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We present optical spectra of the nuclei of seven luminous (P[sub 178 MHz]&gsim;10[sup 25] W Hz[sup -1] Sr[sup -1]) nearby (z<0.08) radio galaxies, which mostly correspond to the FR II class. In two cases, Hydra A and 3C 285, the Balmer and λ4000-Å break indices constrain the spectral types and luminosity classes of the stars involved, revealing that the blue spectra are dominated by blue supergiant and/or giant stars. The ages derived for the last burst of star formation in Hydra A are between 7 and 40 Myr, and in 3C 285 about 10 Myr. The rest of the narrow-line radio galaxies (four) have a λ4000-Å break and metallic indices consistent with those of elliptical galaxies. The only broad-line radio galaxy in our sample, 3C 382, has a strong featureless blue continuum and broad emission lines that dilute the underlying blue stellar spectra. We are able to detect the Ca ii triplet in absorption in the seven objects, with good quality data for only four of them. The strengths of the absorptions are similar to those found in normal elliptical galaxies, but these values are consistent both with stellar populations of roughly similar ages (as derived from the Balmer absorption and break strengths) andQ2 with mixed young+old populations. [ABSTRACT FROM AUTHOR]

The analysis of the four-colour maps of galaxies in the Hubble Deep Field (HDF) has revealed, in the sample of 0.4

We investigate the nature of Lyman break galaxies (LBGs) using a chemodynamical model for the evolution of galaxies. Our model predicts an early (the first Gyr) stage of intense star formation in the evolution of massive spheroids which could be identified with the LBGs, observed at redshift ∼ 3 with strong ongoing star formation. In particular, we are successful in reproducing the properties of the LBG DSF 2237+116 C2 with a model describing a young ∼ L[sup *] spheroid. The comparison of the predictions of our models with the observations gives support to the scenario in which LBGs are the progenitors of present-day massive spheroids, i.e. bulges of luminous early-type spirals or luminous elliptical galaxies. [ABSTRACT FROM AUTHOR]

We present an analysis of quasar variability from data collected during a photometric monitoring of 50 objects carried out at CNPq/Laboratório Nacional de Astrofísicá, Brazil, between 1993 March and 1996 July. A distinctive feature of this survey is its photometric accuracy, ∼0.02 V mag, achieved through differential photometry with CCD detectors, which allows the detection of faint levels of variability. We find that the relative variability, δσL, observed in the V band is anticorrelated with both luminosity and redshift, although we have no means of discovering the dominant relation, given the strong coupling between luminosity and redshift for the objects in our sample. We introduce a model for the dependence of quasar variability on frequency that is consistent with multiwavelength observations of the nuclear variability of the Seyfert galaxy NGC 4151. We show that correcting the observed variability for this effect slightly increases the significance of the trends of variability with luminosity and redshift. Assuming that variability depends only on the luminosity, we show that the corrected variability is anticorrelated with luminosity and is in good agreement with predictions of a simple Poissonian model. The energy derived for the hypothetical pulses, ∼10[sup 50] erg, agrees well with those obtained in other studies. We also find that the radio-loud objects in our sample tend to be more variable than the radio-quiet ones, for all luminosities and redshifts. [ABSTRACT FROM AUTHOR]

We present a spectro-photometric catalogue of the evolution of supernova 1988Z, which combines new and published observations in the radio, optical and X-ray bands, with the aim of offering a comprehensive view of the evolution of this object and deriving the total energy radiated since its discovery. The major contribution to the total radiated energy comes at optical to X-ray frequencies, with a total emission of at least 2×10[sup 51] erg (for H[sub 0]=50 km s[sup -1] Mpc[sup -1]) in 8.5 yr. A model-dependent extrapolation of this value indicates that the total radiated energy may be as high as 10[sup 52] erg. The high value of the radiated energy supports a scenario in which most of the kinetic energy of the ejecta is thermalized and radiated in a short interaction with a dense circumstellar medium of nearly constant density. In this sense, 1988Z is not a supernova but a young and compact supernova remnant. [ABSTRACT FROM AUTHOR]