérsic shape indices are. For 1/4 r AnS p 1.7 ‫ע‬ 0.7 the same sample, fits to near-infrared ground-based profiles yield Sérsic indices up to. The high n of n p 4-6 ground-based profiles are the result of nuclear point sources blending with the extended light of the bulge because of seeing. The low Sérsic indices are not expected from violent relaxation in mergers.

Some recent observations seem to disagree with hierarchical theories of galaxy formation about the role played by major mergers in the late buildup of massive E-S0's. We re-address this question by analysing the morphology, structural distortion level, and star formation enhancement of a sample of massive galaxies (M * > 5 × 10 10 M ⊙ ) lying on the Red Sequence and its surroundings at 0.3 < z < 1.5. We have used an initial sample of ∼1800 sources with K s < 20.5 mag over an area ∼ 155 arcmin 2 on the Groth Strip, combining data from the Rainbow Extragalactic Database and the GOYA Survey. Red galaxy classes that can be directly associated to intermediate stages of major mergers and to their final products have been defined. We report observational evidence of the existence of a dominant evolutionary path among massive red galaxies at 0.6 < z < 1.5, consisting in the conversion of irregular disks into irregular spheroids, and of these ones into regular spheroids. This result implies: 1) the massive red regular galaxies at low redshifts derive from the irregular ones populating the Red Sequence and its neighbourhood at earlier epochs up to z ∼ 1.5; 2) the progenitors of the bulk of present-day massive red regular galaxies have been disks that seem to have migrated to the Red Sequence mostly through major mergers at 0.6 < z < 1.2 (these mergers thus starting at z ∼ 1.5); and 3) the formation of E-S0's that end up with M * > 10 11 M ⊙ at z = 0 through gas-rich major mergers has frozen since z ∼ 0.6. All these facts support that major mergers have played a dominant role in the definitive buildup of present-day E-S0's with M * > 10 11 M ⊙ at 0.6 < z < 1.2, in good agreement with hierarchical scenarios of galaxy formation.

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Context. The chronology of bulge and disk formation is a major unsolved issue in galaxy formation, which impacts on our global understanding of the Hubble sequence. Aims. We analyse colours of the nuclear regions of intermediate redshift disk galaxies, with the aim of obtaining empirical information of relative ages of bulges and disks at 0.1 < z < 1.3. Methods. We work with an apparent-diameter limited parent sample of 248 galaxies from the HST Groth Strip Survey. We apply a conservative criterion to identify bulges and potential precursors of present-day bulges based on nuclear surface brightness excess above the exponential profile of the outer parts and select a sample of 56 galaxies with measurable bulges. We measure bulge colours on wedge profiles opening on the semi-minor axis least affected by dust in the disk, and compare them to disk, and global galaxy colours.Results. For 60% of galaxies with bulges, the rest-frame nuclear colour distribution shows a red sequence that is well fit by passive evolution models of various ages, while the remainder 40% scatters towards bluer colours. In contrast, galaxies without central brightness excess show typical colours of star forming population and lack a red sequence. We also see that, as in the local Universe, most of the minor axis colour profiles are negative (bluer outward), and fairly gentle, indicating that nuclear colours are not distinctly different from disk colours. This is corroborated when comparing nuclear, global and disk colours: these show strong correlations, for any value of the central brightness prominence of the bulge. No major differences are found between the low and high inclination samples, both for the bulge and non-bulge samples. Conclusions.Comparison with synthetic models of red sequence bulge colours suggests that such red bulges have stopped forming stars at an epoch earlier than ∼1 Gyr before the observation. The correlation between nuclear and disk colours and the small colour gradients hints at an intertwined star formation history for bulges and disks: probably, most of our red bulges formed in a process in which truncation of star formation in the bulge did not destroy the disk.

Context. It is thought that fossil systems are relics of structure formation in the primitive Universe. They are galaxy aggregations that have assembled their mass at high redshift with few or no subsequent accretion. Observationally these systems are selected by large magnitude gaps between their 1st and 2nd ranked galaxies (∆m 12 ). Nevertheless, there is still debate over whether or not this observational criterium selects dynamically evolved ancient systems. Aims. We have studied the properties of the nearby fossil group RXJ075243.6+455653 in order to understand the mass assembly of this system. Methods. Deep spectroscopic observations allow us to construct the galaxy luminosity function (LF) of RXJ075243.6+455653 down to M * r + 6. The analysis of the faint-end of the LF in groups and clusters provides valuable information about the mass assembly of the system. In addition, we have analyzed the nearby large-scale structure around this group. The LF of the group shows a flat faint-end slope (α = −1.08 ± 0.33). This low density of dwarf galaxies is confirmed by the low value of the dwarf-to-giant ratio (DGR = 0.99 ± 0.49) for this system. Both the lack of dwarf galaxies and the low luminosity of the BGG suggests that RXJ075243.6+455653 still has to accrete mass from its nearby environment. This mass accretion will be achieved because it is the dominant structure of a rich environment formed by several groups of galaxies (15) within ∼ 7 Mpc from the group center and with ±1000 km s −1 . Conclusions. RXJ075243.6+455653 is a group of galaxies that has not yet completed the process of its mass assembly. This new mass accretion will change the fossil state of the group. This group is an example of a galaxy aggregation selected by a large magnitude gap but still in the process of the accretion of its mass.