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| Cosmological Constraints from Strong Gravitational Lensing in Clusters of Galaxies |
| Current efforts in observational cosmology are focused on characterizing the mass-energy content of the universe. We present results from a geometric test based on strong lensing in galaxy clusters. Based on Hubble Space Telescope images and extensive ground-based spectroscopic follow-up of the massive galaxy cluster Abell 1689, we used a parametric model to simultaneously constrain the cluster mass distribution and dark energy equation of state. Combining our cosmological constraints with those from X-ray clusters and the Wilkinson Microwave Anisotropy Probe 5-year data gives Omegam = 0.25 ±0.05 and wx = -0.97 ±0.07, which are consistent with results from other methods. Inclusion of our method with all other available techniques brings down the current 2-sigma contours on the dark energy equation-of-state parameter wx by ~30%. |
| Publication date: 19 Aug 2010 |
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| Seeing with Hubble-Vision: 20 years of science with the Hubble Space Telescope |
| The NASA/ESA Hubble Space Telescope is one of the greatest scientific projects of all time. For 20 years, Hubble has opened our eyes to the wonders of our 'planetary' backyard and beyond, and has made a number of fundamental discoveries in astronomy and physics. |
| Publication date: 16 Jul 2010 |
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| Two distinct sequences of blue straggler stars in the globular cluster M30 |
| Stars in globular clusters are generally believed to have all formed at the same time, early in the Galaxy's history. 'Blue stragglers' are stars massive enough that they should have evolved into white dwarfs long ago. Two possible mechanisms have been proposed for their formation: mass transfer between binary companions and stellar mergers resulting from direct collisions between two stars. Recently the binary explanation was claimed to be dominant. Here we report that there are two distinct parallel sequences of blue stragglers in M 30. This globular cluster is thought to have undergone 'core collapse', during which both the collision rate and the mass transfer activity in binary systems would have been enhanced. We suggest that the two observed sequences are a consequence of cluster core collapse, with the bluer population arising from direct stellar collisions and the redder one arising from the evolution of close binaries that are probably still experiencing an active phase of mass transfer. |
| Publication date: 24 Dec 2009 |
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| A single sub-kilometre Kuiper belt object from a stellar occultation in archival data |
| The Kuiper belt is a remnant of the primordial Solar System. Measurements of its size distribution constrain its accretion and collisional history, and the importance of material strength of Kuiper belt objects. Small, sub-kilometre-sized, Kuiper belt objects elude direct detection, but the signature of their occultations of background stars should be detectable. Observations at both optical and X-ray wavelengths claim to have detected such occultations, but their implied abundances are inconsistent with each other and far exceed theoretical expectations. Here we report an analysis of archival data that reveals an occultation by a body with an approximately 500-metre radius at a distance of 45 astronomical units. The probability of this event arising from random statistical fluctuations within our data set is about two per cent. Our survey yields a surface density of Kuiper belt objects with radii exceeding 250 metres of 2.1+4.8-1.7 x 107 deg-2, ruling out inferred surface densities from previous claimed detections by more than 5 sigma. The detection of only one event reveals a deficit of sub-kilometre-sized Kuiper belt objects compared to a population extrapolated from objects with radii exceeding 50 kilometres. This implies that sub-kilometre-sized objects are undergoing collisional erosion, just like debris disks observed around other stars. |
| Publication date: 17 Dec 2009 |
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| The Disappearance of the Progenitors of Supernovae 1993J and 2003gd |
| Using images from the Hubble Space Telescope and the Gemini Telescope, we confirmed the disappearance of the progenitors of two type II supernovae (SNe) and evaluated the presence of other stars associated with them. We found that the progenitor of SN 2003gd, an M-supergiant star, is no longer observed at the SN location and determined its intrinsic brightness using image subtraction techniques. The progenitor of SN 1993J, a K-supergiant star, is also no longer present, but its B-supergiant binary companion is still observed. The disappearance of the progenitors confirms that these two supernovae were produced by red supergiants. |
| Publication date: 24 Apr 2009 |
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| A massive hypergiant star as the progenitor of the supernova SN 2005gl |
| Our understanding of the evolution of massive stars before their final explosions as supernovae is incomplete, from both an observational and a theoretical standpoint. A key missing piece in the supernova puzzle is the difficulty of identifying and studying progenitor stars. In only a single case - that of supernova SN 1987A in the Large Magellanic Cloud - has a star been detected at the supernova location before the explosion, and been subsequently shown to have vanished after the supernova event. The progenitor of SN 1987A was a blue supergiant, which required a rethink of stellar evolution models. The progenitor of supernova SN 2005gl was proposed to be an extremely luminous object, but the association was not robustly established (it was not even clear that the putative progenitor was a single luminous star). Here we report that the previously proposed object was indeed the progenitor star of SN 2005gl. This very massive star was likely a luminous blue variable that standard stellar evolution predicts should not have exploded in that state. |
| Publication date: 16 Apr 2009 |
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| A binary origin for 'blue stragglers' in globular clusters |
| Blue stragglers in globular clusters are abnormally massive stars that should have evolved off the stellar main sequence long ago. There are two known processes that can create these objects: direct stellar collisions and binary evolution. However, the relative importance of these processes has remained unclear. In particular, the total number of blue stragglers found in a given cluster does not seem to correlate with the predicted collision rate, providing indirect support for the binary-evolution model. Yet the radial distributions of blue stragglers in many clusters are bimodal, with a dominant central peak: this has been interpreted as an indication that collisions do dominate blue straggler production, at least in the high-density cluster cores. Here we report that there is a clear, but sublinear, correlation between the number of blue stragglers found in a cluster core and the total stellar mass contained within it. From this we conclude that most blue stragglers, even those found in cluster cores, come from binary systems. The parent binaries, however, may themselves have been affected by dynamical encounters. This may be the key to reconciling all of the seemingly conflicting results found to date. |
| Publication date: 15 Jan 2009 |
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| 18 years of science with the Hubble Space Telescope |
| After several decades of planning, the Hubble Space Telescope (HST) was launched in 1990 as the first of NASA's Great Observatories. After a rocky start arising from an error in the fabrication of its main mirror, it went on to change forever many fields of astronomy, and to capture the public's imagination with its images. An ongoing programme of servicing missions has kept the telescope on the cutting edge of astronomical research. Here I review the advances made possible by the HST over the past 18 years. |
| Publication date: 01 Jan 2009 |
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| ST-ECF Newsletter 45 |
Contents:- Hubble Status
- The Hubble Cache
- Footprint Finder
- HLA ACS Grism Data
- Scisoft 7.2
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| Publication date: 15 Dec 2008 |
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| Optical Images of an Exosolar Planet 25 Light-Years from Earth |
| Fomalhaut, a bright star 7.7 parsecs (25 light-years) from Earth, harbors a belt of cold dust with a structure consistent with gravitational sculpting by an orbiting planet. Here, we present optical observations of an exoplanet candidate, Fomalhaut b. Fomalhaut b lies about 119 astronomical units (AU) from the star and 18 AU of the dust belt, matching predictions of its location. Hubble Space Telescope observations separated by 1.73 years reveal counterclockwise orbital motion. Dynamical models of the interaction between the planet and the belt indicate that the planet's mass is at most three times that of Jupiter; a higher mass would lead to gravitational disruption of the belt, matching predictions of its location. The flux detected at 0.8 µm is also consistent with that of a planet with mass no greater than a few times that of Jupiter. The brightness at 0.6 µm and the lack of detection at longer wavelengths suggest that the detected flux may include starlight reflected off a circumplanetary disk, with dimension comparable to the orbits of the Galilean satellites. We also observe variability of unknown origin at 0.6 µm. |
| Publication date: 28 Nov 2008 |
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| Magnetic support of the optical emission line filaments in NGC 1275 |
| The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments, which are plausibly in excess of 108 years old. The filaments are dragged out from the centre of the galaxy by radio-emitting 'bubbles' rising buoyantly in the hot intracluster gas, before later falling back. They act as markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding extremely hot (4x107 K) gas has been unclear. Here we report observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation. |
| Publication date: 21 Aug 2008 |
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| The ACS Virgo Cluster Survey. XV. The Formation Efficiencies of Globular Clusters in Early-Type Galaxies: The Effects of Mass and Environment |
The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy's early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey, the largest homogeneous catalog of its kind.
[This is an extract of the original abstract.]
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| Publication date: 01 Jul 2008 |
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| ST-ECF Newsletter 44 |
Contents: - Hubble Legacy Archive - Data Release One
- The WFC3 Slitless Spectroscopy Simulator aXeSIMweb
- Why Should We Bother to Communicate Astronomy?
- Tiny Tim PSF Simulator - with WFC3 Support
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| Publication date: 15 Jun 2008 |
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| Discovery of a Very Bright Strongly Lensed Galaxy Candidate at z ~ 7.6 |
| Using Hubble Space Telescope (HST) and Spitzer IRAC imaging, we report the discovery of a very bright strongly lensed Lyman break galaxy (LBG) candidate at z ~ 7.6 in the field of the massive galaxy cluster Abell 1689 (z=0.18). The galaxy candidate, which we refer to as A1689-zD1, shows a strong z850-J110 break of at least 2.2 mag and is completely undetected (<1 sigma) in HST Advanced Camera for Surveys (ACS) g475, r625, i775, and z850 data. These properties, combined with the very blue J110-H160 and H160-4.5 micron colors, are exactly the properties of an z ~ 7.6 LBG, and can only be reasonably fit by a star-forming galaxy at z=7.6±0.4 (Chimu-square=1.1). Attempts to reproduce these properties with a model galaxy at z < 4 yield particularly poor fits (Chimu-square=25). A1689-zD1 has an observed (lensed) magnitude of 24.7 AB (8 sigma) in the NICMOS H160 band and is 1.3 mag brighter than the brightest known z850-dropout galaxy. When corrected for the cluster magnification of ~9.3 at z~7.6, the candidate has an intrinsic magnitude of H160=27.1 AB, or about an L* galaxy at z~7.6. The source-plane deprojection shows that the star formation is occurring in compact knots of size <~300 pc. The best-fit stellar population synthesis models yield a median redshift of 7.6, stellar masses (1.6-3.9)×109 Msolar, stellar ages 45-320 Myr, star formation rates <~7.6 Msolar yr-1, and low reddening with AV<=0.3. These properties are generally similar to those of LBGs found at z~5-6. The inferred stellar ages suggest a formation redshift of z~8-10 (t<~0.63Gyr). A1689-zD1 is the brightest observed, highly reliable z>7.0 galaxy candidate found to date. |
| Publication date: 10 May 2008 |
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| A New Red Giant–based Distance Modulus of 13.3 Mpc to the Antennae Galaxies and Its Consequences |
| The Antennae galaxies are the closest example of an ongoing major galaxy merger and, as such, represent a unique laboratory for furthering the understanding of the formation of exotic objects (e.g., tidal dwarf galaxies, ultraluminous X-ray sources, super stellar clusters). In a previous paper HST WFPC2 observations were used to demonstrate that the Antennae system might be at a distance considerably less than that conventionally assumed in the literature. Here we report new, much deeper HST ACS imaging that resolves the composite stellar populations and, most importantly, reveals a well-defined red giant branch. The tip of this red giant branch (TRGB) is unambiguously detected at I0TRGB=26.65+-0.09 mag. Adopting the most recent calibration of the luminosity of the TRGB then yields a distance modulus for the Antennae of (m-M)0 = 30.62+-0.17 corresponding to a distance of 13.3+-1.0 Mpc. This is consistent with our earlier result, once the different calibrations for the standard candle are considered. We briefly discuss the implications of this now well-determined shorter distance. |
| Publication date: 09 May 2008 |
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| Confirmation of the Remarkable Compactness of Massive Quiescent Galaxies at z~2.3: Early-Type Galaxies Did not Form in a Simple Monolithic Collapse |
| Using deep near-infrared spectroscopy, Kriek et al. found that 45% of massive galaxies at z~2.3 have evolved stellar populations and little or no ongoing star formation. Here we determine the sizes of these quiescent galaxies using deep, high-resolution images obtained with HST/NIC2 and laser guide star (LGS)-assisted Keck/adaptive optics (AO). Considering that their median stellar mass is 1.7 x 1011 MSun, the galaxies are remarkably small, with a median effective radius re = 0.9 kpc. Galaxies of similar mass in the nearby universe have sizes of H5 kpc and average stellar densities that are 2 orders of magnitude lower than the z~2.3 galaxies. These results extend earlier work at z~1.5 and confirm previous studies at z>2 that lacked spectroscopic redshifts and imaging of sufficient resolution to resolve the galaxies. Our findings demonstrate that fully assembled early-type galaxies make up at most 10% of the population of K-selected quiescent galaxies at z~2.3, effectively ruling out simple monolithic models for their formation. The galaxies must evolve significantly after z~2.3, through dry mergers or other processes, consistent with predictions from hierarchical models. |
| Publication date: 10 Apr 2008 |
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| Gemini and Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in omega Centauri |
| The globular cluster omega Centauri is one of the largest and most massive members of the galactic system. However, its classification as a globular cluster has been challenged making it a candidate for being the stripped core of an accreted dwarf galaxy; this together with the fact that it has one of the largest velocity dispersions for star clusters in our galaxy makes it an interesting candidate for harboring an intermediate-mass black hole. We measure the surface brightness profile from integrated light on an HSTACS image of the center, and find a central power-law cusp of logarithmic slope
-0.08.We also analyze Gemini GMOS-IFU kinematic data for a 5"x5" field centered on the nucleus of the cluster, as well as for a field 14" away.We detect a clear rise in the velocity dispersion from 18.6 km s-1 at 14" to 23 km s-1 in the center. A rise in the velocity dispersion could be due to a central black hole, a central concentration of stellar remnants, or a central orbital structure that is radially biased.We discuss each of these possibilities. An isotropic, spherical dynamical model implies a black hole mass of 4.0+0.75-1.0 x 104 MSun, and excludes the no black hole case at greater than 99% significance. We have also run flattened, orbit-based models and find similar results. While our preferred model is the existence of a central black hole, detailed numerical simulations are required to confidently rule out the other possibilities. |
| Publication date: 02 Apr 2008 |
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| Methane present in an extrasolar planet atmosphere |
| Molecules present in exoplanetary atmospheres are expected to strongly influence the atmospheric radiation balance, trace dynamical and chemical processes, and indicate the presence of disequilibrium effects. Since molecules have the potential to reveal the exoplanet atmospheric conditions and chemistry, searching for them is a high priority. The rotational-vibrational transition bands of water, carbon monoxide, and methane are anticipated to be the primary sources of non-continuum opacity in hot-Jovian planets. Since these bands overlap in wavelength, and the corresponding signatures from them are weak, decisive identification requires precision infrared spectroscopy. Here we report on a near-infrared transmission spectrum of the planet HD 189733b showing the presence of methane. Additionally, a resolved water-vapour band at 1.9 micron confirms the recent claim of water in this object. On thermochemical grounds, carbon-monoxide is expected to be abundant in the upper atmosphere of hot-Jovian exoplanets; thus the detection of methane rather than carbon-monoxide in such a hot planet could signal the presence of a horizontal chemical gradient away from the permanent dayside, or it may imply an ill-understood photochemical mechanisms that leads to an enhancement of methane. |
| Publication date: 20 Mar 2008 |
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| Depth of a strong jovian jet from a planetary-scale disturbance driven by storms |
| The atmospheres of the gas giant planets (Jupiter and Saturn) contain jets that dominate the circulation at visible levels. The power source for these jets (solar radiation, internal heat, or both) and their vertical structure below the upper cloud are major open questions in the atmospheric circulation and meteorology of giant planets. Several observations and in situ measurements found intense winds at a depth of 24 bar, and have been interpreted as supporting an internal heat source. This issue remains controversial, in part because of effects from the local meteorology. Here we report observations and modelling of two plumes in Jupiter's atmosphere that erupted at the same latitude as the strongest jet (23° N). The plumes reached a height of 30 km above the surrounding clouds, moved faster than any other feature (169 m s-1), and left in their wake a turbulent planetary-scale disturbance containing red aerosols. On the basis of dynamical modelling, we conclude that the data are consistent only with a wind that extends well below the level where solar radiation is deposited. |
| Publication date: 24 Jan 2008 |
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| The dark matter environment of the Abell 901/902 supercluster: a weak lensing analysis of the HST STAGES survey |
| We present a high resolution dark matter reconstruction of the z = 0.16 Abell 901/902 supercluster from a weak lensing analysis of the STAGES HST survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901-alpha. We present mass, mass-to-light and massto-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'. |
| Publication date: 08 Jan 2008 |
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