1,702 research outputs found
The Sloan Lens ACS Survey. VIII. The relation between environment and internal structure of early-type galaxies
We study the relation between the internal structure of early-type galaxies
and their environment using 70 strong gravitational lenses from the Sloan ACS
Lens Survey. The Sloan database is used to determine two measures of
overdensity of galaxies around each lens: the projected number density of
galaxies inside the tenth nearest neighbor (\Sigma_{10}) and within a cone of
radius one h^{-1} Mpc (D_1). Our main results are: 1) The average overdensity
is somewhat larger than unity, consistent with lenses preferring overdense
environments as expected for massive early-type galaxies (12/70 lenses are in
known groups/clusters). 2) The distribution of overdensities is
indistinguishable from that of "twin" non-lens galaxies selected from SDSS to
have the same redshift and stellar velocity dispersion \sigma_*. Thus, within
our errors, lens galaxies are an unbiased population, and the SLACS results can
be generalized to the overall population of early-type galaxies. 3) Typical
contributions from external mass distribution are no more than a few per cent,
reaching 10-20% (~0.05-0.10 external convergence) only in the most extreme
overdensities. 4) No significant correlation between overdensity and slope of
the mass density profile of the lens is found. 5) Satellite galaxies (those
with a more luminous companion) have marginally steeper mass density profiles
than central galaxies (those without). This result suggests that tidal
stripping may affect the mass structure of early-type galaxies down to kpc
scales probed by strong lensing, when they fall into larger structures
[ABRIDGED].Comment: ApJ, in press; minor changes with respect to v
Exposure of Escherichia coli to low-frequency vibrations
Low frequency mechanical vibration effects on biochemical mutant formation in E. col
Digital Affordances on Speaking English Fluently: Learning English as a Second Language
Speaking is a skill that involves verbal communication, such as deriving meaning between two or more people as a speaker and listener. The pread of technology make people easier to study. On of the digital tool to improve speaking ability is by using digital story telling or called DST. Lots of researcs about DST have been published and told how effective it is. The effectiveness of DST as a tool to support the learning of all four English language skills. In this study, a descriptive research technique, case studies, was used. The case study aims to explore that examine the characteristics of existing cases and try to describe them in detail. In this study used digital media in teaching speaking. The two most important methods used by Toontastic turned out to be scaffolding and subject familiarity. In their learning Regarding protocol, more than 50% of students said the gradual development of the story was driven by: Toontastic makes it easy to create your own stories. From a social constructivist perspective, Toontastic's resources were the bridge that took ZPD's performance to the next level
S4N: A Spectroscopic Survey of Stars in the Solar Neighborhood
[ABRIDGED] We report the results of a high-resolution spectroscopic survey of
all the stars more luminous than Mv = 6.5 mag within 14.5 pc from the Sun. We
derive stellar parameters and perform a preliminary abundance and kinematic
analysis of the F-G-K stars in the sample. The inferred metallicity ([Fe/H])
distribution is centered at about -0.1 dex, and shows a standard deviation of
0.2 dex.
We identify a number of metal-rich K-type stars which appear to be very old,
confirming the claims for the existence of such stars in the solar
neighborhood. With atmospheric effective temperatures and gravities derived
independently of the spectra, we find that our classical LTE model-atmosphere
analysis of metal-rich (and mainly K-type) stars provides discrepant abundances
from neutral and ionized lines of several metals. Based on transitions of
majority species, we discuss abundances of 16 chemical elements.
In agreement with earlier studies we find that the abundance ratios to iron
of Si, Sc, Ti, Co, and Zn become smaller as the iron abundance increases until
approaching the solar values, but the trends reverse for higher iron
abundances. At any given metallicity, stars with a `low' galactic rotational
velocity tend to have high abundances of Mg, Si, Ca, Sc, Ti, Co, Zn, and Eu,
but low abundances of Ba, Ce, and Nd. The Sun appears deficient by roughly 0.1
dex in O, Si, Ca, Sc, Ti, Y, Ce, Nd, and Eu, compared to its immediate
neighbors with similar iron abundances.Comment: 24 pages, 19 figures, to appear in A&A; data can be accessed from
http://hebe.as.utexas.edu/s4n/ or http://www.astro.uu.se/~s4n
Hole density dependence of effective mass, mobility and transport time in strained Ge channel modulation-doped heterostructures
We performed systematic low-temperature (T = 350 mK–15 K) magnetotransport measurements on the two-dimensional hole gas with various sheet carrier densities Ps = (0.57–2.1)×1012 cm–2 formed in the strained Ge channel modulation-doped (MOD) SiGe heterostructures grown on Si substrates. It was found that the effective hole mass deduced by temperature dependent Shubnikov–de Hass oscillations increased monotonically from (0.087±0.05)m0 to (0.19±0.01)m0 with the increase of Ps, showing large band nonparabolicity in strained Ge. In contrast to this result, the increase of the mobility with increasing Ps (up to 29 000 cm2/V s) was observed, suggesting that Coulomb scattering played a dominant role in the transport of the Ge channel at low temperatures. In addition, the Dingle ratio of the transport time to the quantum lifetime was found to increase with increasing Ps, which was attributed to the increase of remote impurity scattering with the increase of the doping concentration in MOD SiGe layers
The initial mass function of early-type galaxies
We determine an absolute calibration of the initial mass function (IMF) of
early-type galaxies, by studying a sample of 56 gravitational lenses identified
by the SLACS Survey. Under the assumption of standard Navarro, Frenk & White
dark matter halos, a combination of lensing, dynamical, and stellar population
synthesis models is used to disentangle the stellar and dark matter
contribution for each lens. We define an "IMF mismatch" parameter
\alpha=M*(L+D)/M*(SPS) as the ratio of stellar mass inferred by a joint lensing
and dynamical models (M*(L+D)) to the current stellar mass inferred from
stellar populations synthesis models (M*(SPS)). We find that a Salpeter IMF
provides stellar masses in agreement with those inferred by lensing and
dynamical models (=0.00+-0.03+-0.02), while a Chabrier IMF
underestimates them (=0.25+-0.03+-0.02). A tentative trend is
found, in the sense that \alpha appears to increase with galaxy velocity
dispersion. Taken at face value, this result would imply a non universal IMF,
perhaps dependent on metallicity, age, or abundance ratios of the stellar
populations. Alternatively, the observed trend may imply non-universal dark
matter halos with inner density slope increasing with velocity dispersion.
While the degeneracy between the two interpretations cannot be broken without
additional information, the data imply that massive early-type galaxies cannot
have both a universal IMF and universal dark matter halos.Comment: 10 pages 4 figures. Resubmitted to ApJ taking into account referee's
comment
Overlapping-gate architecture for silicon Hall bar MOSFET devices in the low electron density regime
We report the fabrication and study of Hall bar MOSFET devices in which an
overlapping-gate architecture allows four-terminal measurements of low-density
2D electron systems, while maintaining a high density at the ohmic contacts.
Comparison with devices made using a standard single gate show that
measurements can be performed at much lower densities and higher channel
resistances, despite a reduced peak mobility. We also observe a voltage
threshold shift which we attribute to negative oxide charge, injected during
electron-beam lithography processing.Comment: 4 pages, 4 figures, submitted for Applied Physics Letter
On the quantum and classical scattering times due to charged dislocations in an impure electron gas
We derive the ratio of transport and single particle relaxation times in
three and two - dimensional electron gases due to scattering from charged
dislocations in semiconductors. The results are compared to the respective
relaxation times due to randomly placed charged impurities. We find that the
ratio is larger than the case of ionized impurity scattering in both three and
two-dimensional electron transport.Comment: 4 pages, 3 figure
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