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| A Wide Field Hubble Space Telescope Study of the Cluster Cl0024+1654 at z=0.4 II: The Cluster Mass Distribution |
| We present a comprehensive lensing analysis of the rich cluster Cl0024+1654 (z=0.395) based on
panoramic sparse-sampled imaging conducted with the WFPC2 and STIS cameras on board the Hubble
Space Telescope. By comparing higher fidelity signals in the limited STIS data with the wider field data available from WFPC2, we demonstrate an ability to detect reliably weak lensing signals to a cluster radius of ~5 (h65)-1 Mpc where the mean shear is around 1%. |
| Publication date: 10 Nov 2003 |
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| Massive Star Formation in a Gravitationally Lensed H II Galaxy at z = 3.357 |
| The Lynx arc, with a redshift of 3.357, was discovered during spectroscopic follow-up of the z=0.570 cluster RX J0848+4456 from the ROSAT Deep Cluster Survey. The arc is characterized by a very red R-K color and strong, narrow emission lines. Analysis of HST WFPC2 imaging and Keck optical and infrared spectroscopy shows that the arc is an H II galaxy magnified by a factor of ~10 by a complex cluster environment. The high intrinsic luminosity, the emission-line spectrum, the absorption components seen in Lyalpha and C IV, and the rest-frame ultraviolet continuum are all consistent with a simple H II region model containing ~106 hot O stars. The best-fit parameters for this model imply a very hot ionizing continuum (TBB~= 80 000 K), a high ionization parameter (logU~=-1), and a low nebular metallicity (Z/Zsolar~=0.05). The narrowness of the emission lines requires a low mass-to-light ratio for the ionizing stars, suggestive of an extremely low metallicity stellar cluster. The apparent overabundance of silicon in the nebula could indicate enrichment by past pair-instability supernovae, requiring stars more massive than ~140 Msolar. |
| Publication date: 20 Oct 2003 |
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| ST-ECF Newsletter 34 |
| The last few months have proved to be a time when both the Hubble project and the ST-ECF are considering the transition period when Hubble will end its life and the James Webb Space Telescope (JWST) will begin operations. |
| Publication date: 01 Sep 2003 |
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| A Scattered Light Echo around SN 1993J in M81 |
| A light echo around SN 1993J was observed 8.2 yr after explosion by a Hubble Space Telescope Wide Field Planetary Camera 2 observation, adding to the small family of supernovae with light echoes. The light echo was formed by supernova light scattered from a dust sheet that lies 220 pc away from the supernova, 50 pc thick along the line of sight, as inferred from the radius and width of the light echo. The dust inferred from the light echo surface brightness is 1000 times denser than the intercloud dust. The graphite-to-silicate fraction cannot be determined by our BVI photometric measurements. However, a pure graphite model can be excluded on the basis of comparison with the data. With future observations, it will be possible to measure the expansion rate of the light echo, from which an independent distance to M81 can be obtained. |
| Publication date: 15 Jan 2003 |
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| Multiple Light Echoes from SN 1993J |
| Using the technique of point-spread function-matched image subtraction, we have analyzed archival Hubble Space Telescope (HST) Wide Field Planetary Camera 2 data to reveal details of at least two light echo structures, including some unknown before now, around SN 1993J in the galaxy M81. In particular, we see one partial sheet of material 81 pc in front of the supernova (SN) and tilted ~60° relative to the disk plane of M81 and another 220 pc in front of the SN, roughly parallel to the disk. The inferred echoing material is consistent with the H I surface density detected in this region of M81's disk; however, these data imply a fragmented covering factor for the echoing structures. We discuss prospects for future (roughly annual) visits by HST to image these and yet undiscovered echoes in the interstellar and circumstellar environment of SN 1993J. |
| Publication date: 15 Dec 2002 |
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| ESA SP-1259: ESA's Report to the 34th COSPAR Meeting |
Scientific editor: B. Foing
Editor: A. Wilson
The report to the 34th COSPAR Meeting covers the missions of the Scientific Programme of ESA in the areas of astronomy, Solar System exploration and fundamental physics. |
| Publication date: 01 Oct 2002 |
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| Structure and Dynamics of Candidate O Star Bubbles in N44 |
| Dynamical studies of superbubbles and Wolf-Rayet ring nebulae show discrepancies from the standard adiabatic model for windblown bubbles. We therefore study the physical properties and kinematics of three candidate bubbles blown by single O stars to evaluate whether these discrepancies are also found in these simpler objects. Our sample candidates are N44 F, N44 J, and N44 M, in the outskirts of the H II complex N44 in the Large Magellanic Cloud. We have obtained ground-based and Hubble Space Telescope emission-line images and high-dispersion echelle spectra for these objects. From the Halpha luminosities and the [O III]/Halpha ratios of these nebulae, we estimate the spectral types of the ionizing stars to be O7 V, O9.5 V, and O9.5 V for N44 F, N44 J, and N44 M, respectively. We find that the observed expansion velocity of 12 kms-1 for N44 F is consistent with the stellar wind luminosity expected from the central ionizing star, as predicted by the standard bubble model. The observed upper limits for the expansion velocities of N44 J and N44 M are also compatible with the expected values, within the uncertainties. We also report the discovery in N44 F of strongly defined dust columns, similar to those seen in the Eagle Nebula. The photoevaporation of these dense dust features may be kinematically important and may actually govern the evolution of the shell. The inclusion of photoevaporation processes may thus undermine the apparent agreement between the observed bubble dynamics and the simple adiabatic models. |
| Publication date: 23 Aug 2002 |
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| Interstellar Bubbles in Two Young HII Regions |
| Massive stars are expected to produce wind-blown bubbles in the interstellar medium; however, ring nebulae, suggesting the existence of bubbles, are rarely seen around main-sequence O stars. To search for wind-blown bubbles around main-sequence O stars, we have obtained high-resolution Hubble Space Telescope WFPC2 images and high-dispersion echelle spectra of two pristine H II regions, N11B and N180B, in the Large Magellanic Cloud. These H II regions are ionized by OB associations that still contain O3 stars, suggesting that the H II regions are young and have not hosted any supernova explosions. Our observations show that wind-blown bubbles in these H II regions can be detected kinematically, but not morphologically, because their expansion velocities are comparable to or only slightly higher than the isothermal sound velocity in the H II regions. Bubbles are detected around concentrations of massive stars, individual O stars, and even an evolved red supergiant (a fossil bubble). Comparisons between the observed bubble dynamics and model predictions show a large discrepancy (1-2 orders of magnitude) between the stellar wind luminosity derived from bubble observations and models and that derived from observations of stellar winds. The number and distribution of bubbles in N11B differ from those in N180B, which can be explained by the difference in the richness of stellar content between these two H II regions. Most of the bubbles observed in N11B and N180B show a blister structure, indicating that the stars were formed on the surfaces of dense clouds. Numerous small dust clouds, similar to Bok globules or elephant trunks, are detected in these H II regions, and at least one of them hosts on-going star formation. |
| Publication date: 15 Aug 2001 |
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| The Hubble Space Telescope - 10 Years On |
| Last Christmas Eve was very special one for ESA astronauts Claude Nicollier and Jean-François Clervoy: together with their American colleagues, they spent it aboard the Space Shuttle 'Discovery', after concluding the latest scheduled repair mission to the orbiting Hubble Space Telescope (HST). This third Shuttle refurbishment mission to HST was, like its two predecessors, a resounding success. Only days later, as Hubble entered the new millennium, came the first beautiful images of a complex gravitationally lensing cluster of galaxies.
The astronauts' visit took place shortly before the 10th Anniversary of the launch of Hubble, which was first placed in orbit on 26 April 1990. Since then, HST has become the leading tool in ultraviolet, optical and near-infrared astronomy and is now looking forward to another decade
of exciting discoveries and sharp views of the Universe. |
| Publication date: 01 Nov 2000 |
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| Evidence for a 20 parsec disk at the Nucleus of Centaurus A |
| We report Hubble Space Telescope NICMOS observations of the central region of NGC 5128 at 2.2 mm and
in Paa. The continuum images show extended emission typical of an elliptical galaxy and a strong unresolved
central source we identify as the nucleus of the galaxy. Its position is consistent with ground-based IR and radio
data, and with the peak of reddening found with the first wide field planetary camera. In Paa , we detect a
prominent elongated structure, centered on the nucleus, extended by ~2" at a position angle of ~33°, and with
a major to minor axis ratio of ~2. We interpret this as an inclined, ~40 pc diameter, thin nuclear disk of ionized
gas rather than a jet-gas cloud interaction. We do see several weaker Paa features, some of which may be
circumnuclear gas clouds shocked by the X-ray/radio jet. The disk is one of the smallest ever observed at the
nucleus of an active galactic nucleus (AGN). It is not perpendicular to the jet but is consistent with being oriented
along the major axis of the bulge. If it represents the warped outer portion of an accretion disk around a black
hole, we conclude that even on the scale of a few parsecs, the disk is dominated by the galaxy gravitational
potential and is not directly related to the symmetry axis of the AGN. |
| Publication date: 01 Jun 1998 |
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