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| Optical to mid-IR observations of Lyman-alpha galaxies at redshift 5 in the HUDF: a young and low mass population |
| To appear in The Astrophysical Journal
High redshift galaxies selected on the basis of their strong Lyman-alpha emission tend to be young ages and small physical sizes. We show this by analyzing the spectral energy distribution (SED) of 9 Lyman-alpha emitting (LAE) galaxies at 4.0 < z < 5.7 in the Hubble Ultra Deep Field (HUDF). Rest-frame UV to optical 700A < lambda < 7500A luminosities, or upper limits, are used to constrain old stellar populations. We derive best fit, as well as maximally massive and maximally old, properties of all 9 objects. We show that these faint and distant objects are all very young, being most likely only a few millions years old, and not massive, the mass in stars being ~106-108 MSun. Deep Spitzer Infrared Array Camera (IRAC) observations of these objects, even in cases where objects were not detected, were crucial in constraining the masses of these objects. The space density of these objects, ~1.25x10-4 Mpc-3 is comparable to previously reported space density of LAEs at moderate to high redshifts. These Lyman-alpha galaxies show modest star formation rates of ~8 MSun yr-1, which is nevertheless strong enough to have allowed these galaxies to assemble their stellar mass in less than a few x106 years. These sources appear to have small physical sizes, usually smaller than 1 Kpc, and are also rather concentrated. They are likely to be some of the least massive and youngest high redshift galaxies observed to date. |
| Publication date: 13 Aug 2007 |
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| ST-ECF Newsletter 42 |
Contents: - Hubble Status
- Spectral Signal-to-Noise
- Staff Update
- Re-activation of the ACS Solar Blind Channel (SBC)
- Scisoft VII - with VO Support
- Hubblecast: A Video Podcast from ST-ECF
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| Publication date: 15 Jun 2007 |
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| Discovery of a ringlike dark matter structure in the core of the galaxy cluster Cl 0024+17 [proof article] |
| We present a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z ~ 0.4 from ACS data, unifying both strong- and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (~120 arcmin-2) across the cluster enables the derivation of a high-resolution parameter-free mass map. The strongly lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17 obtained in such a way is remarkable. It reveals a ringlike dark matter substructure at r ~ 75" surrounding a soft, dense core at r <= 50". We interpret this peculiar substructure as the result of a high-speed line-of-sight collision of two massive clusters ~1-2 Gyr ago. Such an event is also indicated by the cluster velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the precollision cores. Cl 0024+17 can be likened to the bullet cluster 1E 0657-56, but viewed along the collision axis at a much later epoch. In addition, we show that the long-standing mass discrepancy for Cl 0024+17 between X-ray and lensing can be resolved by treating the cluster X-ray emission as coming from a superposition of two X-ray systems. The cluster's unusual X-ray surface brightness profile that requires a two isothermal sphere description supports this hypothesis. |
| Publication date: 15 May 2007 |
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| The strong transformation of spiral galaxies infalling into massive clusters at z ~ 0.2 |
| We describe two peculiar galaxies falling into the massive galaxy clusters Abell 1689 (z ~ 0.18) and Abell 2667 (z ~ 0.23), respectively. Hubble Space Telescope (HST) images show extraordinary trails composed of bright blue knots (-16.5 < M < -11.5 mag) and stellar streams associated with each of these systems. Combining optical, near- and mid-infrared and radio observations we prove that while both galaxies show similar extended trails of star-forming knots, their recent star formation histories are different. One (~L*) is experiencing a strong burst of star formation, appearing as a rare example of a luminous infrared cluster galaxy. In comparison, the other (~0.1L*) has recently ceased its star formation activity. Our model suggests that the morphologies and star formation in these galaxies have been influenced by the combined action of tidal interaction (likely with the cluster potential) and of ram pressure with the intracluster medium (ICM). These results can be used to gain more insights to the origin of S0s, dwarf and ultracompact dwarf (UCD) cluster galaxies. |
| Publication date: 22 Feb 2007 |
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| Dark matter maps reveal cosmic scaffolding |
| Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious 'dark matter' component, which does not interact via electromagnetism and thus neither emits nor reflects light. As dark matter cannot be seen directly using traditional observations, very little is currently known about its properties. It does interact via gravity, and is most effectively probed through gravitational lensing: the deflection of light from distant galaxies by the gravitational attraction of foreground mass concentrations. This is a purely geometrical effect that is free of astrophysical assumptions and sensitive to all matter - whether baryonic or dark. Here we show high-fidelity maps of the large-scale distribution of dark matter, resolved in both angle and depth. We find a loose network of filaments, growing over time, which intersect in massive structures at the locations of clusters of galaxies. Our results are consistent with predictions of gravitationally induced structure formation, in which the initial, smooth distribution of dark matter collapses into filaments then into clusters, forming a gravitational scaffold into which gas can accumulate, and stars can be built. |
| Publication date: 07 Jan 2007 |
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| ST-ECF Newsletter 41 |
Contents: - Hubble's Bequest to Astronomy
- PHLAG: Pipeline for Hubble Legacy Archive Grism Data
- ESA-ESO Topical Science Working Groups
- Solar System Bodies in Hubble Observations
- Staff Update
- Communication of the "Pluto Affair"
- NASA Award for ST-ECF Staff
- The International Year of Astronomy 2009
- Servicing Mission 4
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| Publication date: 15 Dec 2006 |
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| Large structures and galaxy evolution in COSMOS at z < 1.1 |
| E-print arXiv:astro-ph/0612384, for ApJS COSMOS Special Issue, 2007 in press We present the first identification of large-scale structures (LSS) at z < 1.1 in the Cosmic Evolution Survey (COSMOS). The structures are identified from adaptive smoothing of galaxy counts in the pseudo-3d space (R.A., Dec., z) using the COSMOS photometric redshift catalog. The technique is tested on a simulation including galaxies distributed in model clusters and a field galaxy population - recovering structures on all scales from 1 to 202 without a priori assumptions for the structure size or density profile. Our procedure makes no a priori selection on galaxy spectral energy distribution (SED, for example the Red Sequence), enabling an unbiased investigation of environmental effects on galaxy evolution.
The COSMOS photometric redshift catalog yields a sample of 1.5 × 105 galaxies with redshift accuracy, delta zFWHM/(1 + z) d 0.1 at z < 1.1 down to IAB d 25 mag. Using this sample of galaxies, we identify 42 large-scale structures and clusters. Projected surface-density maps for the structures indicate multiple peaks and internal structure in many of the most massive LSS. The stellar masses (determined from the galactic SEDs) for the LSS range from M* ~ 1011 up to ~ 3 × 1013 Msun. Five LSS have total stellar masses exceeding 1013 Msun. (Total masses including non-stellar baryons and dark matter are expected to be ~ 50 - 100 times greater.) The derived mass function for the LSS is consistent (within the expected Poisson and cosmic variances) with those derived from optical and X-ray studies at lower redshift. |
| Publication date: 14 Dec 2006 |
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| The Spiderweb Galaxy: A Forming Massive Cluster Galaxy at z~2 |
| We present a deep image of the radio galaxy MRC 1138-262 taken with the Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized structure, identified with a protocluster. The morphology shown on the new deep HST ACS image is reminiscent of a spider's web. More than 10 individual clumpy features are observed, apparently star-forming satellite galaxies in the process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago. There is an extended emission component, implying that star formation was occurring over a 50 × 40 kpc region at a rate of more than 100 Msolar yr-1. A striking feature of the newly named "Spiderweb galaxy" is the presence of several faint linear galaxies within the merging structure. The dense environments and fast galaxy motions at the centers of protoclusters may stimulate the formation of these structures, which dominate the faint resolved galaxy populations in the Hubble Ultra Deep Field. The new image provides a unique testbed for simulations of forming dominant cluster galaxies. |
| Publication date: 10 Oct 2006 |
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| HST STIS Spectra of Nuclear Star Clusters in Spiral Galaxies |
| We study the nuclear star clusters in spiral galaxies of various Hubble types using spectra obtained with STIS on-board HST. We observed the nuclear clusters in 40 galaxies, selected from two previous HST/WFPC2 imaging surveys. At a spatial resolution of about 0.2", the spectra provide a better separation of cluster light from underlying galaxy light than is possible with ground-based spectra. Approximately half of the spectra have sufficient signal-to-noise ratio for detailed stellar population analysis. For the other half we only measure the continuum slope, as quantified by the B-V color. To infer the star formation history, metallicity and dust extinction, we fit weighted superpositions of single-age stellar population templates to the high signal-to-noise spectra. We use the results to determine the luminosity-weighted age, mass-to-light ratio, and masses of the clusters. The models provide excellent fits to the data and generally require a mixture of populations of different ages. Approximately half of the sample clusters contain a population younger than 1 Gyr. |
| Publication date: 15 Sep 2006 |
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| Hubble Space Telescope Proper Motions and Stellar Dynamics in the Core of the Globular Cluster 47 Tucanae |
| We have used HST imaging of the central regions of the globular cluster 47 Tucanae (=NGC 104), taken with the WFPC2 and ACS instruments between 1995 and 2002, to derive proper motions and U- and V-band magnitudes for 14 366 stars within 100" (about 5 core radii) of the cluster center. This represents the largest set of member velocities collected for any globular cluster. The stars involved range in brightness from just fainter than the horizontal branch of the cluster to more than 2.5 mag below the main-sequence turnoff. In the course of obtaining these kinematic data, we also use a recent set of ACS images to define a list of astrometrically calibrated positions (and F475W magnitudes) for nearly 130 000 stars in a larger, ~3'×3' central area. We describe our data reduction procedures in some detail and provide the full position, photometric, and velocity data in the form of downloadable electronic tables. We have used the star counts to obtain a new estimate for the position of the cluster center and to define the density profile of main-sequence turnoff and giant branch stars in to essentially zero radius, thus constraining the global spatial structure of the cluster better than before. A single-mass, isotropic King model fit to it is then used as a rough point of reference against which to compare the gross characteristics of our proper-motion data. We search in particular for any evidence of very fast-moving stars, in significantly greater numbers than expected for the extreme tails of the velocity distribution in a sample of our size. We find that likely fewer than 0.1%, and no more than about 0.3%, of stars with measured proper motions have total speeds above the nominal central escape velocity of the cluster. At lower speeds, the proper-motion velocity distribution very closely matches that of a regular King model (which is itself nearly Gaussian given the high stellar density) at all observed radii. |
| Publication date: 15 Sep 2006 |
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| ST-ECF Newsletter 40 |
| Contents: Hubble News Update; Staff Changes; The AstroAsciiData Python Module; Creating a Legacy Archive for Hubble; The New ST-ECF Web; Scisoft VI |
| Publication date: 15 Jul 2006 |
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| ESA SP-1296: ESA's Report to the 36th COSPAR Meeting |
Scientific editor: R. Marsden
Editor: A. Wilson
The report for the 36th COSPAR Meeting covers, as in previous issues, the missions of the Scientific Programme of ESA in the areas of astronomy, Solar System science and fundamental physics. This year's COSPAR meeting will take place only weeks before the end of the SMART-1 mission to the Moon, a technology project that provided the first European look at our natural satellite from lunar orbit.In October of this year, a new mission will be launched: COROT. ESA, together with a number of countries, is contributing to this unique, French-led project that will provide an insight into the interior of the stars, by means of the asteroseismology technique successfully applied by SOHO. COROT will also perform a systematic search for new extrasolar planets using photometric transits.
The record number of ESA Science Programme missions in operation established at the time of the last report was maintained in 2006 (Huygens having been replaced in the list by Venus Express). Eleven different missions, involving 14 operating spacecraft, are providing excellent science to the worldwide scientific community. The Research and Scientific Support Department (RSSD) is responsible for the science operations of these missions and makes every effort to ensure the best possible science return. The Department also supports the realisation of approved projects in all phases of their development. |
| Publication date: 15 Jun 2006 |
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| Multi-wavelength study of the gravitational lens system RXS J113155.4-123155 |
| We present a follow-up imaging study (HST + ground based observations) of the complex gravitational lens system RXS J113155.4-123155. The latter consists of a quadruply imaged QSO and of an Einstein ring. Thanks to the MCS deconvolution technique (Magain, Courbin, Sohy 1998), we retrieve accurate relative positions for the lensed QSO images and photometry in the Bessel B, V, R, I filters and in the J-SOFI, F160W and Ks filters. The HST frames unveil structures in the Einstein ring as well as an unidentified object X in the vicinity of the lensing galaxy. We discuss the lightcurves and the chromatic flux ratio variations observed in this system and deduce that both intrinsic variability and microlensing took place during a span longer than one year. We also demonstrate that microlensing may easily account for the so called anomalous flux ratios presented in the discovery paper. However, the actual flux ratios are still poorly reproduced when modelling the lens potential with a Singular Isothermal Ellipsoid+shear. We argue that this disagreement can hardly be explained by milli-lensing due to substructures in the lensing galaxy. A solution proposed elsewhere (Claeskens et al. 2005) consists in a more complex lens model including an octupole term to the lens gravitational potential. |
| Publication date: 12 Apr 2006 |
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| Abundances in the ISM of Star-Forming Galaxies from FUSE Observations: the case of I Zw 18 |
| We report on the analysis of the FUSE spectrum of the
most metal-poor blue compact dwarf (BCD) galaxy I Zw 18. From a
simultaneous fitting of HI and metal absorption lines arising from the
neutral interstellar medium (ISM), we infer abundances of heavy elements
(Fe, O, Si, Ar, and N) which are several times lower than in the HII regions.
The only exception is Fe, whose abundance is the same. The
abundance pattern of the ISM suggests ancient star formation (SF) activity
with an age of at least a Gyr that enriched the H i phase. A more
recent episode that started 10 to several 100 Myr ago is responsible for
the additional enrichment of alpha-elements and N in the HII regions. |
| Publication date: 01 Apr 2006 |
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| Metals in the Neutral Interstellar Medium of Dwarf Star-Forming Galaxies |
| The determination of the metal abundances in the neutral interstellar medium (ISM) of dwarf star-forming galaxies is a key step in understand-
ing their physical and chemical evolution. This type of investigation has been
possible in the last 5 years thanks to FUSE. We will give a flavor of the issues involved by presenting the work that we are performing in this astrophysical field. |
| Publication date: 10 Jan 2006 |
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| ST-ECF Newsletter 39 |
| Contents: Ageing of Spectral Lamps in Space; Extracting Spectra with Optimal Weights in aXe1.5; ESA-ESO Topical Science Working Groups; NEON Observing Schools |
| Publication date: 15 Dec 2005 |
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| Hubble Space Telescope spectroscopy of the Balmer lines in Sirius B |
| Sirius B is the nearest and brightest of all white dwarfs, but it is very difficult to observe at visible wavelengths due to the overwhelming scattered light contribution from Sirius A. However, from space we can take advantage of the superb spatial resolution of the Hubble Space Telescope (HST) to resolve the A and B components. Since the closest approach in 1993, the separation between the two stars has become increasingly favourable and we have recently been able to obtain a spectrum of the complete Balmer line series for Sirius B using the HST Space Telescope Imaging Spectrograph (STIS). The quality of the STIS spectra greatly exceeds that of previous ground-based spectra, and can be used to provide an important determination of the stellar temperature (Teff= 25193 K) and gravity (log g= 8.556). In addition, we have obtained a new, more accurate, gravitational redshift of 80.42 +/- 4.83 km s-1 for Sirius B. Combining these results with the photometric data and the Hipparcos parallax, we obtain new determinations of the stellar mass for comparison with the theoretical mass-radius relation. However, there are some disparities between the results obtained independently from log g and the gravitational redshift which may arise from flux losses in the narrow 50 × 0.2 arcsec² slit. Combining our measurements of Teff and log g with the Wood evolutionary mass-radius relation, we obtain a best estimate for the white dwarf mass of 0.978 Msolar. Within the overall uncertainties, this is in agreement with a mass of 1.02 Msolar obtained by matching our new gravitational redshift to the theoretical mass-radius relation. |
| Publication date: 15 Oct 2005 |
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| Do Young Galaxies Exist in the Local Universe? Red Giant Branch Detection in the Metal-Poor Dwarf Galaxies SBS 1415+437 |
| We present results from an HST/ACS imaging study of the metal-poor blue compact dwarf galaxy SBS 1415+437.
It has been argued previously that this is a very young galaxy that started to form stars only ~100 Myr ago.
However, we find that the optical color-magnitude diagram prominently reveals asymptotic giant branch and red
giant branch (RGB) stars. The brightness of the RGB tip yields a distance D H 13.6 Mpc. The color of the RGB
implies that its stars must be older than <1.3 Gyr, with the exact age depending on the assumed metallicity and
dust extinction. The number of RGB stars implies that most of the stellar mass resides in this evolved population.
In view of these and other HST results for metal-poor galaxies, it seems that the local universe simply may not
contain any galaxies that are currently undergoing their first burst of star formation. |
| Publication date: 20 Sep 2005 |
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| Discovery of Multiply Imaged Galaxies behind the Cluster and Lensed Quasar SDSS J1004+4112 |
| We have identified three multiply imaged galaxies in Hubble Space Telescope images of the redshift z=0.68 cluster responsible for the large-separation quadruply lensed quasar, SDSS J1004+4112. Spectroscopic redshifts have been secured for two of these systems using the Keck I 10 m telescope. The most distant lensed galaxy, at z=3.332, forms at least four images, and an Einstein ring encompassing 3.1 times more area than the Einstein ring of the lensed QSO images at z=1.74, due to the greater source distance. For a second multiply imaged galaxy, we identify Lyalpha emission at a redshift of z=2.74. The cluster mass profile can be constrained from near the center of the brightest cluster galaxy, where we observe both a radial arc and the fifth image of the lensed quasar, to the Einstein radius of the highest redshift galaxy, ~110 kpc. Our preliminary modeling indicates that the mass approximates an elliptical body, with an average projected logarithmic gradient of ~=-0.5. The system is potentially useful for a direct measurement of world models in a previously untested redshift range.
Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. |
| Publication date: 15 Aug 2005 |
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| The Stellar Content of Nearby Star-forming Galaxies (III) |
| We investigate the nature of the diffuse intracluster ultraviolet light seen in 12 local starburst galaxies, using longslit
ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble
Space Telescope (HST ). We take this faint intracluster light to be the field in each galaxy and compare its spectroscopic
signature with Starburst99 evolutionary synthesis models and with neighboring star clusters. Our main
result is that the diffuse ultraviolet light in 11 of the 12 starbursts lacks the strong O star wind features that are clearly
visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and
field spectra indicates that the field light comes primarily from a different stellar population and not from scattering
of UV photons originating in the massive clusters. |
| Publication date: 20 Jul 2005 |
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