2,031 research outputs found
Towards the First Galaxies
The formation of the first galaxies at redshifts z~10-15 signaled the
transition from the simple initial state of the universe to one of ever
increasing complexity. We here review recent progress in understanding their
assembly process with numerical simulations, starting with cosmological initial
conditions and modelling the detailed physics of star formation. In particular,
we study the role of HD cooling in ionized primordial gas, the impact of UV
radiation produced by the first stars, and the propagation of the supernova
blast waves triggered at the end of their brief lives. We conclude by
discussing promising observational diagnostics that will allow us to probe the
properties of the first galaxies, such as their contribution to reionization
and the chemical abundance pattern observed in extremely low-metallicity stars.Comment: 12 pages, 14 figures, appeared in "First Stars III", eds. B. O'Shea,
A. Heger and T. Abel, a high resolution version (highly recommended) can be
found at http://www.ita.uni-heidelberg.de/~tgreif/files/gjb07.pd
Large-scale variations of the dust optical properties in the Galaxy
We present an analysis of the dust optical properties at large scale, for the
whole galactic anticenter hemisphere. We used the 2MASS Extended Source Catalog
to obtain the total reddening on each galaxy line of sight and we compared this
value to the IRAS 100 microns surface brightness converted to extinction by
Schlegel et al (1998). We performed a careful examination and correction of the
possible systematic effects resulting from foreground star contamination,
redshift contribution and galaxy selection bias. We also evaluated the
contribution of dust temperature variations and interstellar clumpiness to our
method. The correlation of the near-infrared extinction to the far-infrared
optical depth shows a discrepancy for visual extinction greater than 1 mag with
a ratio A_V(FIR) / A_V(gal) = 1.31 +- 0.06. We attribute this result to the
presence of fluffy/composite grains characterized by an enhanced far--infrared
emissivity. Our analysis, applied to half of the sky, provides new insights on
the dust grains nature suggesting fluffy grains are found not only in some very
specific regions but in all directions for which the visual extinction reaches
about 1 mag.Comment: 10 pages, 11 figures, accepted for publication in A&
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The First Supernova Explosions: Energetics, Feedback, And Chemical Enrichment
We perform three-dimensional smoothed particle hydrodynamics simulations in a realistic cosmological setting to investigate the expansion, feedback, and chemical enrichment properties of a 200M(circle dot) pair-instability supernova in the high-redshift universe. We find that the SN remnant propagates for a Hubble time at z similar or equal to 20 to a final mass-weighted mean shock radius of 2.5 kpc (proper), roughly half the size of the H II region, and in this process sweeps up a total gas mass of 2: 5; 10(5) M-circle dot. The morphology of the shock becomes highly anisotropic once it leaves the host halo and encounters filaments and neighboring minihalos, while the bulk of the shock propagates into the voids of the intergalactic medium. The SN entirely disrupts the host halo and terminates further star formation for at least 200 Myr, while in our specific case it exerts positive mechanical feedback on neighboring minihalos by shock-compressing their cores. In contrast, we do not observe secondary star formation in the dense shell via gravitational fragmentation, due to the previous photoheating by the progenitor star. We find that cooling by metal lines is unimportant for the entire evolution of the SN remnant, while the metal-enriched, interior bubble expands adiabatically into the cavities created by the shock, and ultimately into the voids with a maximum extent similar to the final mass-weighted mean shock radius. Finally, we conclude that dark matter halos of at least M-vir greater than or similar to 10(8) M-circle dot must be assembled to recollect all components of the swept-up gas.Astronom
The 2MASS Tully-Fisher Survey : Mapping the Mass in the Universe
The 2MASS Tully-Fisher Survey (2MTF) aims to measure Tully-Fisher (TF)
distances for all bright inclined spirals in the 2MASS Redshift Survey (2MRS)
using high quality HI widths and 2MASS photometry. Compared with previous
peculiar velocity surveys, the 2MTF survey provides more accurate width
measurements and more uniform sky coverage, combining observations with the
Green Bank, Arecibo and Parkes telescopes. With this new redshift-independent
distance database, we will significantly improve our understanding of the mass
distribution in the local universe.Comment: 4 pages, 3 figures, IAU Symposium 289 proceedin
Forecasting Design Day Demand Using Extremal Quantile Regression
Extreme events occur rarely, making them difficult to predict. Extreme cold events strain natural gas systems to their limits. Natural gas distribution companies need to be prepared to satisfy demand on any given day that is at or warmer than an extreme cold threshold. The hypothetical day with temperature at this threshold is called the Design Day. To guarantee Design Day demand is satisfied, distribution companies need to determine the demand that is unlikely to be exceeded on the Design Day.
We approach determining this demand as an extremal quantile regression problem. We review current methods for extremal quantile regression. We implement a quantile forecast to estimate the demand that has a minimal chance of being exceeded on the design day. We show extremal quantile regression to be more reliable than direct quantile estimation. We discuss the difficult task of evaluating a probabilistic forecast on rare events.
Probabilistic forecasting is a quickly growing research topic in the field of energy forecasting. Our paper contributes to this field in three ways. First, we forecast quantiles during extreme cold events where data is sparse. Second, we forecast extremely high quantiles that have a very low probability of being exceeded. Finally, we provide a real world scenario on which to apply these techniques
Theoretical simulation of the anisotropic phases of antiferromagnetic thin films
We simulate antiferromagnetic thin films. Dipole-dipole and antiferromagnetic
exchange interactions as well as uniaxial and quadrupolar anisotropies are
taken into account. Various phases unfold as the corresponding parameters, J, D
and C, as well as the temperature T and the number n of film layers vary. We
find (1) how the strength Delta_m of the anisotropy arising from dipole-dipole
interactions varies with the number of layers m away from the film's surface,
with J and with n; (2) a unified phase diagram for all n-layer films and bulk
systems; (3) a layer dependent spin reorientation (SR) phase in which spins
rotate continuously as T, D, C and n vary; (4) that the ratio of the SR to the
ordering temperature depends (approximately) on n only through (D+Delta/n)/C,
and hardly on J; (5) a phase transformation between two different magnetic
orderings, in which spin orientations may or may not change, for some values of
J, by varying n.Comment: 10 LaTeX pages, 13 eps figures. Submitted to PRB on 30 June 2006.
Accepted on 10 October 200
The First Stars
The formation of the first generations of stars at redshifts z > 15-20
signaled the transition from the simple initial state of the universe to one of
increasing complexity. We here review recent progress in understanding the
assembly process of the first galaxies, starting with cosmological initial
conditions and modelling the detailed physics of star formation. In particular,
we study the role of HD cooling in ionized primordial gas, the impact of UV
radiation produced by the first stars, and the propagation of the supernova
blast waves triggered at the end of their brief lives. We conclude by
discussing how the chemical abundance patterns observed in extremely
low-metallicity stars allow us to probe the properties of the first stars.Comment: 12 pages, 9 figures, proceedings of the IAU Symposium 250 "Massive
stars as cosmic engines
Optically controlled spin-glasses in multi-qubit cavity systems
Recent advances in nanostructure fabrication and optical control, suggest
that it will soon be possible to prepare collections of interacting two-level
systems (i.e. qubits) within an optical cavity. Here we show theoretically that
such systems could exhibit novel phase transition phenomena involving
spin-glass phases. By contrast with traditional realizations using magnetic
solids, these phase transition phenomena are associated with both matter and
radiation subsystems. Moreover the various phase transitions should be tunable
simply by varying the matter-radiation coupling strength.Comment: 4 pages, 3 figure
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