543 research outputs found
Enhanced heterogeneous nucleation on oxides in Al alloys by intensive shearing
Oxides, in liquid aluminium alloys, can cause severe difficulties during casting, contribute to the formation of cast defects and degrade the mechanical properties of cast components. In this paper, microstructural characteristics of naturally occurring oxides in the melts of commercial purity aluminium and Al-Mg binary alloys have been investigated. They are characterised by densely populated oxide particles within liquid oxide films. With intensive shearing, the particle agglomerates are dispersed into uniformly distributed individual particles. It was found that with intensive melt shearing, grain refinement of α-Al can be achieved by the dispersed oxide particles. The smaller lattice misfit between the oxide particles and the α-Al phase is characterised by a well defined crystallographic orientation relationship. And the mechanisms of grain refinement are discussed.The EPSR
A Study of Lyman-Alpha Quasar Absorbers in the Nearby Universe
Spectroscopy of ten quasars obtained with the Goddard High Resolution
Spectrograph (GHRS) of the Hubble Space Telescope (HST) is presented. A
clustering analysis reveals an excess of nearest neighbor line pairs on
velocity scales of 250-750 km/s at a 95-98% confidence level. The hypothesis
that the absorbers are randomly distributed in velocity space can be ruled out
at the 99.8% confidence level. No two-point correlation power is detected (xi <
1 with 95% confidence). Lyman-alpha absorbers have correlation amplitudes on
scales of 250-500 km/s at least 4-5 times smaller than the correlation
amplitude of bright galaxies. A detailed comparison between absorbers in nearby
galaxies is carried out on a limited subset of 11 Lyman- alpha absorbers where
the galaxy sample in a large contiguous volume is complete to M_B = -16.
Absorbers lie preferentially in regions of intermediate galaxy density but it
is often not possible to uniquely assign a galaxy counterpart to an absorber.
This sample provides no explicit support for the hypothesis that absorbers are
preferentially associated with the halos of luminous galaxies. We have made a
preliminary comparison of the absorption line properties and environments with
the results of hydrodynamic simulations. The results suggest that the
Lyman-alpha absorbers represent diffuse or shocked gas in the IGM that traces
the cosmic web of large scale structure. (abridged)Comment: 36 pages of text, 15 figures, 4 tables, 36 file
A sharp look at the gravitationally lensed quasar SDSS J0806+2006 with Laser Guide Star Adaptive Optics
We present the first VLT near-IR observations of a gravitationally lensed
quasar, using adaptive optics and laser guide star. These observations can be
considered as a test bench for future systematic observations of lensed quasars
with adaptive optics, even when bright natural guide stars are not available in
the nearby field. With only 14 minutes of observing time, we derived very
accurate astrometry of the quasar images and of the lensing galaxy, with 0.05
\arcsec spatial resolution, comparable to the Hubble Space Telescope (HST). In
combination with deep VLT optical spectra of the quasar images, we use our
adaptive optics images to constrain simple models for the mass distribution of
the lensing galaxy. The latter is almost circular and does not need any strong
external shear to fit the data. The time delay predicted for SDSS0806+2006,
assuming a singular isothermal ellipsoid model and the concordance cosmology,
is Delta t \simeq 50 days. Our optical spectra indicate a flux ratio between
the quasar images of A/B=1.3 in the continuum and A/B=2.2 in both the MgII and
in the CIII] broad emission lines. This suggests that microlensing affects the
continuum emission. However, the constant ratio between the two emission lines
indicates that the broad emission line region is not microlensed. Finally, we
see no evidence of reddening by dust in the lensing galaxy.Comment: 4 pages, Published in Astronomy and Astrophysics. Discussion slightly
expanded with respect to v1. Typos correcte
The Quasar Pair Q 1634+267 A, B and the Binary QSO vs. Dark Lens Hypotheses
Deep HST/NICMOS H (F160W) band observations of the z=1.96 quasar pair Q
1634+267A,B reveal no signs of a lens galaxy to a 1 sigma threshold of
approximately 22.5 mag. The minimum luminosity for a normal lens galaxy would
be a 6L_* galaxy at z > 0.5, which is 650 times greater than our detection
threshold. Our observation constrains the infrared mass-to-light ratio of any
putative, early-type, lens galaxy to (M/L)_H > 690h_65 (1200h_65) for
Omega_0=0.1 (1.0) and H_0=65h_65 km/s/Mpc. We would expect to detect a galaxy
somewhere in the field because of the very strong Mg II absorption lines at
z=1.1262 in the Q 1634+267 A spectrum, but the HST H-band, I-band (F785LP) and
V-band (F555W) images require that any associated galaxy be very under-luminous
less than 0.1 L^*_H (1.0 L^*_I) if it lies within less than 40 h^{-1} (100
h^{-1}) kpc from Q 1634+267 A,B.
While the large image separation (3.86 arcsec) and the lack of a lens galaxy
strongly favor interpreting Q 1634+267A,B as a binary quasar system, the
spectral similarity remains a puzzle. We estimate that at most 0.06% of
randomly selected quasar pairs would have spectra as similar to each other as
the spectra of Q 1634+267 A and B. Moreover, spectral similarities observed for
the 14 quasar pairs are significantly greater than would be expected for an
equivalent sample of randomly selected field quasars. Depending on how strictly
we define similarity, we estimate that only 0.01--3% of randomly drawn samples
of 14 quasar pairs would have as many similar pairs as the observational
sample.Comment: 24 pages, including 4 figures, LaTex, ApJ accepted, comments from the
editor included, minor editorial change
EGRET Spectral Index and the Low-Energy Peak Position in the Spectral Energy Distribution of EGRET-Detected Blazars
In current theoretical models of the blazar subclass of active galaxies, the
broadband emission consists of two components: a low-frequency synchrotron
component with a peak in the IR to X-ray band, and a high-frequency inverse
Compton component with a peak in the gamma-ray band. In such models, the
gamma-ray spectral index should be correlated with the location of the
low-energy peak, with flatter gamma-ray spectra expected for blazars with
synchrotron peaks at higher photon energies and vice versa. Using the
EGRET-detected blazars as a sample, we examine this correlation and possible
uncertainties in its construction.Comment: 17 pages including 1 figure, accepted for publication in The
Astrophysical Journa
The Evolution of a Mass-Selected Sample of Early-Type Field Galaxies
We investigate the evolution of mass-selected early-type field galaxies using
a sample of 28 gravitational lenses spanning the redshift range 0 < z < 1.
Based on the redshift-dependent intercept of the fundamental plane in the rest
frame B band, we measure an evolution rate of d log (M/L)_B / dz = -0.56 +/-
0.04 (all errors are 1 sigma unless noted) if we directly compare to the local
intercept measured from the Coma cluster. Re-fitting the local intercept helps
minimize potential systematic errors, and yields an evolution rate of d log
(M/L)_B / dz = -0.54 +/- 0.09. An evolution analysis of properly-corrected
aperture mass-to-light ratios (defined by the lensed image separations) is
closely related to the Faber-Jackson relation. In rest frame B band we find an
evolution rate of d log (M/L)_B / dz = -0.41 +/- 0.21, a present-day
characteristic magnitude of M_{*0} = -19.70 + 5 log h +/- 0.29 (assuming a
characteristic velocity dispersion of sigma_{DM*} = 225 km/s), and a
Faber-Jackson slope of gamma_{FJ} = 3.29 +/- 0.58. The measured evolution rates
favor old stellar populations (mean formation redshift z_f > 1.8 at 2 sigma
confidence for a Salpeter initial mass function and a flat Omega_m =0.3
cosmology) among early-type field galaxies, and argue against significant
episodes of star formation at z < 1.Comment: 38 pages; 9 figs; ApJ accepted; REVISION: erroneous image separation
corrected for one lens, another lens removed; results recalculated and
slightly modifie
The Fundamental Plane of Gravitational Lens Galaxies and The Evolution of Early-Type Galaxies in Low Density Environments
Most gravitational lenses are early-type galaxies in relatively low density
environments -- a ``field'' rather than a ``cluster'' population. We show that
field early-type galaxies with 0 < z < 1, as represented by the lens galaxies,
lie on the same fundamental plane as those in rich clusters at similar
redshifts. We then use the fundamental plane to measure the combined
evolutionary and K-corrections for early-type galaxies in the V, I and H bands.
Only for passively evolving stellar populations formed at z > 2 (H_0=65 km/s
Mpc, Omega_0=0.3, Lambda_0=0.7) can the lens galaxies be matched to the local
fundamental plane. The high formation epoch and the lack of significant
differences between the field and cluster populations contradict many current
models of the formation history of early-type galaxies. Lens galaxy colors and
the fundamental plane provide good photometric redshift estimates with an
empirical accuracy of -0.03 +/- 0.11 for the 17 lenses with known redshifts. A
mass model dominated by dark matter is more consistent with the data than
either an isotropic or radially anisotropic constant M/L mass model, and a
radially anisotropic model is better than an isotropic model.Comment: 36 pages, 9 figures, 6 tables. ApJ in press. Final version contains
more observational dat
The XMM-Newton wide-field survey in the COSMOS field. IV: X-ray spectral properties of Active Galactic Nuclei
We present a detailed spectral analysis of point-like X-ray sources in the
XMM-COSMOS field. Our sample of 135 sources only includes those that have more
than 100 net counts in the 0.3-10 keV energy band and have been identified
through optical spectroscopy. The majority of the sources are well described by
a simple power-law model with either no absorption (76%) or a significant
intrinsic, absorbing column (20%).As expected, the distribution of intrinsic
absorbing column densities is markedly different between AGN with or without
broad optical emission lines. We find within our sample four Type-2 QSOs
candidates (L_X > 10^44 erg/s, N_H > 10^22 cm^-2), with a spectral energy
distribution well reproduced by a composite Seyfert-2 spectrum, that
demonstrates the strength of the wide field XMM/COSMOS survey to detect these
rare and underrepresented sources.Comment: 16 pages, ApJS COSMOS Special Issue, 2007 in press. The
full-resolution version is available at
http://www.mpe.mpg.de/XMMCosmos/PAPERS/mainieri_cosmos.ps.g
High Frequency Peakers: young radio sources or flaring blazars?
We present new, simultaneous, multifrequency observations of 45 out of the 55
candidate High Frequency Peakers (HFP) selected by Dallacasa et al. (2000),
carried out 3 to 4 years after a first set of observations. Our sub-sample
consists of 10 galaxies, 28 stellar objects (``quasars'') and 7 unidentified
sources. Both sets of observations are sensitive enough to allow the detection
of variability at the 10% level or lower. While galaxies do not show
significant variability, most quasars do. Seven of them no longer show the
convex spectrum which is the defining property of Gigahertz Peaked Spectrum
(GPS)/HFP sources and are interpreted as blazars caught by Dallacasa et al.
(2000) during a flare, when a highly self-absorbed component dominated the
emission. In general, the variability properties (amplitude, timescales,
correlation between peak luminosity and peak frequency of the flaring
component) of the quasar sub-sample resemble those of blazars. We thus conclude
that most HFP candidates identified with quasars may well be flaring blazars.Comment: 20 pages, 18 figures, accepted for publication in Astronomy &
Astrophysic
The K20 survey. III. Photometric and spectroscopic properties of the sample
The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey
aimed at obtaining spectral information and redshifts of a complete sample of
about 550 objects to K_s\leq20.0 over two independent fields with a total area
of 52 arcmin^2. In this paper we discuss the scientific motivation of such a
survey, we describe the photometric and spectroscopic properties of the sample,
and we release the -band photometric catalog. Extensive simulations showed
that the sample is photometrically highly complete to K_s=20. The observed
galaxy counts and the R-K_s color distribution are consistent with literature
results. We observed spectroscopically 94% of the sample, reaching a
spectroscopic redshift identification completeness of 92% to K_s\leq20.0 for
the observed targets, and of 87% for the whole sample (i.e. counting also the
unobserved targets). Deep spectroscopy was complemented with multi-band deep
imaging in order to derive tested and reliable photometric redshifts for the
galaxies lacking spectroscopic redshifts. The results show a very good
agreement between the spectroscopic and the photometric redshifts with
=0.01 and with a dispersion of \sigma_{\Delta z}=0.09. Using
both the spectroscopic and the photometric redshifts, we reached an overall
redshift completeness of about 98%. The size of the sample, the redshift
completeness, the availability of high quality photometric redshifts and
multicolor spectral energy distributions make the K20 survey database one of
the most complete samples available to date for constraining the currently
competing scenarios of galaxy formation and for a variety of other galaxy
evolution studies.Comment: 13 pages, 10 figures, to appear in Astronomy & Astrophysic
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