274 research outputs found
Extracting local surface charges and charge regulation behavior from atomic force microscopy measurements at heterogeneous solid-electrolyte interfaces
We present a method to determine the local surface charge of solidâliquid interfaces from Atomic Force Microscopy (AFM) measurements that takes into account shifts of the adsorption/desorption equilibria of protons and ions as the cantilever tip approaches the sample. We recorded AFM force distance curves in dynamic mode with sharp tips on heterogeneous silica surfaces partially covered by gibbsite nano-particles immersed in an aqueous electrolyte with variable concentrations of dissolved NaCl and KCl at pH 5.8. Forces are analyzed in the framework of DerjaguinâLandauâVerweyâOverbeek (DLVO) theory in combination with a charge regulation boundary that describes adsorption and desorption reactions of protons and ions. A systematic method to extract the equilibrium constants of these reactions by simultaneous least-squared fitting to experimental data for various salt concentrations is developed and is shown to yield highly consistent results for silica-electrolyte interfaces. For gibbsite-electrolyte interfaces, the surface charge can be determined, yet, an unambiguous identification of the relevant surface speciation reactions is not possible, presumably due to a combination of intrinsic chemical complexity and heterogeneity of the nano-particle surfaces
Cosmic Chronometers: Constraining the Equation of State of Dark Energy. II. A Spectroscopic Catalog of Red Galaxies in Galaxy Clusters
We present a spectroscopic catalog of (mostly) red galaxies in 24 galaxy
clusters in the redshift range 0.17 < z < 0.92 obtained with the LRIS
instrument on the Keck I telescope. Here we describe the observations and the
galaxy spectra, including the discovery of three cD galaxies with LINER
emission spectra, and the spectroscopic discovery of four new galaxy-galaxy
lenses in cluster environments.Comment: Submitted to ApJ
The Observed Growth of Massive Galaxy Clusters III: Testing General Relativity on Cosmological Scales
This is the third of a series of papers in which we derive simultaneous
constraints on cosmological parameters and X-ray scaling relations using
observations of the growth of massive, X-ray flux-selected galaxy clusters. Our
data set consists of 238 clusters drawn from the ROSAT All-Sky Survey, and
incorporates extensive follow-up observations using the Chandra X-ray
Observatory. Here we present improved constraints on departures from General
Relativity (GR) on cosmological scales, using the growth index, gamma, to
parameterize the linear growth rate of cosmic structure. Using the method of
Mantz et al. (2009a), we simultaneously and self-consistently model the growth
of X-ray luminous clusters and their observable-mass scaling relations,
accounting for survey biases, parameter degeneracies and systematic
uncertainties. We combine the cluster growth data with gas mass fraction, SNIa,
BAO and CMB data. This combination leads to a tight correlation between gamma
and sigma_8. Consistency with GR requires gamma~0.55. Under the assumption of
self-similar evolution and constant scatter in the scaling relations, and for a
flat LCDM model, we measure gamma(sigma_8/0.8)^6.8=0.55+0.13-0.10, with
0.79<sigma_8<0.89. Relaxing the assumptions on the scaling relations by
introducing two additional parameters to model possible evolution in the
normalization and scatter of the luminosity-mass relation, we obtain consistent
constraints on gamma that are only ~20% weaker than those above. Allowing the
dark energy equation of state, w, to take any constant value, we simultaneously
constrain the growth and expansion histories, and find no evidence for
departures from either GR or LCDM. Our results represent the most robust
consistency test of GR on cosmological scales to date. (Abridged)Comment: Accepted for publication in MNRAS. 11 pages, 5 figures, 1 table. New
figure added: Fig. 4 shows the tight constraints on gamma from the cluster
growth data alone compared with those from the other data sets combined
SHELS: Testing Weak Lensing Maps with Redshift Surveys
Weak lensing surveys are emerging as an important tool for the construction
of "mass selected" clusters of galaxies. We evaluate both the efficiency and
completeness of a weak lensing selection by combining a dense, complete
redshift survey, the Smithsonian Hectospec Lensing Survey (SHELS), with a weak
lensing map from the Deep Lens Survey (DLS). SHELS includes 11,692 redshifts
for galaxies with R < 20.6 in the four square degree DLS field; the survey is a
solid basis for identifying massive clusters of galaxies with redshift z <
0.55. The range of sensitivity of the redshift survey is similar to the range
for the DLS convergence map. Only four the twelve convergence peaks with
signal-to-noise > 3.5 correspond to clusters of galaxies with M > 1.7 x 10^14
solar masses. Four of the eight massive clusters in SHELS are detected in the
weak lensing map yielding a completeness of roughly 50%. We examine the seven
known extended cluster x-ray sources in the DLS field: three can be detected in
the weak lensing map, three should not be detected without boosting from
superposed large-scale structure, and one is mysteriously undetected even
though its optical properties suggest that it should produce a detectable
lensing signal. Taken together, these results underscore the need for more
extensive comparisons among different methods of massive cluster
identification.Comment: 34 pages, 16 figures, ApJ accepte
An Empirically-Calibrated Model For Interpreting the Evolution of Galaxies During the Reionization Era
[Abridged] We develop a simple star formation model whose goal is to
interpret the emerging body of observational data on star-forming galaxies at
z>~6. The efficiency and duty cycle of the star formation activity within dark
matter halos are determined by fitting the luminosity functions of Lya emitter
and Lyman-break galaxies at redshifts z~5-6. Using our model parameters we
predict the likely abundance of star forming galaxies at earlier epochs and
compare these to the emerging data in the redshift interval 7<z<10. We find
that the abundance of luminous Lyman-break galaxies in the 500 Myr between z~6
and 10 can be naturally explained by the hierarchical assembly of dark matter
haloes; there is only marginal evidence for strong physical evolution. In
contrast, the first estimates of the abundance of less luminous star forming
galaxies at z=9-10 are higher than predicted and, if verified by further data,
may suggest a top-heavy stellar mass function at these early epochs. Although
these abundances remain uncertain because of the difficulty of spectroscopic
confirmation and cosmic variance, even a modest improvement in survey
capability with present or upcoming facilities should yield great progress. In
this context, we use our model to consider those observational techniques that
hold the most promise and make predictions for specific surveys that are, or
will soon be, underway. We conclude that narrowband Lya emitter surveys should
be efficient on searches at z~7-8; however, such conventional surveys are
unlikely to detect sufficient galaxies at z~10 to provide useful constraints.
For this reason, gravitational lensing offers the best prospect for probing the
z~10 universe prior to JWST.Comment: 17 pages, 12 figures, submitted to Ap
Gravitational Lensing by Nearby Clusters of Galaxies
We present an estimation of the expected number of arcs and arclets in a
sample of nearby (z<0.1) clusters of galaxies, that takes into account the
magnitude limit of the objects as well as seeing effects. We show that strong
lensing effects are not common, but also they are not as rare as usually
stated. Indeed, for a given cluster, they present a strong dependence with the
magnitude limit adopted in the analysis and the seeing of the observations. We
also describe the procedures and results of a search for lensing effects in a
sample of 33 clusters spanning the redshift range of 0.014 to 0.076,
representative of the local cluster distribution. This search produced two arc
candidates. The first one is in A3408 (z=0.042), the same arc previously
discovered by Campusano & Hardy (1996), with z=0.073 and associated to the
brightest cluster galaxy. The second candidate is in the cluster A3266
(z=0.059) and is near a bright elliptical outside the cluster center, requiring
the presence of a very massive sub-structure around this galaxy to be produced
by gravitational lensing.Comment: 22 pages including 9 Figures and 2 Tables, submitted to A
Discovery of a possibly old galaxy at , multiply imaged by the massive cluster Abell 383
We report the discovery of a unique 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. Taking into account the magnification factor of
11.4+/-1.9 (2.65+/-0.17 mag) for the brightest image, the unlensed AB magnitude
for the source is 27.2+/-0.05 in the H band, corresponding to a 0.4 L* galaxy,
and 25.7+/-0.08 at 3.6 um. The UV slope is consistent with beta~2.0, and from
the rest-frame UV continuum we measure a current star formation rate of
2.4+/-1.1 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" 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 with simple SED models, we derive the following properties: very
little reddening, an inferred stellar mass of M*=6e9 Msol, and an inferred age
of ~800 Myrs (corresponding to a redshift of formation of ~18). The
star-formation rate of this object was likely 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.Comment: 6 pages, 3 figures, MNRAS in press, replaced with accepted version
including minor comment
Circulating 25-Hydroxyvitamin D Concentration and Risk of Breast, Prostate, and Colorectal Cancers: The Melbourne Collaborative Cohort Study.
BACKGROUND: The role of vitamin D in cancer risk remains controversial, and limited data exist on associations between vitamin D and subtypes of specific cancers. We investigated associations between circulating 25-hydroxyvitamin D (25(OH)D) and risk of colorectal, breast, and prostate cancers, including subtypes. METHODS: A case-cohort study within the Melbourne Collaborative Cohort Study included 547 colorectal, 634 breast, and 824 prostate cancers, and a sex-stratified random sample of participants (n = 2,996). Concentration of 25(OH)D in baseline-dried blood spots was measured using LC-MS/MS. Cox regression yielded adjusted HRs and 95% confidence intervals (CI) for each cancer in relation to plasma-equivalent 25(OH)D concentration. Associations by stage and BRAF/KRAS status for colorectal cancer, estrogen receptor status for breast cancer, and aggressiveness for prostate cancer were examined in competing risks models. RESULTS: 25(OH)D concentrations were inversely associated with risk of colorectal cancer [highest vs. lowest 25(OH)D quintile: HR, 0.71; 95% confidence interval (CI), 0.51-0.98], which was limited to women (HR, 0.52; 95% CI, 0.33-0.82). Circulating 25(OH)D was also inversely associated with BRAF V600E-positive colorectal cancer (per 25 nmol/L increment: HR, 0.71; 95% CI, 0.50-1.01). There were no inverse associations with breast cancer (HR, 0.98; 95% CI, 0.70-1.36) or prostate cancer (HR, 1.11; 95% CI, 0.82-1.48). CONCLUSIONS: Circulating 25(OH)D concentration was inversely associated with colorectal cancer risk for women, but not with risk of breast cancer or prostate cancer. IMPACT: Vitamin D might play a role in preventing colorectal cancer. Further studies are required to confirm whether vitamin D is associated with specific tumor subtypes
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