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Abell 383: Getting a Full Picture of an Elusive Subject

A838.jpg

Two teams of astronomers have used data from NASA's Chandra X-ray Observatory and other telescopes to map the distribution of dark matter in a galaxy cluster known as Abell 383, which is located about 2.3 billion light years from Earth. Not only were the researchers able to find where the dark matter lies in the two dimensions across the sky, they were also able to determine how the dark matter is distributed along the line of sight.
Galaxy clusters are the largest gravitationally-bound structures in the universe, and play an important role in research on dark matter and cosmology, the study of the structure and evolution of the universe. The use of clusters as dark matter and cosmological probes hinges on scientists' ability to use objects such as Abell 383 to accurately determine the three-dimensional structures and masses of clusters.

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Discovery of a possibly old galaxy at z=6.027, multiply imaged by the massive cluster Abell 383

Astronomers have uncovered one of the youngest galaxies in the distant universe, with stars that formed 13.5 billion years ago, a mere 200 million years after the Big Bang. The finding addresses questions about when the first galaxies arose, and how the early universe evolved.
NASA's Hubble Space Telescope was the first to spot the newfound galaxy. Detailed observations from the W.M. Keck Observatory on Mauna Kea in Hawaii revealed the observed light dates to when the universe was only 950 million years old; the universe formed about 13.7 billion years ago.

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NASA Telescopes Help Discover Surprisingly Young Galaxy

Astronomers have uncovered one of the youngest galaxies in the distant universe, with stars that formed 13.5 billion years ago, a mere 200 million years after the Big Bang. The finding addresses questions about when the first galaxies arose, and how the early universe evolved.
NASA's Hubble Space Telescope was the first to spot the newfound galaxy. Detailed observations from the W.M. Keck Observatory on Mauna Kea in Hawaii revealed the observed light dates to when the universe was only 950 million years old; the universe formed about 13.7 billion years ago.

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First galaxies were born much earlier than expected

Using the amplifying power of a cosmic gravitational lens, astronomers have discovered a distant galaxy whose stars were born unexpectedly early in cosmic history. This result sheds new light on the formation of the first galaxies, as well as on the early evolution of the Universe.

"We have discovered a distant galaxy that began forming stars just 200 million years after the Big Bang. This challenges theories of how soon galaxies formed and evolved in the first years of the Universe. It could even help solve the mystery of how the hydrogen fog that filled the early Universe was cleared" - Johan Richard, the lead author of a new study.

Richard's team spotted the galaxy in recent observations from the NASA/ESA Hubble Space Telescope, verified it with observations from the NASA Spitzer Space Telescope and measured its distance using W. M. Keck Observatory in Hawaii.

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A383_sm.gif
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The central region of A383 as observed with HST WFPC2, overlaid with an isodensity representation of our lens model.

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Title: Discovery of an old galaxy at z=6.027, multiply imaged by the massive cluster Abell 383
Authors: Johan Richard (1,2), Jean-Paul Kneib (3), Harald Ebeling (4), Dan Stark (5), Eiichi Egami (6), Andrew K. Fiedler (6) ((1) CRAL/Lyon (2) Dark Cosmology Centre (3) LAM/OAMP (4) U.Hawaii (5) IoA, Cambdrige (6) Steward Observatory)

We report the discovery of an old z=6.027 galaxy, multiply imaged by the cluster Abell 383 and detected in new Hubble Space Telescope ACS and WFC3 imaging, as well as in Warm Spitzer observations. This galaxy was selected as a pair of i-dropouts; its suspected high redshift was confirmed by the measurement of a strong Lyman-alpha line in both images using Keck/DEIMOS. Combining Hubble and Spitzer photometry after correcting for contamination by line emission (estimated to be a small effect), we identify a strong Balmer break of 1.5 magnitudes, suggesting the presence of old stars from an early episode of star formation. Taking into account the magnification factor of 11.41.9 (2.650.17 mag) for the brightest image, the unlensed AB magnitude for the source is 27.20.05 in the H band and 25.70.08 at 3.6 um, corresponding to a 0.4 L* galaxy. The UV slope is consistent with beta~2.0, and from the rest-frame UV continuum we measure a current star formation rate of 2.41.1 Msol/yr. The unlensed half-light radius is measured to be 300 pc, from which we deduce a star-forming surface density of ~10 Msol/yr/kpc2. The Lyman-alpha emission is found to be extended over ~3 arcsec along the slit, corresponding to ~5 kpc in the source plane. This can be explained by the presence of a much larger envelope of neutral hydrogen around the star-forming region. Finally, fitting the spectral energy distribution using 7 photometric data points, we derive the following properties: an intrinsic stellar mass of M*=6.3+2.8 -1.2 10^9 Msol, an age of 640-940 Myrs (corresponding to a redshift of formation of 184, and very little reddening. The star-formation rate of this object was much stronger in the past than at the time of observation, suggesting that we may be missing a fraction of galaxies at z~6 which have already faded in rest-frame UV wavelengths.

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Title: A weak lensing analysis of the Abell 383 cluster
Authors: Zhuoyi Huang, Mario Radovich, Aniello Grado, Emanuella Puddu, Anna Romano, Luca Limatola, Liping Fu

In this paper we use deep CFHT and SUBARU uBVRIz archival images of the Abell 383 cluster (z=0.187) to estimate its mass by weak lensing. To this end, we first use simulated images to check the accuracy provided by our KSB pipeline. Such simulations include both the STEP 1 and 2 simulations, and more realistic simulations of the distortion of galaxy shapes by a cluster with a Navarro-Frenk-White (NFW) profile. From such simulations we estimate the effect of noise on shear measurement and derive the correction terms. The R-band image is used to derive the mass by fitting the observed tangential shear profile with a NFW mass profile. Photometric redshifts are computed from the uBVRIz catalogs. Different methods for the foreground/background galaxy selection are implemented, namely selection by magnitude, color and photometric redshifts, and results are compared. In particular, we developed a semi-automatic algorithm to select the foreground galaxies in the color-color diagram, based on observed colors. Using color selection or photometric redshifts improves the correction of dilution from foreground galaxies: this leads to higher signals in the inner parts of the cluster. We obtain a cluster mass that is ~ 20% higher than previous estimates, and is more consistent the mass expected from X--ray data. The R-band luminosity function of the cluster is finally computed.

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Title: The Dark Matter Distribution in Abell 383: Evidence for a Shallow Density Cusp from Improved Lensing, Stellar Kinematic and X-ray Data
Authors: Andrew B. Newman, Tommaso Treu, Richard S. Ellis, David J. Sand

We extend our analyses of the dark matter (DM) distribution in relaxed clusters to the case of Abell 383, a luminous X-ray cluster at z=0.189 with a dominant central galaxy and numerous strongly-lensed features. Following our earlier papers, we combine strong and weak lensing constraints secured with Hubble Space Telescope and Subaru imaging with the radial profile of the stellar velocity dispersion of the central galaxy, essential for separating the baryonic mass distribution in the cluster core. Hydrostatic mass estimates from Chandra X-ray observations further constrain the solution. These combined datasets provide nearly continuous constraints extending from 2 kpc to 1.5 Mpc in radius, allowing stringent tests of results from recent numerical simulations. Two key improvements in our data and its analysis make this the most robust case yet for a shallow slope \beta of the DM density profile
ho_DM ~ r^-\beta on small scales. First, following deep Keck spectroscopy, we have secured the stellar velocity dispersion profile to a radius of 26 kpc for the first time in a lensing cluster. Secondly, we improve our previous analysis by adopting a triaxial DM distribution and axisymmetric dynamical models. We demonstrate that in this remarkably well-constrained system, the logarithmic slope of the DM density at small radii is \beta < 1.0 (95% confidence). An improved treatment of baryonic physics is necessary, but possibly insufficient, to reconcile our observations with the recent results of high-resolution simulations.

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