We have imaged more than thirty early-type galaxies in the K-band to investigate their stellar populations. Our surface brightness fluctuations (SBF) measurements for the nearest 14 galaxies produce a mean fluctuations magnitude in the K-band of The scatter in the Virgo cluster is small at 0.18 mag, which implies that infrared SBF is potentially a good distance indicator (Pahre & Mould 1994). Inspection of the simple stellar population tracks of Worthey (1994) suggests that a plot of the fluctuation color against broadband color (V – I) might be useful in discriminating between age and metallicity effects in elliptical galaxies. We have measured (r – K) color gradients for the entire sample utilizing three methods, two of them independent of sky-subtraction errors, as found in Sparks & Jørgensen (1993). Our color gradients are consistent with a mean metallicity gradient of 0.14 mag dex−1, which is somewhat smaller than that implied by optical color and line gradients alone, suggesting that age gradients may also be important. Finally, we have constructed an infrared Fundamental Plane (FP) which is consistent with its optical counterpart; continued work will determine if there is a significant change in the tilt of the FP between the optical and infrared.

Direct measures of luminosity functions, UV fluxes and colors of the hot stars that produce the UV turn–up in the SED of ellipticals and spiral bulges is the scientific goal of the HST observations presented here (see Bertola et al. 1995 for details). We concentrated our analysis on the hot stellar content of the M31 bulge. HST/FOC f/48 images were analyzed as observed (before the repair mission) through the F150W broad-band UV filter. We find that both individual stars and unresolved objects contribute to about 50% of the UV (1200-2450 Å) flux. Making use of the isochrones calculated by Bertelli et al. (1994) and Chiosi et al. (1994) together with the models of population synthesis presented by Bressan et al. (1994), we constructed theoretical CMDs to be compared with the CMD obtained by combining our F150W observations with the F175W observations of King et al. (1992). We conclude that the stars we resolved in M31 are classical P-AGB stars belonging to an old standard metallicity population. However, the P-EAGB and AGB-manqué stages as well as H-HB stars could be the source of the diffuse UV emission.

HST images of the E galaxies NGC 1399 and NGC 4552 and of the Sa NGC 2681 were obtained with FOC f/96 through the filters F175W, F220W, F275W and F342W. The most important discovery of these observations is the clear detection of central spikes in NGC 2681 and NGC 4552, that are prominent in each of the FOC images. No spike is detected in NGC 1399. The spikes appear unresolved by the 0″.07 core of the Pre-COSTAR PSF, corresponding to a physical size <~2-3 pc. We estimate V = 16.4 for the spike in NGC 2681 and V = 18.4 for that in NGC 4552, or ~ 107L⊙ in both cases. The spikes contribute less than a few percent of the UV flux sampled by IUE. We can offer three options for their nature: a central burst of star formation; effects of stellar interaction in a very dense environment; and a low level AGN. The first two options are currently entertained for the origin of the very dense star cluster at the center of our own Galaxy, whose size and luminosity are comparable to the corresponding limits for the two UV spikes (Genzel et al. 1994). A much fainter central spike (~ 105L⊙) is also seen at the center of NGC 205, and appears to be consistent with a modest burst of star formation (Bertola et al. 1995).

Recent observations with the Australia Telescope reveal that the elliptical galaxy NGC 5266 has a disk like structure of neutral hydrogen extending as far as almost 10 Re which approximatively lies along the galaxy's major axis, at 65° apart from the inner minor–axis dust lane (Varnas et al 1987). From the present data is not clear whether the HI structure and the dust lane are two distinct disks or a single warped structure. The regularity of the velocity field of the HI structure allow us to use it as a probe of the potential of NGC 5266. The velocity curve along the major axis is flat till the last measured point (rmax ~ 10′) at Vrot = 200km/s. Assuming that the gas in moving in circular orbits, we can derive the mass of the galaxy inside to this radius. The mass–to–light ratio M/LB rises from about 3 in the central regions to 12 at 9 Re (D = 57.6 Mpc), thus indicating that NGC 5266 is embedded in a dark massive halo. Moreover the representative point (cumulative M/LB within the last measured point) of NGC 5266 in the diagram log(M/LB) – log(Re) falls well within the region characteristic of spiral galaxies (Figure 2, Bertola et al. 1993), as do ellipticals previously studied in HI, thus reinforcing the suggestion (Bertola et al. 1993) of a parallel behaviour of the dark matter in elliptical and spiral galaxies.

We have analyzed a selection of globular clusters based on the indications of high metallicity from previous photometric studies.

Recent UV observations of elliptical galaxies are interpreted as evidence for the global second parameter phenomenon of horizontal-branch (HB) morphology within, as well as between, these galaxies. In this picture, the origin of the UV radiation is mostly due to hot HB stars and their post-HB progeny produced by the metal-poor tail of the wide metallicity distribution expected to be present in these systems. The attractive feature of this model is that the bimodal temperature distributions of HB stars (and their progeny), required to generate the 2000 Å dip of the spectral energy distribution (SED), can naturally be reproduced from the standard HB population models with large range of metal abundance (see Lee 1994, ApJ, 430, L113). Detailed population synthesis models are presented, which reproduce the systematic variation of UV upturn among elliptical galaxies (Fig 1). If age is the major second parameter, as suggested by the fossil record in our Galaxy, the observed UV color gradient and the UV upturn-total mass (mean metallicity) correlation, within and between the early-type systems, would imply, respectively, (1) that most galaxies formed from the inside out, and (2) that there is age spread among galaxies, in the sense that more massive galaxies are older (and more metal-rich in the mean) than less massive galaxies as a result of more efficient star formation (and metal enrichment) in denser environments.

I present a preliminary analysis of three UV images of the Virgo cluster region, obtained by the FAUST experiment (Bowyer et al. 1993) in April 1992. FAUST is a wide-field imaging telescope covering an 8° area with effective resolution of 3′.5. The experiment was constructed by the Berkeley Space Astronomy Group together with the Laboratoire d'Astrophysique Spatiale of the CNES. It operated on board the Shuttle during the mission ATLAS-1.

We study the stellar orbits, as a function of the binary motion of two identical ellipticals, under initial conditions marginally sufficient for strong interaction. The stars were initially given circularly symmetric velocities. The tidal effects cause a redistribution of stellar orbits, resulting in crowding of stars in shells; the same attaining its maximum intensity slightly after a pericentric passage. As the galaxies recede, the structure disperses gradually by expanding; but is restored, intensified and forms at a shorter radial distance as the galaxies return for a subsequent approach in a shrinking orbit. We give the stellar positions, projected perpendicular to the orbital plane, shortly after the first (t≈0.5) and second (t≈6) pericentric passages in the figures; (time being given in dimensionless units corresponding to mass=l, radius=l, G=4.50). On the basis of the cooling gas inflow model, the gas will be compressed and shocked in these regions of enhanced stellar density, leading to bursts of star formation. The interval between the two successive starbursts is found to be of the same order as the trapping time needed by the galaxy to incorporate the gas ejected by stars in its cooling flow.

Line-strengths and their gradients in Mg, Fe, and Hβ have been determined for a sample of 9 brightest cluster (BCG), 7 elliptical, and 15 S0 galaxies in order to study their stellar populations and investigate their relationship to one another. We find that BCGs follow the same relationship between central Mgb line-strength and central velocity dispersion found for ellipticals while the S0 galaxies show significant scatter with respect to this relation. Brightest cluster galaxies are in agreement with the known trend towards more massive ellipticals having larger [Mg/Fe] ratios while the internal gradients within our BCG and E galaxies are consistent with a roughly constant [Mg/Fe] ratio. We find that a correlation exists between the central [Mg/Fe] ratio and average Hβ line-strength in the sense that BCG and E galaxies with larger [Mg/Fe] ratios have lower Hβ strengths. For our BCG and E galaxies, Hβ is the best predictor of [Mg/Fe] ratio. The Mgb metallicity gradients for BCGs and ellipticals are similar and consistent with a reduction in the mean metallicity of the stellar population by about a factor of 2 over a factor of ten in radius. No strong correlations are found between the metallicity gradient sizes and either kinematic or line-strength parameters of the E and BCG galaxies. The S0 disks display roughly constant Mg, Fe, and Hβ line-strengths with radius indicating that they have uniform age and metallicity throughout. S0 galaxy minor axes ‘bulge’ metal line-strength gradients are similar to elliptical gradients and fall to values lower than those found in the disks.

Samples of groups, clusters and superclusters of galaxies have been collected from existing catalogs. The group sample consisted of 70 systems, the list of clusters contains 50 systems, and there are five superclusters. Kinematical and dynamical characteristics are obtained — elongation (along the line of sight, RA and Dec), velocity statistics, total luminosity, harmonic radius, mean pair separation, crossing time, virial mass … We have compared some of the observational features with dynamical models of groups, clusters and superclusters. Aarseth's (1985) code is used.

Analysis of cosmological evolution effects in the low-frequency source counts at 25 MHz obtained with the UTR-2 radio telescope and conclusions on the large-scale extragalactic radio source space distribution are presented. The data require the existence of a decrease in the space distribution of the most distant extended radio sources.

The tidal effects of disruption and merger of a massive perturber-quasar on a galaxy cluster have been studied by computer simulations using Aarseth's NBODY2 code. The model consists of a spherical N-body galaxy cluster and a point-mass perturber. A wide range of initial conditions (mass ratio of the galaxy and the perturber, the virial coefficient q of the N-body system, and the pericentric distances of orbits) have been considered.

Our chemical evolution models describe in detail the enrichment process of a number of individual elements from 12C to 56Fe, including delayed SNI contributions. Variation of the characteristic timescale of star formation (SF) t∗ not only results in differences in the abundance evolution but also changes abundance ratios of elements originating from different nucleosvnthetic sites, as e.g. [C/O], [O/Fe], or [Mg/Fe].

Spectra of single-aged old stellar populations of metallicities in the range −1.0 ≤ [Fe/H] ≤ +0.5 are built in the wavelength range 6950-7550 Å including TiO bands, by combining synthetic spectra of individual stars. Two approaches are applied: one for moderately metal-poor populations, entirely based on isochrones, and the other one applied to the metal-rich Galactic globular clusters, based on isochrones for the main sequence and data of observed colour-magnitude diagrams for the evolved stages. Abundance ratios of [α/Fe] = +0.3 are adopted for populations of [Fe/H] = −1.0, −0.5, and both 0.0 and +0.3 for the more metal-rich ones.

We analyzed – in terms of age and metallicity – the stellar content of blue galaxies in 3 distant clusters at z ~ 0.3. We found that they contain a more metal-rich population than nearby late-type spirals.

The “Butcher–Oemler” effect was originally defined as the excess of blue galaxies observed in distant rich clusters when compared to local counterparts. Subsequent observations revealed that a larger fraction of objects in clusters between 0.1 < z < 1 show signs in their spectra of the presence of young stars, i.e. of a recent (during the last 2 Gyrs) or current burst of star formation.

Starburst galaxies are ideal laboratories to study both the physics of massive stars and processes important in galaxy formation and evolution. Observations of starburst galaxies at UV wavelengths are crucial because only in this spectral regime can we directly observe the spectroscopic signatures of the hot, massive stars that power the emission at other wavebands. UV signatures from massive stars are mainly broad spectral features (e.g. P Cygni profiles of SiIVλ1400, and CIVλ1550) formed in the stellar winds.

The nature of the emission-line galaxies up to z ≈ 0.3 in the CFRS was analysed. The (4500–8500Å) spectral range allowed us to measure the Hα line intensity until z ≈ 0.3 and to correct for reddening. This permitted us to examine the physical properties of these galaxies in the light of a new grid of photoionisation models, obtained with the code PHOTO (Stasińska 1990). This grid was used to define the loci for photoionisation by hot main sequence stars in two diagnostic diagrams. As ionisation source we used the Kurucz (1992) log g = 5 stellar model atmospheres with abundances consistent with those of the model nebulae. The photoionisation models reproduce the separation zone between the objects thermally and non-thermally excited in the [S II] λ6725/Hα versus [O III]λ5007/Hβ diagnostic diagram, which Veilleux and Osterbrock (1987) determined empirically. A equivalent separation zone was defined for the [O III] λ5007/Hβ versus [O II] λ3727/Hβ diagram. In spite of its reddening dependence, this diagram is useful to separate the H II region-like galaxies from the non-thermally excited ones. Using both diagrams the analysis of the CFRS data allowed us to obtain the following statistics about the nature of the objects: about 20 % of all the galaxies with z ≤ 0.3 have spectra with properties intermediate between Seyfert 2 galaxies and LINERs (compared to 2 % found in the local Universe; Huchra & Burg 1992). More details of this work are given in Tresse et al. (1994). A full version will be submitted to MNRAS.

We have spectra, colours and morphologies from the CFHT for 240 cluster members and 80 field galaxies over a 7 × 46 arcmin field. The cluster galaxies show strong radial gradients in colour, morphology, and spectrum. The central group has only red early-type galaxies of high central concentration, which must have formed at least 5 Gy ago. The population becomes bluer and shows an increasing fraction of disk morphology and recent star-formation with clustocentric radius, and eventually blends into the field. However, the fraction of blue galaxies is significantly lower than in Butcher-Oemler clusters, and there are only two starburst galaxies (one of which is the cluster cD). Fitting of line measures with stellar population models indicates that star-formation has been truncated in the 15-20% of the cluster galaxies that have strong Hδ absorption, and that these galaxies are not all blue. There is evidence that cluster galaxies are dusty compared with the field. The morphology gradient can be fit with models of disk fading. However, there is a small fraction of interacting and merging galaxies, which must play a part in the population evolution. The cluster is accreting from the field in a non-violent way, including some distinct subgroups. This is different from what is seen at both higher and lower redshift, and if generally true, suggests a high Omega universe.

These results are being published in detail elsewhere.