13,692 research outputs found
A New Measurement of the Stellar Mass Density at z~5: Implications for the Sources of Cosmic Reionization
We present a new measurement of the integrated stellar mass per comoving
volume at redshift 5 determined via spectral energy fitting drawn from a sample
of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS
field. Following procedures introduced by Eyles et al. (2005), we estimate
stellar masses for various sub-samples for which reliable and unconfused
Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most
luminous sources with =4.92 provides a firm lower limit to the stellar mass
density of 1e6 Msun/Mpc^3. Several galaxies in this sub-sample have masses of
order 10^11 Msun implying significant earlier activity occurred in massive
systems. We then consider a larger sample whose photometric redshifts in the
publicly-available GOODS-MUSIC catalog lie in the range 4.4 <z 5.6. Before
adopting the GOODS-MUSIC photometric redshifts, we check the accuracy of their
photometry and explore the possibility of contamination by low-z galaxies and
low-mass stars. After excising probable stellar contaminants and using the z'-J
color to exclude any remaining foreground red galaxies, we conclude that 196
sources are likely to be at z~5. The implied mass density from the unconfused
IRAC fraction of this sample, scaled to the total available, is 6e6 Msun/Mpc^3.
We discuss the uncertainties as well as the likelihood that we have
underestimated the true mass density. Including fainter and quiescent sources
the total integrated density could be as high as 1e7 Msun/Mpc^3. Using the
currently available (but highly uncertain) rate of decline in the star
formationhistory over 5 <z< 10, a better fit is obtained for the assembled mass
at z~5 if we admit significant dust extinction at early times or extend the
luminosity function to very faint limits. [abridged]Comment: Accepted for Publication in ApJ, 39 page
Chemistry and kinematics of the pre-stellar core L1544: Constraints from H2D+
This paper explores the sensitivity of line profiles of H2D+, HCO+ and N2H+,
observed towards the center of L1544, to various kinematic and chemical
parameters. The total width of the H2D+ line can be matched by a static model
and by models invoking ambipolar diffusion and gravitational collapse. The
derived turbulent line width is b=0.15 km/s for the static case and <~ 0.05
km/s for the collapse case. However, line profiles of HC18O+ and N2H+ rule out
the static solution. The double-peaked H2D+ line shape requires either infall
speeds in the center that are much higher than predicted by ambipolar diffusion
models, or a shell-type distribution of H2D+, as is the case for HCO+ and N2H+.
At an offset of ~20 arcsec from the dust peak, the H2D+ abundance drops by a
factor of ~5.Comment: four pages, two colour figures; to appear in The Dense Interstellar
Medium in Galaxies, proceedings of the fourth Cologne-Bonn-Zermatt Symposium,
Sept 22-26, 200
Soliton blue-shift in tapered photonic crystal fiber
We show that solitons undergo a strong blue shift in fibers with a dispersion
landscape that varies along the direction of propagation. The experiments are
based on a small-core photonic crystal fiber, tapered to have a core diameter
that varies continuously along its length, resulting in a zero-dispersion
wavelength that moves from 731 nm to 640 nm over the transition. The central
wavelength of a soliton translates over 400 nm towards shorter wavelength. This
accompanied by strong emission of radiation into the UV and IR spectral region.
The experimental results are confirmed by numerical simulation.Comment: 10 pages, 4 figure
Oscillator strengths and line widths of dipole-allowed transitions in ¹⁴N₂ between 89.7 and 93.5 nm
Line oscillator strengths in the 20 electric dipole-allowed bands of ¹⁴N₂ in the 89.7–93.5nm (111480–106950cm⁻¹) region are reported from photoabsorptionmeasurements at an instrumental resolution of ∼6mÅ (0.7cm⁻¹) full width at half maximum. The absorptionspectrum comprises transitions to vibrational levels of the 3pσᵤc′₄¹Σᵤ⁺, 3pπᵤc³Πᵤ, and 3sσgo₃¹ΠᵤRydberg states and of the b′¹Σᵤ⁺ and b¹Πᵤ valence states. The J dependences of band f values derived from the experimental line f values are reported as polynomials in J′(J′+1) and are extrapolated to J′=0 in order to facilitate comparisons with results of coupled Schrödinger-equation calculations. Most bands in this study are characterized by a strong J dependence of the band f values and display anomalous P-, Q-, and R-branch intensity patterns. Predissociation line widths, which are reported for 11 bands, also exhibit strong J dependences. The f value and line width patterns can inform current efforts to develop comprehensive spectroscopic models that incorporate rotational effects and predissociation mechanisms, and they are critical for the construction of realistic atmospheric radiative-transfer models.This work was supported in part by NASA Grant No.
NNG05GA03G to Wellesley College and Australian Research
Council Discovery Program Grant No. DP0558962
Experiencing the digital world: The cultural value of digital engagement with heritage
Since the late 1990s the potential of the digital world for generating new ways of engaging with heritage, broadly defined, has been a key focus of academic work and cultural practice. At times, the emphasis has been on how the internet can provide a ‘shop window’ for the sector, and how this might be translated into physical visits to sites. Elsewhere, scholars have argued that the digital sphere can provide a dynamic space for two-way engagement with heritage culture, aimed at providing a complementary experience to physical visits through a range of phenomena (e.g. user-generated content, online communities, crowdsourcing projects). Alongside such discussions, questions have also been raised about how to measure the value of this activity and what we mean by value in this context. This paper brings together key literature on digital engagement, interactivity and participation within heritage, case studies of current digital heritage practice, and an online survey of heritage professionals to focus on six key areas: 1. Financial resources 2. Relative value of the digital experience 3. The location of culture value 4. Cultural value and time 5. Enhanced value through participation 6. Cultural value, space and place By exploring these themes we present strategies which heritage organisations of different scales might consider incorporating into new digital resources to enhance their public offering, whilst also suggesting further areas for research. Primarily, we suggest that there is substantial untapped potential to better understand the experience of end users by harnessing the vast amount of data available within heritage institutions, but which organisations frequently do not have the resources to exploit
Pupillometry, a bioengineering overview
The pupillary control system is examined using a microprocessor based integrative pupillometer. The real time software functions of the microprocessor include: data collection, stimulus generation and area to diameter conversion. Results of an analysis of linear and nonlinear phenomena are presented
The Jabal Akhdar Dome in the Oman Mountains : evolution of a dynamic fracture system
Acknowledgments: This study was carried out within the framework of DGMK (German Society for Petroleum and Coal Science and Technology) research project 718 “Mineral Vein Dynamics Modelling,” which is funded by the companies ExxonMobil Production Deutschland GmbH, GDF SUEZ E&P Deutschland GmbH, RWE Dea AG and Wintershall Holding GmbH, within the basic research program of the WEG Wirtschaftsverband Erdo¨l- und Erdgasgewinnung e.V. We thank the companies for their financial support and their permission to publish these results. The German University of Technology in Oman (GU-Tech) is acknowledged for its logistic support. We gratefully acknowledge the reviewers Andrea Billi and Jean-Paul Breton, whose constructive reviews greatly improved the manuscriptPeer reviewedPreprin
Nonlinear wavelength conversion in photonic crystal fibers with three zero dispersion points
In this theoretical study, we show that a simple endlessly single-mode
photonic crystal fiber can be designed to yield, not just two, but three
zero-dispersion wavelengths. The presence of a third dispersion zero creates a
rich phase-matching topology, enabling enhanced control over the spectral
locations of the four-wave-mixing and resonant-radiation bands emitted by
solitons and short pulses. The greatly enhanced flexibility in the positioning
of these bands has applications in wavelength conversion, supercontinuum
generation and pair-photon sources for quantum optics
Closed-cycle, low-vibration 4 K cryostat for ion traps and other applications
In-vacuo cryogenic environments are ideal for applications requiring both low
temperatures and extremely low particle densities. This enables reaching long
storage and coherence times for example in ion traps, essential requirements
for experiments with highly charged ions, quantum computation, and optical
clocks. We have developed a novel cryostat continuously refrigerated with a
pulse-tube cryocooler and providing the lowest vibration level reported for
such a closed-cycle system with 1 W cooling power for a <5 K experiment. A
decoupling system suppresses vibrations from the cryocooler by three orders of
magnitude down to a level of 10 nm peak amplitudes in the horizontal plane.
Heat loads of about 40 W (at 45 K) and 1 W (at 4 K) are transferred from an
experimental chamber, mounted on an optical table, to the cryocooler through a
vacuum-insulated massive 120 kg inertial copper pendulum. The 1.4 m long
pendulum allows installation of the cryocooler in a separate, acoustically
isolated machine room. In the laser laboratory, we measured the residual
vibrations using an interferometric setup. The positioning of the 4 K elements
is reproduced to better than a few micrometer after a full thermal cycle to
room temperature. Extreme high vacuum on the mbar level is achieved.
In collaboration with the Max-Planck-Intitut f\"ur Kernphysik (MPIK), such a
setup is now in operation at the Physikalisch-Technische Bundesanstalt (PTB)
for a next-generation optical clock experiment using highly charged ions
Detection of HI 21 cm-line absorption in the Warm Neutral Medium and in the Outer Arm of the Galaxy
Using the Westerbork Synthesis Radio Telescope, we have detected HI 21
cm-line absorption in the Warm Neutral Medium of the Galaxy toward the
extragalactic source 3C147. This absorption, at an LSR velocity of -29+/-4 km/s
with a full width at half maximum of 53+/-6 km/s, is associated with the
Perseus Arm of the Galaxy. The observed optical depth is (1.9+/-0.2)*10**(-3).
The estimated spin temperature of the gas is 3600+/-360 K. The volume density
is 0.4 per cc assuming pressure equilibrium. Toward two other sources, 3C273
and 3C295, no wide HI 21 cm-line absorption was detected. The highest of the
3sigma lower limits on the spin temperature of the Warm Neutral Medium is 2600
K. In addition, we have also detected HI 21 cm-line absorption from high
velocity clouds in the Outer Arm toward 3C147 and 3C380 at LSR velocities of
-117.3, -124.5 and -113.7 km/s respectively. We find two distinct temperature
components in the high velocity clouds with spin temperatures of greater than
1000 K and less than 200 K, respectively.Comment: 21 pages inclusive of 7 figures and 2 table
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