Showing papers by "European Southern Observatory published in 2003"

Cosmological Results from High-z Supernovae* **

Abstract: The High-z Supernova Search Team has discovered and observed eight new supernovae in the redshift interval z = 0.3-1.2. These independent observations, analyzed by similar but distinct methods, confirm the results of Riess and Perlmutter and coworkers that supernova luminosity distances imply an accelerating universe. More importantly, they extend the redshift range of consistently observed Type Ia supernovae (SNe Ia) to z ≈ 1, where the signature of cosmological effects has the opposite sign of some plausible systematic effects. Consequently, these measurements not only provide another quantitative confirmation of the importance of dark energy, but also constitute a powerful qualitative test for the cosmological origin of cosmic acceleration. We find a rate for SN Ia of (1.4 ± 0.5) × 10-4 h3 Mpc-3 yr-1 at a mean redshift of 0.5. We present distances and host extinctions for 230 SN Ia. These place the following constraints on cosmological quantities: if the equation of state parameter of the dark energy is w = -1, then H0t0 = 0.96 ± 0.04, and ΩΛ - 1.4ΩM = 0.35 ± 0.14. Including the constraint of a flat universe, we find ΩM = 0.28 ± 0.05, independent of any large-scale structure measurements. Adopting a prior based on the Two Degree Field (2dF) Redshift Survey constraint on ΩM and assuming a flat universe, we find that the equation of state parameter of the dark energy lies in the range -1.48 -1, we obtain w < -0.73 at 95% confidence. These constraints are similar in precision and in value to recent results reported using the WMAP satellite, also in combination with the 2dF Redshift Survey.

New Constraints on ΩM, ΩΛ, and w from an Independent Set of 11 High-Redshift Supernovae Observed with the Hubble Space Telescope*

Abstract: We report measurements of $\Omega_M$, $\Omega_\Lambda$, and w from eleven supernovae at z=0.36-0.86 with high-quality lightcurves measured using WFPC-2 on the HST. This is an independent set of high-redshift supernovae that confirms previous supernova evidence for an accelerating Universe. Combined with earlier Supernova Cosmology Project data, the new supernovae yield a flat-universe measurement of the mass density $\Omega_M=0.25^{+0.07}_{-0.06}$ (statistical) $\pm0.04$ (identified systematics), or equivalently, a cosmological constant of $\Omega_\Lambda=0.75^{+0.06}_{-0.07}$ (statistical) $\pm0.04$ (identified systematics). When the supernova results are combined with independent flat-universe measurements of $\Omega_M$ from CMB and galaxy redshift distortion data, they provide a measurement of $w=-1.05^{+0.15}_{-0.20}$ (statistical) $\pm0.09$ (identified systematic), if w is assumed to be constant in time. The new data offer greatly improved color measurements of the high-redshift supernovae, and hence improved host-galaxy extinction estimates. These extinction measurements show no anomalous negative E(B-V) at high redshift. The precision of the measurements is such that it is possible to perform a host-galaxy extinction correction directly for individual supernovae without any assumptions or priors on the parent E(B-V) distribution. Our cosmological fits using full extinction corrections confirm that dark energy is required with $P(\Omega_\Lambda>0)>0.99$, a result consistent with previous and current supernova analyses which rely upon the identification of a low-extinction subset or prior assumptions concerning the intrinsic extinction distribution.

A very energetic supernova associated with the |[gamma]|-ray burst of 29 March 2003

Abstract: Over the past five years evidence has mounted that long-duration (greater than 2s) gamma-ray bursts (GRBs), the most brilliant of all astronomical explosions, signal the collapse of massive stars in our Universe. This evidence, originally based on the probable association of one unusual GRB with a supernova, now includes the association of GRBs with regions of massive star-formation in distant galaxies, tantalizing evidence of supernova-like light-curve 'bumps' in the optical afterglows of several bursts, and lines of freshly synthesized elements in the spectra of a few X-ray afterglows. These observations support, but do not yet conclusively validate, models based upon the deaths of massive stars, presumably associated with core collapse. Here we report evidence for a very energetic supernova (a hypernova), temporally and spatially coincident with a GRB at redshift z=0.1685. The timing of the supernova indicates that it exploded within a few days of the GRB, strongly suggesting that core-collapse events can give rise to GRBs. Amongst the GRB central engine models proposed to-date, the properties of this supernova thus favour the collapsar model.

The Stellar Cusp around the Supermassive Black Hole in the Galactic Center

Abstract: We analyze deep near-IR adaptive optics imaging (taken with NAOS/CONICA on the Very Large Telescope at the European Southern Observatory, Chile), as well as new proper-motion data of the nuclear star cluster of the Milky Way The surface density distribution of faint (H ≤ 20, Ks ≤ 19) stars peaks within 02 of the black hole candidate Sgr A* The radial density distribution of this stellar "cusp" follows a power law of exponent α ~ 13-14 The K-band luminosity function of the overall nuclear stellar cluster (within 9'' of Sgr A*) resembles that of the large-scale Galactic bulge but shows an excess of stars at Ks≤ 14 It fits population synthesis models of an old, metal-rich stellar population with a contribution from young, early, and late-type stars at the bright end In contrast, the cusp within ≤15 of Sgr A* appears to have a featureless luminosity function, suggesting that old, low-mass, horizontal-branch/red-clump stars are lacking Likewise, there appear to be fewer late-type giants The innermost cusp also contains a group of moderately bright, early-type stars that are tightly bound to the black hole We interpret these results as evidence that the stellar properties change significantly from the outer cluster (≥a few arcseconds) to the dense innermost region around the black hole We find that most of the massive early-type stars at distances of 1''-10'' from Sgr A* are located in two rotating and geometrically thin disks These disks are inclined at large angles and counterrotate with respect to each other Their stellar content is essentially the same, indicating that they formed at the same time We conclude that of the possible formation scenarios for these massive stars the most probable one is that 5-8 million years ago two clouds fell into the center, collided, were shock compressed, and then formed two rotating (accretion) disks orbiting the central black hole For the OB stars in the central arcsecond, on the other hand, a stellar merger model is the most appealing explanation These stars may thus be "super-blue stragglers," formed and "rejuvenated" through mergers of lower mass stars in the very dense (≥108 M☉ pc-3) environment of the cusp The "collider model" also accounts for the lack of giants within the central few arcseconds The star closest to Sgr A* in 2002, S2, exhibits a 38 μm excess We propose that the mid-IR emission comes either from the accretion flow around the black hole itself or from dust in the accretion flow that is heated by the ultraviolet emission of S2

The Extremely Metal-poor, Neutron Capture-rich Star CS 22892-052: A Comprehensive Abundance Analysis

Abstract: High-resolution spectra obtained with three ground-based facilities and the Hubble Space Telescope (HST) have been combined to produce a new abundance analysis of CS 22892-052, an extremely metal-poor giant with large relative enhancements of neutron capture elements. A revised model stellar atmosphere has been derived with the aid of a large number of Fe peak transitions, including both neutral and ionized species of six elements. Several elements, including Mo, Lu, Au, Pt, and Pb, have been detected for the first time in CS 22892-052, and significant upper limits have been placed on the abundances of Ga, Ge, Cd, Sn, and U in this star. In total, abundance measurements or upper limits have been determined for 57 elements, far more than previously possible. New Be and Li detections in CS 22892-052 indicate that the abundances of both these elements are significantly depleted compared to unevolved main-sequence turnoff stars of similar metallicity. Abundance comparisons show an excellent agreement between the heaviest n-capture elements (Z ≥ 56) and scaled solar system r-process abundances, confirming earlier results for CS 22892-052 and other metal-poor stars. New theoretical r-process calculations also show good agreement with CS 22892-052 abundances and the solar r-process abundance components. The abundances of lighter elements (40 ≤ Z ≤ 50), however, deviate from the same scaled abundance curves that match the heavier elements, suggesting different synthesis conditions or sites for the low-mass and high-mass ends of the abundance distribution. The detection of Th and the upper limit on the U abundance together imply a lower limit of 10.4 Gyr on the age of CS 22892-052, quite consistent with the Th/Eu age estimate of 12.8± 3 Gyr. An average of several chronometric ratios yields an age 14.2± 3 Gyr.

VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. I. Nucleosynthesis and Abundance Ratios*

Abstract: We have used the Ultraviolet Echelle Spectrograph (UVES) on Kueyen (UT2) of the Very Large Telescope to take spectra of 15 individual red giants in the Sculptor, Fornax, Carina, and Leo I dwarf spheroidal galaxies (dSph's). We measure the abundances of α-, iron peak, first s-process, second s-process, and r-process elements. No dSph giants in our sample show the deep mixing abundance pattern (O and sometimes Mg depleted, while Na and Al are enhanced) seen in nearly all globular clusters. At a given metallicity the dSph giants exhibit lower [el/Fe] abundance ratios for the α-elements than stars in the Galactic halo. The low α abundances at low metallicities can be caused by a slow star formation rate and contribution from Type Ia SNe, and/or a small star formation event (low total mass) and mass-dependent Type II SN yields. In addition, Leo I and Sculptor exhibit a declining even-Z [el/Fe] pattern with increasing metallicity, while Fornax exhibits no significant slope. In contrast, Carina shows a large spread in the even-Z abundance pattern, even over small metallicity ranges, as might be expected from a bursting star formation history. The metal-poor stars in these dSph galaxies ([Fe/H] < -1) have halo-like s- and r-process abundances, but not every dSph exhibits the same evolution in the s- and r-process abundance pattern. Carina, Sculptor, and Fornax show a rise in the s-/r-process ratio with increasing metallicity, evolving from a pure r-process ratio to a solar-like s- and r-process ratio. On the other hand, Leo I, appears to show an r-process–dominated ratio over the range in metallicities sampled. At present, we attribute these differences in the star formation histories of these galaxies. Comparison of the dSph abundances with those of the halo reveals some consistencies with the Galactic halo. In particular, Nissen & Shuster found that their metal-rich, high Rmax high zmax halo stars exhibited low [α/Fe], [Na/Fe] and [Ni/Fe] abundance ratios. In the same abundance range our dSph exhibit the same abundance pattern, supporting their suggestions that disrupted dSph's may explain up to 50% of the metal-rich halo. Unfortunately, similar comparisons with the metal-poor Galactic halo have not revealed similar consistencies, suggesting that the majority of the metal-poor Galactic halo could not have been formed from objects similar to the dSph studied here. We use the dSph abundances to place new constraints on the nucleosynthetic origins of several elements. We attribute differences in the evolution of [Y/Fe] in the dSph stars versus the halo stars to a very weak AGB or SN Ia yield of Y (especially compared with Ba). That a lower and flatter Ba/Y ratio is seen in the halo is most likely a result of the pattern being erased by the large metallicity dispersion in the halo. Also, we find [Cu/Fe] and [Mn/Fe] are flat and halo-like over the metallicity city range -2 < [Fe/H] < -1.2, and that the [Cu/α] ratios are flat. Combining these abundances with knowledge of the age spread in these galaxies suggests that SNe Ia are not the main site for the production of Cu (and Mn) in very metal-poor stars. We suggest that metallicity-dependent SN yields may be more promising.

A significant population of red, near-infrared-selected high-redshift galaxies

Abstract: We use very deep near-infrared photometry of the Hubble Deep Field-South taken with ISAAC on the Very Large Telescope to identify a population of high-redshift galaxies with rest-frame optical colors similar to those of nearby galaxies. The galaxies are chosen by their infrared colors Js-Ks > 2.3, aimed at selecting galaxies with redshifts above 2. When applied to our data set, we find 14 galaxies with Ks < 22.5, corresponding to a surface density of 3 ± 0.8 arcmin-2. The photometric redshifts all lie above 1.9, with a median of 2.6 and an rms of 0.7. The spectral energy distributions of these galaxies show a wide range. One is very blue in the rest-frame UV and satisfies the normal Lyman break criteria for high-redshift, star-forming galaxies. Others are quite red throughout the observed spectral range and are extremely faint in the optical, with a median V = 26.6. Hence, these galaxies would not be included in photometric samples based on optical ground-based data, and spectroscopic follow-up is difficult. The spectral energy distributions often show a prominent break, identified as the Balmer break or the 4000 A break. The median age is 1 Gyr when fitted with a constant star formation model with dust or 0.7 Gyr when fitted with a single burst model. Although significantly younger ages cannot be excluded when a larger range of models is allowed, the results indicate that these galaxies are among the oldest at these redshifts. The volume density to Ks = 22.5 is half that of Lyman break galaxies at z ≈ 3. Since the mass-to-light ratios of the red galaxies are likely to be higher, the stellar mass density is inferred to be comparable to that of Lyman break galaxies. These red galaxies may be the descendants of galaxies that started to form stars at very high redshifts, and they may evolve into the most massive galaxies at low redshift.

Age and Metallicity Distribution of the Galactic Bulge from Extensive Optical and Near-IR Stellar Photometry

Abstract: We present a new determination of the metallicity distribution, age, and luminosity function of the Galactic bulge stellar population. By combining near-IR data from the 2MASS survey, from the SOFI imager at ESO NTT and the NICMOS camera on board HST we were able to construct color-magnitude diagrams (CMD) and luminosity functions (LF) with large statistics and small photometric errors from the Asymptotic Giant Branch (AGB) and Red Giant Branch (RGB) tip down to ∼0.15 M� . This is the most extended and complete LF so far obtained for the galactic bulge. Similar near-IR data for a disk control field were used to decontaminate the bulge CMDs from foreground disk stars, and hence to set a stronger constraint on the bulge age, which we found to be as large as that of Galactic globular clusters, or >10 Gyr. No trace is found for any younger stellar population. Synthetic CMDs have been constructed to simulate the effect of photometric errors, blending, differential reddening, metallicity dispersion and depth effect in the comparison with the observational data. By combining the near-IR data with optical ones, from the Wide Field Imager at the ESO/MPG 2.2 m telescope, a disk-decontaminated (MK,V-K )C MD has been constructed and used to derive the bulge metallicity distribution, by comparison with empirical RGB templates. The bulge metallicity is found to peak at near solar value, with a sharp cutoff just above solar, and a tail towards lower metallicity that does not appreciably extend below (M/H) ∼− 1.5.

SINFONI: integral field spectroscopy at 50-milli-arcsecond resolution with the ESO VLT

Abstract: SINFONI is an adaptive optics assisted near-infrared integral field spectrometer for the ESO VLT. The Adaptive OPtics Module (built by the ESO Adaptive Optics Group) is a 60-elements curvature-sensor based system, designed for operations with natural or sodium laser guide stars. The near-infrared integral field spectrometer SPIFFI (built by the Infrared Group of MPE) provides simultaneous spectroscopy of 32 x 32 spatial pixels, and a spectral resolving power of up to 3300. The adaptive optics module is in the phase of integration; the spectrometer is presented tested in the laboratory. We provide an overview of the project, with particular emphasis on the problems encountered in designing and building an adaptive optics assisted spectrometer.

Metallicity of the intergalactic medium using pixel statistics. ii. the distribution of metals as traced by c iv

Abstract: We measure the distribution of carbon in the intergalactic medium as a function of redshift z and overdensity δ. Using a hydrodynamical simulation to link the H I absorption to the density and temperature of the absorbing gas, and a model for the UV background radiation, we convert ratios of C IV to H I pixel optical depths into carbon abundances. For the median metallicity this technique was described and tested in Paper I of this series. Here we generalize it to reconstruct the full probability distribution of the carbon abundance and apply it to 19 high-quality quasar absorption spectra. We find that the carbon abundance is spatially highly inhomogeneous and is well described by a lognormal distribution for fixed δ and z. Using data in the range log δ = -0.5-1.8 and z = 1.8-4.1, and a renormalized version of the 2001 Haardt & Madau model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = -3.47 + 0.08(z - 3) + 0.65(log δ - 0.5) and a lognormal scatter of σ([C/H]) = 0.76 + 0.02(z - 3) - 0.23(log δ - 0.5). Thus, we find significant trends with overdensity but no evidence for evolution. These measurements imply that gas in this density range accounts for a cosmic carbon abundance of [C/H] = -2.80 ± 0.13 (ΩC ≈ 2 × 10-7), with no evidence for evolution. The dominant source of systematic error is the spectral shape of the UV background, with harder spectra yielding higher carbon abundances. While the systematic errors due to uncertainties in the spectral hardness may exceed the quoted statistical errors for δ < 10, we stress that UV backgrounds that differ significantly from our fiducial model give unphysical results. The measured lognormal scatter is strictly independent of the spectral shape, provided the background radiation is uniform. We also present measurements of the C III/C IV ratio (which rule out temperatures high enough for collisional ionization to be important for the observed C IV) and of the evolution of the effective Lyα optical depth.

NAOS, the first AO system of the VLT: on-sky performance

Abstract: NAOS is the first adaptive optics system installed at the VLT 8m telescopes. It was designed, manufactured and tested by a french Consortium under an ESO contract, to provide compensated images to the high angular resolution IR spectro-imaging camera (CONICA) in the 1 to 5 μ m spectral range. It is equipped with a 185 actuator deformable mirror, a tip/tilt mirror and two wavefront sensors, one in the visible and one in the near IR spectral range. It has been installed in November at the Nasmyth focus B of the VLT UT4. During the first light run in December 2001, NAOS has delivered a Strehl ratio of 50 under average seeing conditions for bright guide stars. The diffraction limit of the telescope has been achieved at 2.2 μ m . The closed loop operation has been very robust under bad seeing conditions. It was also possible to obtain a substantial correction with m V=17.6 and m K=13.1 reference stars. The on-sky acceptance tests of NAOS-CONICA were completed in May 2002 and the instrument will be made available to the European astronomical community in October by ESO. This paper describes the system and present the on-sky performance in terms of Strehl ratio, seeing conditions and guide star magnitude.

NAOS-CONICA first on sky results in a variety of observing modes

Abstract: The Adaptive Optics NIR Instrument NAOS-CONICA has been commissioned at the VLT (UT4) between November 2001 and March 2002. After summarizing the observational capabilities of this multimode instrument in combination with the powerful AO-system, we will present first on sky results of the instrumental performance for several non-direct imaging modes: High spatial resolution slit-spectroscopy in the optical and thermal NIR region has been tested. For compact sources below 2 arcsec extension, Wollaston prism polarimetry is used. For larger objects the linear polarization pattern can be analyzed by wire grids down to the diffraction limit. Coronographic masks are applied to optimize imaging and polarimetric capabilities. The cryogenic Fabry-Perot Interferometer in combination with an 8m-telescope AO-system is shown to be a powerful tool for imaging spectroscopy (3D-scans).

CRIRES: a high-resolution infrared spectrograph for ESO's VLT

Abstract: CRIRES, a pre-dispersed CRyogenic Infrared Echelle Spectrograph, provides a resolving power λ/Δλ ≈ 105 (or Δv ≈ 1.5km/s per pixel) between 1000 and 5000 nm at the 8m ESO VLT-UT 1. A curvature sensing adaptive optics system feed is used to minimize slit losses and to provide 0.2” spatial resolution along the slit. A mosaic of 4 Aladdin InSb-arrays packaged on custom-fabricated ceramics boards provides for an effective 5k x 0.5k pixel focal plane array. Remote insertion of gas cells to measure high precision radial velocities is possible. A linear and circular polarization mode for magnetic Doppler imaging of stellar surfaces is foreseen with motorized retarders in combination with a Wollaston prism. The major design features of CRIRES and a glimpse tutorial preview of astronomical data are given.

Multiplicity of Nearby Free-floating Ultracool Dwarfs: A Hubble Space Telescope WFPC2 Search for Companions

Abstract: We present Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) observations of a sample of 134 ultracool objects (spectral types later than M7) coming from the Deep Near Infrared Survey (DENIS), Two Micron All Sky Survey (2MASS), and Sloan Digital Sky Survey (SDSS), with distances estimated to range from 7 to 105 pc. Fifteen new ultracool binary candidates are reported here. Eleven known binaries are confirmed, and orbital motion is detected in some of them. We estimate that the closest binary systems in this sample have periods between 5 and 20 yr, and thus dynamical masses will be derived in the near future. For the calculation of binary frequency, we restrict ourselves to systems with distances less than 20 pc. After correction of the binaries bias, we find a ratio of visual binaries (at the HST limit of detection) of around 10%, and that ~15% of the 26 objects within 20 pc are binary systems with separations between 1 and 8 AU. The observed frequency of ultracool binaries is similar to that of binaries with G-type primaries in the separation range from 2.1 to 140 AU. There is also a clear deficit of ultracool binaries with separations greater than 15 AU, and a possible tendency for the binaries to have mass ratios near unity. Most systems have indeed visual and near-infrared brightness ratios between 1 and 0.3. We discuss our results in the framework of current scenarios for the formation and evolution of free-floating brown dwarfs.

The Very Large Telescope Ultraviolet and Visible Echelle Spectrograph survey for molecular hydrogen in high‐redshift damped Lyman α systems

Abstract: We have searched for molecular hydrogen in damped Lyman α (DLA) and sub-DLA systems at high redshift (z abs > 1.8) using the Ultraviolet and Visible Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) down to a detection limit of typically N(H 2 ) = 2 × 10 14 cm -2 . Out of the 33 systems in our sample, eight have firm and two have tentative detections of associated H 2 absorption lines. Considering that three detections were already known from past searches, molecular hydrogen is detected in 13-20 per cent of the newly surveyed systems. We report new detections of molecular hydrogen at z abs = 2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z abs = 3.025 toward Q 0347-383. In all of the systems, we measure metallicities relative to solar, [X/H] (with either X = Zn, S or Si), and depletion factors of Fe, [X/Fe], supposedly on to dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between the metallicity and the depletion factor in both our sample and also the global population of DLA systems. Although H 2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H 2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H 2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H 2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H 2 -detected components even has [Zn/Fe]≥1.4. These are the largest depletion factors ever seen in DLA systems. All of this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H 2 molecular fraction, f= 2N (H 2 )/[2N(H 2 ) +N(H i)], is generally small in DLA systems (typically log f 2 and the H i column density. In fact, two systems where H 2 is detected have log N(H i) 2 on to dust grains is reduced in those systems, probably because the gas is warm (T > 1000 Κ) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.

Photometry and Spectroscopy of the Type IIP SN 1999em from Outburst to Dust Formation

Abstract: We present photometry and spectra of the Type IIP supernova 1999em in NGC 1637 from several days after the outburst till day 642. The radioactive tail of the recovered bolometric light curve of SN 1999em indicates that the amount of the ejected 5 6 Ni is 0.02 M O .. The Ha and HeI 10 830-A lines at the nebular epoch show that the distribution of the bulk of 5 6 Ni can be represented approximately by a sphere of 5 6 Ni with a velocity of 1500 km s - 1 , which is shifted towards the far hemisphere by about 400 km s - 1 . The fine structure of the Ha at the photospheric epoch reminiscent of the 'Bochum event' in SN 1987A is analysed in terms of two plausible models: bipolar 5 6 Ni jets and non-monotonic behaviour of the Ha optical depth combined with the one-sided 5 6 Ni ejection. The late-time spectra show a dramatic transformation of the [O I] 6300-A line profile between days 465 and 510, which we interpret as an effect of dust condensation during this period. Late-time photometry supports the dust formation scenario after day 465. The [O I] line profile suggests that the dust occupies a sphere with velocity 800 km s - 1 and optical depth >> 10. The latter exceeds the optical depth of the dusty zone in SN 1987A by more than 10 times. Use is made of the Expanding Photosphere Method to estimate the distance and the explosion time, D 7.83 Mpc and t 0 ≃ 1999 October 24.5 UT, in accord with observational constraints on the explosion time and with other results of detailed studies of the method. The plateau brightness and duration combined with the expansion velocity suggest a pre-supernova radius of 120-150 R O ., ejecta mass of 10-11 M O . and explosion energy of (0.5-1) x 10 5 1 erg. The ejecta mass combined with the neutron star and a conservative assumption about mass loss implies the main sequence progenitor of M m s 12-14 M O .. The derived mass range is in agreement with the upper limit to the mass found using pre-supernova field images by Smartt et al. From the [OI] 6300, 6364 A doublet luminosity we infer the oxygen mass to be a factor four lower than in SN 1987A which is consistent with the estimated SN 1999em progenitor mass according to nucleosynthesis and stellar evolution theory. We note a 'second-plateau' behaviour of the light curve after the main plateau at the beginning of the radioactive tail. This feature seems to be common to SNe IIP with low 5 6 Ni mass.

Are the hosts of gamma-ray bursts sub-luminous and blue galaxies?

Abstract: We present K-band imaging observations of ten gamma-ray burst (GRB) host galaxies for which an optical and/or radio afterglow associated with the GRB event was clearly identified. Data were obtained with the Very Large Telescope and New Technology Telescope at ESO (Chile), and with the Gemini-North telescope at Mauna Kea (Hawaii). Adding to our sample nine other GRB hosts with K-band photometry and determined redshifts published in the literature, we compare their observed and absolute K magnitudes as well as their R K colours with those of other distant sources detected in various optical, near- infrared, mid-infrared and submillimeter deep surveys. We find that the GRB host galaxies, most of them lying at 0:5< z< 1:5, exhibit very blue colours, comparable to those of the faint blue star-forming sources at high redshift. They are sub-luminous in the K-band, suggesting a low stellar mass content. We do not find any GRB hosts harbouring R -a ndK-band properties similar to those characterizing the luminous infrared/submillimeter sources and the extremely red starbursts. Should GRBs be regarded as an unbiased probe of star-forming activity, this lack of luminous and/or reddened objects among the GRB host sample might reveal that the detection of GRB optical afterglows is likely biased toward unobscured galaxies. It would moreover support the idea that a large fraction of the optically-dark GRBs occur within dust-enshrouded regions of star formation. On the other hand, our result might also simply reflect intrinsic properties of GRB host galaxies experiencing a first episode of very massive star formation and characterized by a rather weak underlying stellar population. Finally, we compute the absolute B magnitudes for the whole sample of GRB host galaxies with known redshifts and detected at optical wavelengths. We find that the latter appear statistically even less luminous than the faint blue sources which mostly contributed to the B-band light emitted at high redshift. This indicates that the formation of GRBs could be favoured in particular systems with very low luminosities and, therefore, low metallicities. Such an intrinsic bias toward metal-poor environments would be actually consistent with what can be expected from the currently-favoured scenario of the "collapsar". The forthcoming launch of the SWIFT mission at the end of 2003 will provide a dramatic increase of the number of GRB-selected sources. A detailed study of the chemical composition of the gas within this sample of galaxies will thus allow us to further analyse the potential eect of metallicity in the formation of GRB events.

The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems

Abstract: We have searched for molecular hydrogen in damped Lyman-alpha (DLA) and sub-DLA systems at z>1.8 using UVES at the VLT. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, H2 is detected in 13 to 20 percent of the newly-surveyed systems. We report new detections of molecular hydrogen at z=2.087 and 2.595 toward, respectively, Q 1444+014 and Q 0405-443, and also reanalyse the system at z=3.025 toward Q 0347-383. We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems (60 systems in total). The DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2-detected components even has [Zn/Fe]>=1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction is generally small in DLA systems and similar to what is observed in the Magellanic Clouds. From 58 to 75 percent of the DLA systems have log f 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.

Commissioning and performances of the VLT-VIMOS

Abstract: The Visible Multi-Object Spectrograph VIMOS is a wide field survey instrument in the process of being commissioned for operations at the ESO-VLT. During the first commissioning period, the instrument has confirmed its excellent performances in its three basic modes of operation: direct imaging, multi-slit spectroscopy, and integral field spectroscopy. VIMOS provides the largest imaging field at the VLT with 224 arcmin2. It offers an unprecedented multiplex gain in multi-slit spectroscopy, with on order 800 slits which can be observed simultaneously. The integral field unit has a field up to 54x54 arcsec2, with 6400 spectra recorded at once. The overall efficiency of VIMOS combined to the Melipal unit #3 is confirmed to be as computed on the basis of the measured transmission of optical elements. Image quality is confirmed to be excellent, providing images limited by natural seeing in most conditions. High quality slit masks cut by the laser machine coupled to excellent geometric mask to CCD mapping lead to multi-slit spectra of excellent quality. VIMOS is expected to be offered to the ESO community for reguglar observations in early 2003.

VLT/UVES Abundances in Four Nearby Dwarf Spheroidal Galaxies. II. Implications for Understanding Galaxy Evolution*

Abstract: We have used the Ultraviolet Visual-Echelle Spectrograph (UVES) on Kueyen (UT2) of the Very Large Telescope to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies (dSph's): Sculptor, Fornax, Carina, and Leo I. We measure the abundance variations of numerous elements in these low-mass stars with a range of ages (1–15 Gyr old). This means that we can effectively measure the chemical evolution of these galaxies with time. Our results show a significant spread in metallicity with age, but an overall trend consistent with what might be expected from a closed- (or perhaps leaky-) box chemical evolution scenario over the last 10–15 Gyr. We make comparisons between the properties of stars observed in dSph's and in our Galaxy's disk and halo, as well as globular cluster populations in our Galaxy and in the Large Magellanic Cloud. We also look for the signature of the earliest star formation in the universe, which may have occurred in these small systems. We notice that each of these galaxies show broadly similar abundance patterns for all elements measured. This suggests a fairly uniform progression of chemical evolution with time, despite quite a large range of star formation histories. It seems likely that these galaxies had similar initial conditions, and that they evolve in a similar manner with star formation occurring at a uniformly low rate, even if at different times. With our accurate measurements we find evidence for small variations in abundances, which seem to be correlated to variations in star formation histories between different galaxies. The α-element abundances suggest that dSph chemical evolution has not been affected by very high mass stars (>15–20 M⊙). The abundance patterns we measure for stars in dSph's are significantly different from those typically observed in the disk, bulge, and inner halo of our Galaxy. This means that, as far as we can tell from the (limited) data available to date, it is impossible to construct a significant fraction of our disk, inner halo, or bulge from stars formed in dSph's such as we see today, which subsequently merged into our own. Any merger scenario involving dSph's has to occur in the very early universe while they are still gas-rich, so the majority of mass transfer is gas and few stars.

The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z = 0 to z = 3

Abstract: We present the evolution of the rest-frame optical luminosity density j, the integrated rest-frame optical color, and the stellar mass density, ?*, for a sample of Ks band-selected galaxies in the Hubble Deep Field-South (HDF-S). We derived j in the rest-frame U, B, and V bands and found that j increases by a factor of 1.9 ? 0.4, 2.9 ? 0.6, and 4.9 ? 1.0 in the V, B, and U rest-frame bands, respectively, between redshifts of 0.1 and 3.2. We derived the luminosity-weighted mean cosmic (U-B)rest and (B-V)rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z = 0.1, the (U-B)rest and (B-V)rest colors are 0.16 and 0.75, respectively, while at z = 2.8 they are -0.39 and 0.29, respectively. We derived the luminosity-weighted mean M/L, using the correlation between (U-V)rest and log M/L that exists for a range in smooth star formation histories (SFHs) and moderate extinctions. We have shown that the mean of individual M/L estimates can overpredict the true value by ~70%, while our method overpredicts the true value by only ~35%. We find that the universe at z ~ 3 had ~10 times lower stellar mass density than it does today in galaxies with L > 1.4 ? 1010 h L?. Half of the stellar mass of the universe was formed by z ~ 1-1.5. The rate of increase in ?* with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but it is slightly less than that predicted by the integral of the SFR(z) curve.

Ultradeep Near-Infrared ISAAC Observations of the Hubble Deep Field South: Observations, Reduction, Multicolor Catalog, and Photometric Redshifts

Abstract: We present deep near-infrared (NIR) Js-, H-, and Ks-band ISAAC imaging of the Wide Field Planetary Camera 2 (WFPC2) field of the Hubble Deep Field South (HDF-S). The 25 × 25 high Galactic latitude field was observed with the Very Large Telescope under the best seeing conditions, with integration times amounting to 33.6 hr in Js, 32.3 hr in H, and 35.6 hr in Ks. We reach total AB magnitudes for point sources of 26.8, 26.2, and 26.2, respectively (3 σ), which make it the deepest ground-based NIR observation to date and the deepest Ks-band data in any field. The effective seeing of the co-added images is ≈045 in Js, ≈048 in H, and ≈046 in Ks. Using published WFPC2 optical data, we constructed a Ks-limited multicolor catalog containing 833 sources down to K 26, of which 624 have seven-band optical-to-NIR photometry. These data allow us to select normal galaxies from their rest-frame optical properties to high redshift (z 4). The observations, data reduction, and properties of the final images are discussed, and we address the detection and photometry procedures that were used in making the catalog. In addition, we present deep number counts, color distributions, and photometric redshifts of the HDF-S galaxies. We find that our faint Ks-band number counts are flatter than published counts in other deep fields, which might reflect cosmic variations or different analysis techniques. Compared to the HDF-N, we find many galaxies with very red V-H colors at photometric redshifts 1.95 2.3 (in Johnson magnitudes). Because they are extremely faint in the observed optical, they would be missed by ultraviolet–optical selection techniques, such as the U-dropout method.

Dynamical Evidence for a Black Hole in GX 339-4

Abstract: We present outburst spectroscopy of GX 339-4 that may reveal the motion of its elusive companion star. N III lines exhibit sharp emission components moving over ~300 km s-1 in a single night. The most plausible interpretation of these components is that they are formed by irradiation of the companion star and the velocities indicate its orbital motion. We also detect motion of the wings of the He II 4686 A line and changes in its morphology. No previously proposed period is consistent with periodic behavior of all of these measures. However, consistent and sensible solutions are obtained for periods around 1.7 days. For the best period, 1.7557 days, we estimate a mass function of 5.8 ± 0.5 M☉. Even allowing for aliases, the 95% confidence lower limit on the mass function is 2.0 M☉. GX 339-4 can therefore be added to the list of dynamical black hole candidates. This is supported by the small motion in the wings of the He II line; if the compact object velocity is not larger than the observed motion, then the mass ratio is q 0.08, similar to other systems harboring black holes. Finally, we note that the sharp components are not always present but do seem to occur within a repeating phase range. This appears to migrate between our epochs of observation and may indicate shielding of the companion star by a variable accretion geometry such as a warp.

Advanced Camera for Surveys Photometry of the Cluster RDCS 1252.9–2927: The Color-Magnitude Relation at z = 1.24

Abstract: We investigate the color-magnitude (CM) relation of galaxies in the distant X-ray selected cluster RDCS 1252.9‐2927 at z = 1.24 using images obtained with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescopein the F775W and F850LP bandpasses. We select galaxies based on morphological classifications extending about 3.5 mag down the galaxy lumi nosity function, augmented by spectroscopic membership information. At the core of the cluster is an extensive early-type galaxy population surrounding a central pair of galaxies that show signs of dynamical interaction. The early-type population defines a tight sequence in the CM diagram, with an intrinsic scatter in observed (i775-z850) of 0.029 ± 0.007 mag based on 52 galaxies, or 0.024 ± 0.008 mag for ∼ 30 ellipticals. Simulations using the latest stellar popul ation models indicate an age

The spinning-top Be star Achernar from VLTI-VINCI

Abstract: We report here the first observations of a rapidly rotating Be star, Eridani, using Earth-rotation synthesis on the Very Large Telescope (VLT) Interferometer. Our measures correspond to a 2a=2b= 1:56 0:05 apparent oblate star, 2a and 2b being the equivalent uniform disc angular diameters in the equatorial and polar direction. Considering the presence of a circum- stellar envelope (CSE) we argue that our measurement corresponds to a truly distorted star since Eridani exhibited negligible H emission during the interferometric observations. In this framework we conclude that the commonly adopted Roche approx- imation (uniform rotation and centrally condensed mass) should not apply to Eridani. This result opens new perspectives to basic astrophysical problems, such as rotationally enhanced mass loss and internal angular momentum distribution. In addition to its intimate relation with magnetism and pulsation, rapid rotation thus provides a key to the Be phenomenon: one of the outstanding non-resolved problems in stellar physics.

The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z=0 to z=3

Abstract: We present the evolution of the rest-frame optical luminosity density, of the integrated rest-frame optical color, and of the stellar mass density for a sample of Ks-band selected galaxies in the HDF-S. We derived the luminosity density in the rest-frame U, B, and V-bands and found that the luminosity density increases by a factor of 1.9+-0.4, 2.9+-0.6, and 4.9+-1.0 in the V, B, and U rest-frame bands respectively between a redshift of 0.1 and 3.2. We derived the luminosity weighted mean cosmic (U-B)_rest and (B-V)_rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z=0.1, the (U-B)_rest and (B-V)_rest colors are 0.16 and 0.75 respectively, while at z=2.8 they are -0.39 and 0.29 respectively. We derived the luminosity weighted mean M/LV using the correlation between (U-V)_rest and log_{10} M/LV which exists for a range in smooth SFHs and moderate extinctions. We have shown that the mean of individual M/LV estimates can overpredict the true value by ~70% while our method overpredicts the true values by only ~35%. We find that the universe at z~3 had ~10 times lower stellar mass density than it does today in galaxies with LV>1.4 \times 10^{10} h_{70}^-2 Lsol. 50% of the stellar mass of the universe was formed by $z~1-1.5. The rate of increase in the stellar mass density with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but is slightly less than that predicted by the integral of the SFR(z) curve.

Spectropolarimetry of SN 2001el in NGC 1448: Asphericity of a normal type Ia supernova

Abstract: High-quality spectropolarimetry (range 417-860 nm; spectral resolution 1.27 nm and 0.265 nm pixel-1) of the Type Ia supernova (SN Ia) 2001el was obtained with the ESO Very Large Telescope Melipal (+FORS1) at five epochs. The spectra a week before maximum and around maximum indicate photospheric expansion velocities of about 10,000 km s-1. Prior to optical maximum, the linear polarization of the continuum was ≈0.2%-0.3% with a constant position angle, showing that SN 2001el has a well-defined axis of symmetry. The polarization was nearly undetectable a week after optical maximum. The spectra are similar to those of the normally bright SN 1994D, with the exception of a strong double-troughed absorption feature seen around 800 nm (FWHM about 22 nm). The 800 nm feature is probably due to the Ca II IR triplet at very high velocities (20,000-26,000 km s-1) involving ~0.004 M☉ of calcium and perhaps 0.1 M☉ total mass. The 800 nm feature is distinct in velocity space from the photospheric Ca II IR triplet and has a significantly higher degree of polarization (≈0.7%) and different polarization angle than the continuum. Taken together, these aspects suggest that this high-velocity calcium is a kinematically distinct feature with the matter distributed in a filament, torus, or array of "blobs" almost edge-on to the line of sight. This feature could thus be an important clue to the binary nature of SNe Ia, perhaps associated with an accretion disk, or to the nature of the thermonuclear burning, perhaps representing a stream of material ballistically ejected from the site of the deflagration to detonation transition. If modeled in terms of an oblate spheroid, the continuum polarization implies a minor to major axis ratio of around 0.9 if seen equator-on; this level of asymmetry would produce an absolute luminosity dispersion of about 0.1 mag when viewed at different viewing angles. If typical for SNe Ia, this would create an rms scatter of several hundredths of a magnitude around the mean brightness-decline relation. We discuss the possible implications of this scatter for the high-precision measurements required to determine the cosmological equation of state.

Spatially Resolved Millimeter Interferometry of SMM J02399–0136: A Very Massive Galaxy at z = 2.8*

Abstract: We report high-resolution millimeter mapping with the IRAM Plateau de Bure interferometer of rest-frame 335 ?m continuum and CO (3-2) line emission from the z = 2.8 submillimeter galaxy SMM J02399-0136. The continuum emission comes from a ~3'' diameter structure whose elongation is approximately east-west and whose centroid is coincident within the astrometric errors with the brightest X-ray and rest-UV peak (L1). The line data show that this structure is most likely a rapidly rotating disk. Its rotation velocity of ?420 km s-1 implies a total dynamical mass of ?3 ? 1011 sin?2 i h M? within an intrinsic radius of 8 h kpc, most of which is plausibly in the form of stars and gas. SMM J02399-0136 is thus a very massive system, whose formation at z ~ 3 is not easy to understand in current cold dark matter hierarchical merger cosmogonies.

Hα Spectroscopy of Galaxies at z > 2: Kinematics and Star Formation*

Abstract: We present near-infrared spectroscopy of H? emission lines in a sample of 16 star-forming galaxies at redshifts 2.0 < z < 2.6. Our targets are drawn from a large sample of galaxies photometrically selected and spectroscopically confirmed to lie in this redshift range. We have obtained this large sample with an extension of the broadband UnG color criteria used to identify Lyman break galaxies at z ~ 3. The primary selection criterion for IR spectroscopic observation was proximity to a QSO sight line; we therefore expect the galaxies presented here to be representative of the sample as a whole. Six of the galaxies exhibit spatially extended, tilted H? emission lines; rotation curves for these objects reach mean velocities of ~150 km s-1 at radii of ~6 kpc, without corrections for inclination or any other observational effect. The velocities and radii give a mean dynamical mass of ? 4 ? 1010 M?. We have obtained archival Hubble Space Telescope images for two of these galaxies; they are morphologically irregular. One-dimensional velocity dispersions for the 16 galaxies range from ~50 to ~260 km s-1, and in cases in which we have both virial masses implied by the velocity dispersions and dynamical masses derived from the spatially extended emission lines, they are in rough agreement. We compare our kinematic results with similar measurements made at z ~ 3 and find that both the observed rotational velocities and velocity dispersions tend to be larger at z ~ 2 than at z ~ 3. We also calculate star formation rates (SFRs) from the H? luminosities and compare them with SFRs calculated from the UV continuum luminosity. We find a mean SFRH? of 16 M? yr-1 and an average SFRH?/SFRUV ratio of 2.4, without correcting for extinction. We see moderate evidence for an inverse correlation between the UV continuum luminosity and the ratio SFRH?/SFRUV, such as might be observed if the UV-faint galaxies suffered greater extinction. We discuss the effects of dust and star formation history on the SFRs and conclude that extinction is the most likely explanation for the discrepancy between the two SFRs.