Showing papers by "European Southern Observatory published in 1999"

Measurements of Omega and Lambda from 42 High-Redshift Supernovae

Abstract: We report measurements of the mass density, Omega_M, and cosmological-constant energy density, Omega_Lambda, of the universe based on the analysis of 42 Type Ia supernovae discovered by the Supernova Cosmology Project. The magnitude-redshift data for these SNe, at redshifts between 0.18 and 0.83, are fit jointly with a set of SNe from the Calan/Tololo Supernova Survey, at redshifts below 0.1, to yield values for the cosmological parameters. All SN peak magnitudes are standardized using a SN Ia lightcurve width-luminosity relation. The measurement yields a joint probability distribution of the cosmological parameters that is approximated by the relation 0.8 Omega_M - 0.6 Omega_Lambda ~= -0.2 +/- 0.1 in the region of interest (Omega_M <~ 1.5). For a flat (Omega_M + Omega_Lambda = 1) cosmology we find Omega_M = 0.28{+0.09,-0.08} (1 sigma statistical) {+0.05,-0.04} (identified systematics). The data are strongly inconsistent with a Lambda = 0 flat cosmology, the simplest inflationary universe model. An open, Lambda = 0 cosmology also does not fit the data well: the data indicate that the cosmological constant is non-zero and positive, with a confidence of P(Lambda > 0) = 99%, including the identified systematic uncertainties. The best-fit age of the universe relative to the Hubble time is t_0 = 14.9{+1.4,-1.1} (0.63/h) Gyr for a flat cosmology. The size of our sample allows us to perform a variety of statistical tests to check for possible systematic errors and biases. We find no significant differences in either the host reddening distribution or Malmquist bias between the low-redshift Calan/Tololo sample and our high-redshift sample. The conclusions are robust whether or not a width-luminosity relation is used to standardize the SN peak magnitudes.

The unusual afterglow of the γ-ray burst of 26 March 1998 as evidence for a supernova connection

Abstract: Cosmic γ-ray bursts have now been firmly established as one of the most powerful phenomena in the Universe, releasing almost the rest-mass energy of a neutron star within the space of a few seconds (ref. 1). The two most popular models to explain γ-ray bursts are the coalescence of two compact objects such as neutron stars or black holes, or the catastrophic collapse of a massive star in a very energetic supernova-like explosion. Here we show that, about three weeks after the gamma-ray burst of 26 March 1998, the transient optical source associated with the burst brightened to about 60 times the expected flux, based upon an extrapolation of the initial light curve. Moreover, the spectrum changed dramatically, with the colour becoming extremely red. We argue that the new source is an underlying supernova. If our hypothesis is true then this provides evidence linking cosmologically located γ-ray bursts with deaths of massive stars.

The giant, horizontal, and asymptotic branches of galactic globular clusters. i. the catalog, photometric observables, and features

Abstract: A catalog including a set of the most recent color-magnitude diagrams (CMDs) is presented for a sample of 61 Galactic globular clusters (GGCs). We used this database to perform a homogeneous systematic analysis of the evolved sequences (namely, the red giant branch [RGB], horizontal branch [HB], and asymptotic giant branch [AGB]). Based on this analysis, we present (1) a new procedure to measure the level of the zero-age horizontal branch (VZAHB) and a homogeneous set of distance moduli obtained by adopting the HB as standard candle; (2) an independent estimate for RGB metallicity indicators and new calibrations of these parameters in terms of both spectroscopic ([Fe/H]CG97) and global metallicity ([M/H], including also the ?-element enhancement), such that the set of equations presented can be used to simultaneously derive a photometric estimate of the metal abundance and the reddening from the morphology and the location of the RGB in the (V, B-V) CMD; and (3) the location of the RGB bump (in 47 GGCs) and the AGB bump (in nine GGCs). The dependence of these features on metallicity is discussed. We find that by using the latest theoretical models and the new metallicity scales, the earlier discrepancy between theory and observations (~0.4 mag) completely disappears.

A Large Mid-Infrared Spectroscopic and Near-Infrared Imaging Survey of Ultraluminous Infrared Galaxies: Their Nature and Evolution* **

Abstract: We present a low-resolution mid-infrared spectroscopic survey of an unbiased sample of 62 ultraluminous infrared galaxies (ULIRGs) (LIR > 1012L⊙, z ≤ 0.3) using ISOPHOT-S on board the Infrared Space Observatory (ISO). For comparison, we also present ISOPHOT-S spectra for 23 active galactic nuclei (AGNs) and 15 starburst and normal galaxies. The line-to-continuum ratio of the 7.7 μm polycyclic aromatic hydrocarbon (PAH) emission feature is used as a discriminator between starburst and AGN activity in ULIRGs. We find that the majority of ULIRGs are predominantly powered by starbursts. The ratio of PAH over infrared luminosities, LPAH/LIR, for starburst-dominated ULIRGs is very similar to the ratio found for template starbursts. The shapes of the PAH features are sometimes unusual. Extinction has a noticeable effect on the PAH spectra of ULIRG starbursts. We have obtained high-resolution near-infrared imaging for the southern ISOPHOT-S ULIRGs in order to investigate their evolution stage. The majority (68%) of the ULIRGs imaged are double systems, and all show distorted morphologies. Of the 23 double-nuclei systems, 17 appear at linear separations between 4 and 14 kpc, with a mean separation of 6.5 kpc. Using the separations measured from our new near-infrared imaging as well as from the literature, we have examined whether ULIRGs that are advanced mergers are more AGN-like. We have found no such evidence, contrary to what is postulated by the classical evolutionary scenario. No correlation is found between the stage of merger in ULIRGs and their infrared luminosity. In fact, we find that systems in the early stages of merging may well put out maximum luminosity. We also find that the total mass of interstellar gas, as estimated from the CO (1 → 0) luminosity, does not decrease with decreasing merger separation. When both an AGN and a starburst occur concurrently in ULIRGs, we find that the starburst dominates the luminosity output. We propose that the available gas reservoir and the individual structure of the interacting galaxies plays a major role in the evolution of the system.

The rosat all-sky survey catalogue of the nearby stars

Abstract: We present X-ray data for all entries of the Third Catalogue of Nearby Stars (Gliese & Jahrei 1991) that have been detected as X-ray sources in the ROSAT all-sky survey. The catalogue contains 1252 entries yield- ing an average detection rate of 32.9 percent. In addi- tion to count rates, source detection parameters, hardness ratios, and X-ray fluxes we also list X-ray luminosities derived from Hipparcos parallaxes.

The 3D structure of the Virgo cluster from H-band Fundamental Plane and Tully—Fisher distance determinations

Abstract: We undertook a surface photometry survey of 200 galaxies in the Virgo cluster (complete to B<14.0 mag) carried out in the near-infrared (NIR) H band. Combining velocity dispersion measurements from the literature with new spectroscopic data for 11 galaxies, we derive distances of 59 early-type galaxies using the Fundamental Plane (FP) method. The distances of another 75 late-type galaxies are determined using the Tully--Fisher (TF) method. For this purpose we use the maximum rotational velocity, as derived from H I spectra from the literature, complemented with new Hα rotation curves of eight highly H I-deficient galaxies. The zero-points of the FP and TF template relations are calibrated assuming the distance modulus of Virgo μ0=31.0, as determined with the Cepheids method. Using these 134 distance determinations (with individual uncertainties of 0.35 mag (TF) and 0.45 mag (FP)) we find that the distance of cluster A, associated with M87, is μ0=30.84 ± 0.06. Cluster B, offset to the south, is found at μ0=31.84 ± 0.10. This subcluster is falling on to A at about 750 km s-1. Clouds W and M are at twice the distance of A. Galaxies on the north-west and south-east of the main cluster A belong to two clouds composed almost exclusively of spiral galaxies with distances consistent with A, but with significantly different velocity distributions, suggesting that they are falling on to cluster A at approximately 770 km s-1 from the far side and at 200 km s-1 from the near side respectively. The mass of Virgo inferred from the peculiar motions induced on its vicinity is consistent with the virial expectation.

New catalogue of Wolf-Rayet galaxies and high-excitation extra-galactic HII regions

Abstract: We present a new compilation of Wolf-Rayet (WR) galaxies and extra-galactic Hii regions showing broad He ii 4686 emission drawn from the literature. Relevant information on the presence of other broad emis- sion lines (N iii 4640, C iv 5808 and others) from WR stars of WN and WC subtypes, and other existing broad nebular lines is provided. In total we include 139 known WR galaxies. Among these, 57 objects show both broad He ii 4686 and C iv 5808 features. In addition to the broad (stellar) He ii 4686 emission, a nebular He ii component is well es- tablished (suspected) in 44 (54) objects. We nd 19 extra-galatic Hii regions without WR detections showing nebular He ii 4686 emission. The present sample can be used for a variety of studies on massive stars, interactions of massive stars with the ISM, stellar populations, starburst galaxies etc. The data is accessible electronically and will be updated periodically.

The Type Ia Supernova 1998bu in M96 and the Hubble Constant

Abstract: We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8 ± 0.8) with B = 12.22 ± 0.03 and V = 11.88 ± 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of AV = 0.94 ± 0.15 mag, and indicates the supernova was of average luminosity compared to other normal Type Ia supernovae. Using the HST Cepheid distance modulus to M96 and the MLCS fitted parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, MV = -19.42 ± 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H0 = 64 -->img1.gif km s-1 Mpc-1, where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.

Extended Lyα emission from a damped Ly α absorber at z=1.93, and the relation between damped Ly α absorbers and Lyman-break galaxies

Abstract: The number of damped Lyα absorbers (DLAs) currently known is about 100, but our knowledge of their sizes and morphologies is still very sparse, as very few have been detected in emission. Here we present narrow-band and broad-band observations of a DLA in the field of the quasar pair Q0151+048A (qA) and Q0151+048B (qB). These two quasars have very similar redshifts, zem=1.922 and 1.937 respectively, and an angular separation of 3.27 arcsec. The spectrum of qA contains a DLA at zabs = 1.9342 (close to the emission redshift) which shows an emission line in the trough, detected at 4σ. Our narrow-band image confirms this detection, and we find Lyα emission from an extended area covering 6 × 3 arcsec2, corresponding to 25 × 12 h-2 kpc2 (q0=0.5, H0 = 100 h km s-1). The total Lyα luminosity from the DLA is 1.2 × 1043h-2 erg s-1, which is several times higher than the Lyα luminosity found from other DLAs. The narrow-band image also indicates that qB is not covered by the DLA. This fact, together with the large equivalent width of the emission line from the Lyα cloud, the large luminosity, and the ∼ 300 km s-1 blueshift relative to the DLA, can plausibly be explained if qB is the source of ionizing photons, lying to the near side of the DLA at a distance from the DLA of <20 h-1 kpc. In this case the size of the emission-line region corresponds to the area over which the cloud is optically thick, i.e., is indicative of the size of a Lyman-limit system. We also consider the relation between DLAs and Lyman-break galaxies (LBGs). If DLAs are gaseous discs surrounding LBGs, and if the apparent brightnesses and impact parameters of the few identified DLAs are representative of the brighter members of the population, then the luminosity distribution of DLAs is nearly flat, and we would expect that some 70 per cent of the galaxy counterparts to DLAs at z≈ 3 are fainter than mR=28.

The fourth catalogue of Population I Wolf-Rayet stars in the Large Magellanic Cloud ?

Abstract: The catalogue provides for each of the 134 W-R stars of Population I presently known in the Large Magellanic Cloud, accurate equatorial coordinates, photometric data, spectral classication, binary status, correlation with OB associations and HII regions. The miscellaneous designations of the stars are also listed. Although completeness is not pretended, results published during the last decade are highlighted in the notes given for each individual star. A uniform set of nding charts is presented.

The Ionizing Stars of Extragalactic H II Regions

Abstract: Medium-resolution spectra from 3650 to 10000 A are presented for 96 giant H II regions distributed in 20 spiral galaxies. In order to interpret the data, we have calculated two separate grids of photoionization models, adopting single-star atmospheres (Kurucz) and star clusters synthesized with different initial mass functions (IMFs) as ionizing sources. Additional models were computed with more recent non-LTE stellar atmospheres, in order to check the effects of different stellar ionizing fluxes. We use the radiation softness parameter η'=([O II]/[O III])/([S II]/[S III]) of Vilchez & Pagel to test for a metallicity dependence of the effective temperatures of the ionizing stars. Our results are consistent with a significant decrease in mean stellar temperatures of the ionizing stars with increasing metallicity. The magnitude of the effect, combined with the behavior of the He I λ5876/Hβ ratio, suggest a smaller upper mass limit for star formation at abundances higher than solar, even when considering the effects of metallicity on stellar evolution and atmospheric line blanketing. However, the exact magnitudes of the stellar temperature and IMF variations are dependent on the choice of stellar atmosphere and evolution models used, as well as on uncertainties in the nebular abundance scale at high metallicities. Our results also constrain the systematic behavior of the ionization parameter and the N/O ratio in extragalactic H II regions. The observed spectral sequences are inconsistent with current stellar evolution models, which predict a luminous, hot W-R stellar population in evolved H II regions older than 2-3 Myr. This suggests either that the hardness of the emitted Lyman continuum spectrum has been overestimated in the models or that some mechanism disrupts the H II regions before the W-R phases become important.

An X-Ray-Selected Galaxy Cluster at Z = 1.26

Abstract: We report the discovery of an X-ray–luminous galaxy cluster at z = 1.26. RX J0848.9+4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray emission, with a significant excess of red objects with J-K colors typical of elliptical galaxies at z > 1. Spectroscopic observations at the Keck II Telescope have secured six galaxy redshifts in the range 1.257 1.

Blue Straggler Stars: The Spectacular Population in M80*

Abstract: Using Hubble Space Telescope WFPC2 observations in two ultraviolet (UV) filters (F225W and F336W) of the central region of the high-density Galactic globular cluster (GGC) M80, we have identified 305 blue straggler stars (BSS), which represents the largest and most concentrated population of BSS ever observed in a GGC. We also identify the largest clean sample of evolved BSS yet found. The high stellar density alone cannot explain the BSS, and we suggest that in M80 we are witnessing a transient dynamical state, during which stellar interactions are delaying the core-collapse process leading to an exceptionally large population of collisional BSS.

Calibration of the MACHO Photometry Database

Abstract: The MACHO Project is a microlensing survey that monitors the brightnesses of ~60 million stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and Galactic bulge. Our database presently contains about 80 billion photometric measurements, a significant fraction of all astronomical photometry. We describe the calibration of MACHO two-color photometry and transformation to the standard Kron-Cousins V and R system. Calibrated MACHO photometry may be properly compared with all other observations on the Kron-Cousins standard system, enhancing the astrophysical value of these data. For ~9 million stars in the LMC bar, independent photometric measurements of ~20,000 stars with V 18 mag in field-overlap regions demonstrate an internal precision σV = 0.021, σR = 0.019, σV-R = 0.028 mag. The accuracy of the zero point in this calibration is estimated to be ±0.035 mag for stars with colors in the range –0.1 mag < V-R < 1.2 mag. A comparison of calibrated MACHO photometry with published photometric sequences and new Hubble Space Telescope observations shows agreement. The current calibration zero-point uncertainty for the remainder of the MACHO photometry database is estimated to be ±0.10 mag in V or R and ±0.04 mag in V–R. We describe the first application of calibrated MACHO data: the construction of a color-magnitude diagram used to calculate our experimental sensitivity for detecting microlensing in the LMC.

The I-Band Tully-Fisher Relation for SC Galaxies: 21 Centimeter H I Line Data

Abstract: A compilation of 21 cm line spectral parameters specifically designed for application of the Tully-Fisher (TF) distance method is presented for 1201 spiral galaxies, primarily field Sc galaxies, for which optical I-band photometric imaging is also available. New H I line spectra have been obtained for 881 galaxies. For an additional 320 galaxies, spectra available in a digital archive have been reexamined to allow application of a single algorithm for the derivation of the TF velocity width parameter. A velocity width algorithm is used that provides a robust measurement of rotational velocity and permits an estimate of the error on that width taking into account the effects of instrumental broadening and signal-to-noise. The digital data are used to establish regression relations between measurements of velocity widths using other common prescriptions so that comparable widths can be derived through conversion of values published in the literature. The uniform H I line widths presented here provide the rotational velocity measurement to be used in deriving peculiar velocities via the TF method.

Doppler Imagery of the Spotted RS Canum Venaticorum Star HR 1099 (V711 Tauri) from 1981 to 1992

Abstract: We present a set of 23 Doppler images of the spotted RS CVn star HR 1099 (V711 Tauri; HD 22468) obtained from 1981 to 1992. HR 1099 shows a large, cool polar spot that has persisted for the 11 yr of this study and other low-latitude spots that come and go on relatively short (less than 1 yr) timescales and can emerge anywhere on the star. The polar spot has variable protuberances that look very similar to the time-variable vertical extensions of the Sun's polar coronal hole. The area of the polar spot and its extensions shows marginal evidence of being variable, with a period of about 3 yr and an amplitude of about 1% that is perhaps indicative of a weak cycle, but this is not yet conclusive. Comparison of our Doppler images with previously published few spot model fits to the light curves shows that such simple spot-model solutions, while sometimes in agreement, are often misleading and nonunique, particularly when the light-curve amplitude is small. Moreover, these spot-model fits do not recover the existence of the polar spot. The Doppler images show quite good agreement among multiple images at a given epoch and between different Doppler imaging research groups using completely independent data sets and imaging software. Our (cool spots only) Doppler imaging solutions, when properly thresholded, generally well reproduce the published light curves. However, in one instance the difficulty of fitting light curves suggests that at least one hot spot was present on HR 1099 during one observing season. Variations in the mean brightness of the system at the observed 0.05 mag level seem to correlate with spot area, particularly the polar spot, indicating that the mean light level is a pretty good proxy of spot area on HR 1099. While the polar spot with variable extensions was always present, isolated spots also frequently appeared at both mid- and low latitudes. On several occasions isolated prominent spots emerged and then disappeared on or near the equator. The migrating photometric wave on HR 1099 is due not to a simple longitudinal migration of spots on a differentially rotating star but rather to changes in the spatial distribution of a few spots (some of which move but most of which are fixed in longitude) that emerge and then disappear. So, at least for HR 1099, the phase drift of this migrating photometric wave minimum contains very little unique information about differential rotation or spot migration. While the tracking of individual features involves some uncertainty and speculation because of our limited time sampling, the tracks of two long-lived spots suggest that some spots that emerge at low or intermediate latitudes may migrate up to the pole in a clockwise spiral (slower than the orbit), then apparently merge with the polar spot. If these dark spots trace magnetic flux, we speculate that some of the magnetic flux emerging at lower latitudes migrates poleward and merges with the polar spot flux. It is not yet clear whether this flux is of the same or opposite polarity to the polar spot and thus whether these poleward-migrating, low-latitude spots reinforce or cancel the polar spot field. One of the high-latitude spots also appeared to get stretched in longitude as it approached the polar spot, and its overall track is quite reminiscent of the annulus of toroidal field found by Donati et al. encircling the polar spot of HR 1099 in 1990.9. In general, the spots appear to be very tightly locked to the orbital frame of the system, and most disappear before they have had a chance to migrate significantly. Like solar coronal holes, they show very little evidence for shear due to differential rotation. A few selected, long-lived features gave longitudinal migration rates of 1 part in 300 to 1 part in 3600 of the rotation period, in the sense that intermediate and low latitudes rotate slightly slower than the orbital angular velocity, while the pole and highest latitudes appear to be synchronized to the orbit. The implied differential rotation is thus of opposite sign and about a factor of 56 less than for the Sun. The rotation rate versus latitude behavior can be well fitted with a variety of formulae, including the Maunder formula. One of the best fits is provided by a rotation period versus latitude that is proportional to the surface strength of a centered axisymmetric magnetic dipole field, with the pole synchronized to the orbit and lower latitudes rotating more slowly. We believe that these starspots are not measuring photospheric differential rotation. Instead, like solar coronal holes, their relatively low degree of shearing and nearly solid body rotation may be enforced by a multikilogauss, axisymmetric, nearly current-free quasi-potential global magnetic field. Our Doppler images also agree very closely with the Zeeman-Doppler imagery of Donati et al. and support their finding that regions around the edge of the polar spot and within bright spots show largely monopolar fields of at least 300-700 G strength. The large, permanent cool polar spots, the very low observable differential rotation and shearing of starspots, and the evidence of strong, essentially unipolar magnetic fields associated with them leads us to believe that HR 1099 and other rapidly rotating RS CVn stars harbor quite strong (multikilogauss) axisymmetric global magnetic dipole fields. These fields have historically been largely hidden from view by their high degree of rotational symmetry, by being concentrated in the low surface brightness dark spots, and by these stars' high degree of rotational line broadening. We propose that the starspots on HR 1099 and other rapidly rotating RS CVn stars are, by analogy with solar coronal holes, large unipolar, magnetic regions that are tightly frozen into multikilogauss, axisymmetric global dipole fields in these stars. Since the large cool polar spots, the signature of these dipoles, are not present on more slowly rotating RS CVn stars, we believe that they must be dynamo-induced fields rather than remnant fossil fields.

Dynamical models of ngc 3115

Abstract: We present new dynamical models of the S0 galaxy N3115, making use of the available published photometry and kinematics as well as two-dimensional TIGER spectrography. The models are based on a detailed model of the luminosity distribution built using an MGE fit on HST/WFPC2 and ground-based photometric data. We first examined the kinematics in the central 40 arcsec in the light of two-integral f(E,J) models. Jeans equations were used to constrain the mass-to-light ratio, and the central dark mass, the existence of which was suggested by previous studies. The even part of the distribution function was then retrieved via the Hunter & Qian formalism. We thus confirmed that the velocity and dispersion profiles in the central region could be well-fitted with a two-integral model, given the presence of a central dark mass of ∼109 M⊙. However, no two-integral model could fit the h3 profile around a radius of about 25 arcsec where the outer disc dominates the surface brightness distribution. Three-integral analytical models were therefore built using a quadratic programming technique. These models showed that three-integral components do indeed provide a reasonable fit to the kinematics, including the higher Gauss--Hermite moments. Again, models without a central dark mass failed to reproduce the observed kinematics in the central arcsec. This clearly supports the presence of a nuclear black hole of at least 6.5×108 M⊙ in the centre of NGC 3115. These models were finally used to estimate the importance of the dark matter in the outer part of NGC 3115, suggested by the flat stellar rotation curve observed by Capaccioli et al. This study finally points out the difficulty of integrating independently published data in a coherent and consistent way, thus demonstrating the importance of taking into account the details of the instrumental setup and the reduction processes.

Searching for very low-mass stars and brown dwarfs with DENIS

Abstract: We present the results of infrared spectroscopic observations of a sample of very low-mass stars and brown dwarf candidates detected in a 230 square degree area by the DENIS (DEep Near Infrared Southern sky; Epchtein, 1997) survey. We nd that objects as faint as the faintest known stars are easily detected by DENIS. This sample also includes three members of the new \L" dwarf class, one of which was the rst conrmed isolated eld brown dwarf. As this data represents 1% of the total DENIS survey area, the completed survey can be expected to have a dramatic impact on the study of the faintest stars and brown dwarfs. In particular, it should detect300 of the new and poorly understood \L" class of dwarfs.

Discovery of a moon orbiting the asteroid 45 Eugenia

Abstract: Evidence for asteroidal satellites (moons) has been sought for decades, because the relative frequency of such satellites will bear on the collisional history of the asteroid belt and the Solar System, yet only one has been detected unambiguously1,2,3. Here we report the discovery of a satellite of the asteroid 45 Eugenia, using an adaptive optics system on a ground-based telescope. The satellite has a diameter of about 13 km, and an orbital period of about 4.7 days with a separation of 1,190 km from Eugenia. Using a previously determined4 diameter for Eugenia, we estimate that its bulk density is about 1.2 g cm-3, which is similar to that of the C-type asteroid Mathilde5,6. This implies that Eugenia, also a low-albedo C-type asteroid, may be a rubble pile, or composed of primitive, icy materials of low bulk density.

Detection of CO (4-3), CO (9-8), and Dust Emission in the Broad Absorption Line Quasar APM 08279+5255 at a Redshift of 3.9

Abstract: With the IRAM interferometer, we detected the lines of CO (4-3) and CO (9-8) from the recently discovered broad absorption line quasar APM 08279+5255. The molecular lines are at a redshift of 3.911, which we take to be the true cosmological redshift of the quasar's host galaxy. This means that the quasar emission lines at z=3.87 are blueshifted by a kinematic component of -2500 km s−1 and, along with the broad absorption lines, are probably emitted in the quasar's wind or jet, moving toward us. The CO line ratios suggest that the molecular gas is at a temperature of ~200 K, at a density of ~4000 cm−3. We also detected the dust emission at 94 and 214 GHz (with emitted wavelengths of 650 and 290 μm). The spectral index of the millimeter/submillimeter continuum is 3.2, indicating that the dust emission is optically thin in this part of the spectrum. The extremely high CO and dust luminosities suggest magnification by gravitational lensing. Using the optical extent and our limit on the size of the CO region, we estimate a magnification of 7-30 for the CO lines and the far-IR continuum and 14-60 for the optical/UV. In this interpretation, the molecular gas and dust are in a nuclear disk of radius 90-270 pc around the quasar. The quasar is 25-100 times stronger than, but otherwise resembles, the nucleus of Mrk 231.

The I-Band Tully-Fisher Relation for Sc Galaxies: Optical Imaging Data

Abstract: Properties derived from the analysis of photometric I-band imaging observations are presented for 1727 inclined spiral galaxies, mostly of types Sbc and Sc. The reduction, parameter extraction, and error estimation procedures are discussed in detail. The asymptotic behavior of the magnitude curve of growth and the radial variation in ellipticity and position angle are used in combination with the linearity of the surface brightness falloff to fit the disk portion of the profile. Total I-band magnitudes are calculated by extrapolating the detected surface brightness profile to a radius of eight disk scale lengths. Errors in the magnitudes, typically ~0.04 mag, are dominated by uncertainties in the sky subtraction and disk-fitting procedures. Comparison is made with the similar imaging database of Mathewson, Ford, & Buchhorn, both as presented originally by those authors and after reanalyzing their digital reduction files using identical disk-fitting procedures. Direct comparison is made of profile details for 292 galaxies observed in common. Although some differences occur, good agreement is found, proving that the two data sets can be used in combination with only minor accommodation of those differences. The compilation of optical properties presented here is optimized for use in applications of the Tully-Fisher relation as a secondary distance indicator in studies of the local peculiar velocity field.

Seyfert 1 Mutation of the Classical Seyfert 2 Nucleus NGC 7582

Abstract: We report the transition toward a type 1 Seyfert experienced by the classical type 2 Seyfert nucleus in NGC 7582. The transition, found at most 20 days from its maximum peak, presents a unique opportunity to study these rare events in detail. At maximum, the Ha line width is about 12,000 km s 21 . We examine three scenarios that could potentially explain the transition: capture of a star by a supermassive black hole, a reddening change in the surrounding torus, and the radiative onset of a Type IIn supernova exploding in a compact nuclear/ circumnuclear starburst. Subject headings: galaxies: active — galaxies: individual (NGC 7582) — galaxies: Seyfert — galaxies: starburst — supernova remnants

Cosmological Constraints from the ROSAT Deep Cluster Survey

Abstract: The ROSAT Deep Cluster Survey (RDCS) has provided a new large deep sample of X-ray selected galaxy clusters. Observables such as the flux number counts n(S), the redshift distribution n(z), and the X-ray luminosity function (XLF) over a large redshift baseline (z0.8) are used here in order to constrain cosmological models. Our analysis is based on the Press-Schechter approach, whose reliability is tested against N-body simulations. Following a phenomenological approach, no assumption is made a priori on the relation between cluster masses and observed X-ray luminosities. As a first step, we use the local XLF from RDCS, along with the high-luminosity extension provided by the XLF from the Brightest Cluster Survey, in order to constrain the amplitude of the power spectrum, ?8, and the shape of the local luminosity-temperature, Lbol-T, relation. We obtain ?8=(0.58 ? 0.06) ? ?0-0.47+0.16? for flat models (??=1-?0) and ?8=(0.58 ? 0.06) ? ?0-0.53+0.27? for open models (??=0) at a 90% confidence level, almost independent of the Lbol-T shape. The density parameter ?0 and the evolution of the Lbol-T relation are constrained by the RDCS XLF at z>0 and the EMSS XLF at =0.33, and by the RDCS n(S) and n(z) distributions. By modeling the evolution for the amplitude of the Lbol-T relation as (1+z)A, an ?0=1 model can be accommodated for the evolution of the XLF with 1A3 at a 90% confidence level, while ?0=0.4+0.3?0.2 and ?00.6 are implied by a nonevolving Lbol-T (A=0) for open and flat models, respectively.

The Redshift of the Gravitationally Lensed Radio Source PKS 1830–211*

Abstract: We report on the spectroscopic identification and the long-awaited redshift measurement of the heavily obscured, gravitationally lensed radio source PKS 1830-211, which was first observed as a radio Einstein ring. The northeast component of the doubly imaged core is identified, in our infrared spectrum covering the wavelength range 1.5-2.5 μm, as an impressively reddened quasar at z=2.507±0.002. The mass contained within the Einstein ring radius is M(r<2.1 h−1 kpc)=6.3×1010 h−1 M☉ for ΩM=1 or M(r<2.4 h−1 kpc)=7.4×1010 h−1 M☉ for ΩM=0.3. Our redshift measurement, together with the recently measured time delay (Lovell et al.), means that we are a step closer to determining H0 from this lens. Converting the time delay into H0 by using existing models leads to high values of the Hubble parameter, H0=65+15−9 for ΩM=1 and H0=76+18−10 for ΩM=0.3. Since the lensing galaxy lies very close to the center of the lensed ring, improving the error bars on H0 will require not only a more precise time delay measurement but also very precise astrometry of the whole system.

Small-Scale Structure at High Redshift. I. Glimpses of the Interstellar Medium at Redshift ~3.5*

Abstract: We obtained Keck High Resolution Echelle Spectrometer (FWHM=44 km s?1) spectra of images A and C of the gravitationally lensed quasar Q1422+231 (zem=3628) The images are separated by 13 on the sky In an absorption system at zabs=3538, gas column density variations by an order of magnitude and velocity shear on the order of 10 km s?1 are observed in the low-ionization (Si II and C II) lines The transverse separation in the absorbing cloud is estimated to be as small as 26 h?150 pc, corresponding to an effective angular resolution of 4 mas as seen from the Earth In contrast, the high-ionization (C IV) gas appears mostly featureless and thus must be considerably more extended The abundances of the elements carbon, silicon, and oxygen appear to be close to the solar values The observation provides the first spatially and kinematically resolved probe of the interstellar medium at high redshift on scales small enough to be influenced by individual stars or star clusters The mass associated with the low-ionization "cloudlets" is likely to be less than about 3000 M? and possibly less than 1 M? The velocity shear that is seen across the lines of sight is too large to be caused by galactic bulk motion, so the velocity field of the low-ionization gas must be strongly influenced by small-scale local gasdynamics While presently it cannot be excluded that the disturbances of the gas are due to high-velocity outflows from the background quasar, the observed velocity and density structure of the z=3538 absorption is consistent with our line of sight running through an expanding shell of gas, possibly a supernova bubble or a stellar wind

The properties of Supernova 1997cy associated to GRB970514

Abstract: The extraordinary SN 1997cy associated with GRB 970514 has been observed photometrically and spectroscopically for nearly 2 yr. At the time of discovery SN 1997cy was the brightest SN ever observed (Mv=<-20.1, v(hel)=19140 km/s, H_0=65 km/s/Mpc). Up to the last available observations (600 days after the GRB) the total time-integrated flux was equal to or larger than that expected from the complete thermalization of the gamma-rays produced by 2.3 Mo of 56Co. However, starting already on day 60 the luminosity decline is slower than the 56Co decay rate, indicating that the SN ejecta was interacting with circumstellar material (CSM). The interaction appeared to weaken around day 550. The spectra of SN 1997cy are dominated at all epochs by Halpha emission, which shows at least 3 components of different widths, as in SN 1988Z. Several other lines with different widths are also visible, especially at early epochs. The entire light curve of SN 1997cy is reproduced by a model of the interaction of the very energetic (E=5 x 10^(52) ergs) ejecta of a massive star (25 Mo) with the CSM, with some contribution from radioactive decays. The CSM could have been ejected with a mass-loss rate of Mdot ~ 4x10^(-4) Mo yr^(-1) as the progenitor star evolved from a BSG to a RSG about 10^4 yr before the explosion. The lack of oxygen and magnesium lines in the spectra at nebular phases poses a problem for models requiring high mass progenitors. The possibility that most of the core material of the progenitor has fallen onto a massive black hole so that the reverse shock dies at the inner edge of the H/He envelope is discussed.

The Age Difference between the Globular Cluster Subpopulations in NGC 4472

Abstract: The age difference between the two main globular cluster subpopulations in the Virgo giant elliptical galaxy NGC 4472 (M49) has been determined using Hubble Space Telescope WFPC2 images in the F555W and F814W filters. Accurate photometry has been obtained for several hundred globular clusters in each of the two main subpopulations, down to more than 1 mag below the turnover of their luminosity functions. This allows precise determinations of both the mean colors and the turnover magnitudes of the two main subpopulations. By comparing the data with various population synthesis models, the age-metallicity pairs that fit both the observed colors and magnitudes have been identified. The metal-poor and the metal-rich globular clusters are found to be coeval within the errors (~3 Gyr). If one accepts the validity of our assumptions, these errors are dominated by model uncertainties. A systematic error of up to 4 Gyr could affect this result if the blue and the red clusters have significantly different mass distributions. However, that one subpopulation is half as old as the other is excluded at the 99% confidence level. The different globular cluster populations are assumed to trace the galaxy's major star formation episodes. Consequently, the vast majority of globular clusters (and by implication the majority of stars) in NGC 4472 formed at high redshifts but by two distinct mechanisms or in two episodes. The distance to NGC 4472 is determined to be 15.8 ± 0.8 Mpc, which is in excellent agreement with six of the seven Cepheid distances to Virgo Cluster spiral galaxies. This implies that the spiral and elliptical galaxies in the main body of Virgo are at the same distance.

HUBBLE SPACE TELESCOPE Observations of Galactic Globular Cluster Cores. II. NGC 6273 and the Problem of Horizontal-Branch Gaps*

Abstract: We present observations of the center of the Galactic globular cluster NGC 6273, obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 as part of the snapshot program GO-7470 A BV color-magnitude diagram (CMD) for ~28,000 stars is presented and discussed The most prominent feature of the CMD, identified for the first time in this paper, is the extended horizontal-branch blue tail (EBT) with a clear double-peaked distribution and a significant gap The EBT of NGC 6273 is compared with the EBTs of seven other globular clusters for which we have a CMD in the same photometric system From this comparison, we conclude that all the globular clusters in our sample with an EBT show at least one gap along the horizontal branch, which could have similar origins A comparison with theoretical models suggests that at least some of these gaps may be occurring at a particular value of the stellar mass, common to a number of different clusters From the CMD of NGC 6273 we obtain a distance modulus (m - M)V = 1627 ± 020 We also estimate an average reddening E(B-V) = 047 ± 003, though the CMD is strongly affected by differential reddening, with the relative reddening spanning a ΔE(B-V) ~ 02 mag in the WFPC2 field A luminosity function for the evolved stars in NGC 6273 is also presented and compared with the most recent evolutionary models

Stellar Populations at the Center of IC 1613

Abstract: We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters We analyze the resulting color-magnitude diagrams (CMDs) using the main-sequence and giant-branch luminosity functions and comparisons with theoretical stellar models to derive a preliminary star formation history for this galaxy We find a dominant old stellar population (age ≈7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -138 ± 031 We confirm a distance of 715 ± 40 kpc using the I magnitude of the RGB tip The main-sequence luminosity function down to I ≈ 25 provides evidence for a roughly constant star formation rate (SFR) of approximately 35 × 10-4 M⊙ yr-1 across the WFPC2 field of view (022 kpc2) during the past 250–350 Myr Structure in the blue loop luminosity function implies that the SFR was ≈50% higher 400–900 Myr ago than today The mean heavy-element abundance of these young stars is around one-tenth solar The best explanation for a red spur on the main sequence at I ≈ 247 is the blue horizontal branch component of a very old stellar population at the center of IC 1613 We have also imaged a broader area of IC 1613 using the 35 m WIYN telescope under excellent seeing conditions The WIYN CMD reveals a prominent sequence of asymptotic giant branch stars and red supergiants that is less prominent in the WFPC2 CMD because of the smaller field of view The asymptotic giant branch star luminosity function is consistent with a period of continuous star formation over at least the age range 2–10 Gyr We present an approximate age-metallicity relation for IC 1613, which appears to be similar to that of the Small Magellanic Cloud We compare the Hess diagram of IC 1613 with similar data for three other Local Group dwarf galaxies and find that IC 1613 most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216)

30 years of multi-wavelength observations of 3C 273

Abstract: We present a wide multi-wavelength database of most observations of the quasar 3C 273 obtained during the last 30 years. This database is the most complete set of observations available for an active galactic nucleus (AGN). It contains nearly 20 000 observations grouped together into 70 light curves covering 16 orders of magnitude in frequency from the radio to the γ -ray domain. The database is constituted of many previously unpublished observations and of most publicly available data gathered in the literature and on the World Wide Web (WWW). It is complete to the best of our knowledge, except in the optical (UBV ) domain where we chose not to add all observations from the literature. In addition to the photometric data, we present the spectra of 3C 273 obtained by the International Ultraviolet Explorer (IUE) satellite. In the X-ray domain, we used the spectral fit parameters from the literature to construct the light curves. Apart from describing the data, we show the most representative light curves and the average spectrum of 3C 273. The database is available on the WWW in a homogeneous and clear form and we wish to update it regularly by adding new observations.