168 research outputs found
Effect of local environment and stellar mass on galaxy quenching and morphology at
We study galactic star-formation activity as a function of environment and
stellar mass over 0.5<z<2.0 using the FourStar Galaxy Evolution (ZFOURGE)
survey. We estimate the galaxy environment using a Bayesian-motivated measure
of the distance to the third nearest neighbor for galaxies to the stellar mass
completeness of our survey, at z=1.3 (2.0). This
method, when applied to a mock catalog with the photometric-redshift precision
(), recovers galaxies in low- and high-density
environments accurately. We quantify the environmental quenching efficiency,
and show that at z> 0.5 it depends on galaxy stellar mass, demonstrating that
the effects of quenching related to (stellar) mass and environment are not
separable. In high-density environments, the mass and environmental quenching
efficiencies are comparable for massive galaxies (
10.5) at all redshifts. For lower mass galaxies (
10), the environmental quenching efficiency is very low at 1.5, but
increases rapidly with decreasing redshift. Environmental quenching can account
for nearly all quiescent lower mass galaxies ( 9-10),
which appear primarily at 1.0. The morphologies of lower mass
quiescent galaxies are inconsistent with those expected of recently quenched
star-forming galaxies. Some environmental process must transform the
morphologies on similar timescales as the environmental quenching itself. The
evolution of the environmental quenching favors models that combine gas
starvation (as galaxies become satellites) with gas exhaustion through
star-formation and outflows ("overconsumption"), and additional processes such
as galaxy interactions, tidal stripping and disk fading to account for the
morphological differences between the quiescent and star-forming galaxy
populations.Comment: 29 pages, 15 figure, accepted for publication in Ap
Electronic polarization in pentacene crystals and thin films
Electronic polarization is evaluated in pentacene crystals and in thin films
on a metallic substrate using a self-consistent method for computing charge
redistribution in non-overlapping molecules. The optical dielectric constant
and its principal axes are reported for a neutral crystal. The polarization
energies P+ and P- of a cation and anion at infinite separation are found for
both molecules in the crystal's unit cell in the bulk, at the surface, and at
the organic-metal interface of a film of N molecular layers. We find that a
single pentacene layer with herring-bone packing provides a screening
environment approaching the bulk. The polarization contribution to the
transport gap P=(P+)+(P-), which is 2.01 eV in the bulk, decreases and
increases by only ~ 10% at surfaces and interfaces, respectively. We also
compute the polarization energy of charge-transfer (CT) states with fixed
separation between anion and cation, and compare to electroabsorption data and
to submolecular calculations. Electronic polarization of ~ 1 eV per charge has
a major role for transport in organic molecular systems with limited overlap.Comment: 10 revtex pages, 6 PS figures embedde
Adenosine Triphosphate (ATP) as a Metric of Microbial Biomass in Aquatic Systems: New Simplified Protocols, Laboratory Validation, and a Reflection on Data From the Literature
The use of adenosine triphosphate (ATP) as a universal biomass indicator is built on the premise that ATP concentration tracks biomass rather than the physiological condition of cells. However, reportedly high variability in ATP in response to environmental conditions is the main reason the method has not found widespread application. To test possible sources of this variability, we used the diatom Thalassiosira weissflogii as a model and manipulated its growth rate through nutrient limitation and through exposure to three different temperatures (15°C, 20°C, and 25°C). We simplified the ATP protocol with hotâwater or chemical extraction methods, modified a commercially available luciferinâluciferase assay, and employed singleâphoton counting in a scintillation counter, all of which increased sensitivity and throughput. Perâcell ATP levels remained relatively constant despite changes in growth rates by approximately 10âfold in the batch culture (i.e., nutrient limitation) experiments, and approximately 2âfold in response to temperature. The reâexamination of related literature values revealed that average cellular ATP levels differed little among taxonomic groups of aquatic microbes, even at the domain level, and correlated well with bulk properties such as elemental carbon or nitrogen. Fulfilling multiple cellular functions in addition to being the universal energy currency requires ATP to be maintained in a millimolar concentration range. Consequently, ATP relates directly to live cytoplasm volume, while elemental carbon and nitrogen are constrained by an indeterminate pool of detrital material and intracellular storage compounds. The ATPâbiomass indicator is sensitive, economical, and can be readily standardized among laboratories and across environments
The first Infrared study of the close environment of a long Gamma-Ray Burst
We present a characterization of the close environment of GRB980425 based on
5-160mic spectro-imaging obtained with Spitzer. The Gamma-Ray Burst GRB980425
occurred in a nearby (z=0.0085) SBc-type dwarf galaxy, at a projected distance
of 900pc from an HII region with strong signatures of Wolf-Rayet (WR) stars.
While this "WR region" produces less than 5% of the B-band emission of the
host, we find that it is responsible for 45+/-10% of the total infrared
luminosity, with a maximum contribution reaching 75% at 25-30mic. This atypical
property is rarely observed among morphologically-relaxed dwarves, suggesting a
strong causal link with the GRB event. The luminosity of the WR region
(L_8-1000mic=4.6x10^8 Lsol), the peak of its spectral energy distribution at
<~100mic and the presence of highly-ionized emission lines (e.g., [NeIII]) also
reveal extremely young (<5Myr) star-forming activity, with a typical time-scale
of only 47Myr to double the stellar mass already built. Finally, the mid-IR
over B-band luminosity ratio in this region is substantially higher than in
star-forming galaxies with similar L_IR, but it is lower than in young
dust-enshrouded stellar clusters. Considering the modest obscuration measured
from the silicate features (tau_9.7mic ~ 0.015), this suggests that the WR
region is dominated by one or several star clusters that have either partly
escaped or cleared out their parent molecular cloud. Combined with the
properties characterizing the whole population of GRB hosts, our results
reinforce the idea that long GRBs mostly happen within or in the vicinity of
relatively unobscured galactic regions harboring very recent star formation.Comment: ApJ in press, 14 pages, 2 tables, 7 figure
Non-volatile Phase-only Transmissive Spatial Light Modulators
Free-space modulation of light is crucial for many applications, from light
detection and ranging to virtual or augmented reality. Traditional means of
modulating free-space light involves spatial light modulators based on liquid
crystals and microelectromechanical systems, which are bulky, have large pixel
areas (~10 micron x 10 micron), and require high driving voltage. Recent
progress in meta-optics has shown promise to circumvent some of the
limitations. By integrating active materials with sub-wavelength pixels in a
meta-optic, the power consumption can be dramatically reduced while achieving a
faster speed. However, these reconfiguration methods are volatile and hence
require constant application of control signals, leading to phase jitter and
crosstalk. Additionally, to control a large number of pixels, it is essential
to implement a memory within each pixel to have a tractable number of control
signals. Here, we develop a device with nonvolatile, electrically programmable,
phase-only modulation of free-space infrared radiation in transmission using
the low-loss phase-change material (PCM) Sb2Se3. By coupling an ultra-thin PCM
layer to a high quality (Q)-factor (Q~406) diatomic metasurface, we demonstrate
a phase-only modulation of ~0.25pi (~0.2pi) in simulation (experiment), ten
times larger than a bare PCM layer of the same thickness. The device shows
excellent endurance over 1,000 switching cycles. We then advance the device
geometry, to enable independent control of 17 meta-molecules, achieving ten
deterministic resonance levels with a 2pi phase shift. By independently
controlling the phase delay of pixels, we further show tunable far-field beam
shaping. Our work paves the way to realizing non-volatile transmissive
phase-only spatial light modulators
Integrated Circuits Based on Bilayer MoS
Two-dimensional (2D) materials, such as molybdenum disulfide (MoS2), have been shown to exhibit excellent electrical and optical properties. The semiconducting nature of MoS2 allows it to overcome the shortcomings of zero-bandgap graphene, while still sharing many of grapheneâs advantages for electronic and optoelectronic applications. Discrete electronic and optoelectronic components, such as field-effect transistors, sensors, and photodetectors made from few-layer MoS2 show promising performance as potential substitute of Si in conventional electronics and of organic and amorphous Si semiconductors in ubiquitous systems and display applications. An important next step is the fabrication of fully integrated multistage circuits and logic building blocks on MoS2 to demonstrate its capability for complex digital logic and high-frequency ac applications. This paper demonstrates an inverter, a NAND gate, a static random access memory, and a five-stage ring oscillator based on a direct-coupled transistor logic technology. The circuits comprise between 2 to 12 transistors seamlessly integrated side-by-side on a single sheet of bilayer MoS2. Both enhancement-mode and depletion-mode transistors were fabricated thanks to the use of gate metals with different work functions.United States. Office of Naval Research (Young Investigator Program)Microelectronics Advanced Research Corporation (MARCO) (Focus Center for Materials, Structure and Device (MARCO MSD))National Science Foundation (U.S.) (NSF DMR 0845358)United States. Army Research Offic
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