11,294 research outputs found
How cold is cold dark matter? Small scales constraints from the flux power spectrum of the high-redshift Lyman-alpha forest
We present constraints on the mass of warm dark matter (WDM) particles
derived from the Lyman-alpha flux power spectrum of 55 high- resolution HIRES
spectra at 2.0 < z < 6.4. From the HIRES spectra, we obtain a lower limit of
mwdm > 1.2 keV 2 sigma if the WDM consists of early decoupled thermal relics
and mwdm > 5.6 keV (2 sigma) for sterile neutrinos. Adding the Sloan Digital
Sky Survey Lyman-alpha flux power spectrum, we get mwdm > 4 keV and mwdm > 28
keV (2 sigma) for thermal relics and sterile neutrinos. These results improve
previous constraints by a factor two.Comment: Some issues clarified (especially resolution related). Conclusions
unchanged. Accepted version by PR
B2 and G2 Toda systems on compact surfaces: a variational approach
We consider the B2 and G2 Toda systems on compact surfaces. We attack the
problem using variational techniques. We get existence and multiplicity of
solutions under a topological assumption on the surface and some generic
conditions on the parameters. We also extend some of the results to the case of
general systems.Comment: 28 pages, accepted on Journal of Mathematical Physic
A Search for Nitrogen Enriched Quasars in the Sloan Digital Sky Survey Early Data Release
A search for nitrogen-rich quasars in the Sloan Digital Sky Survey Early Data
Release (SDSS EDR) catalog has yielded 16 candidates, including five with very
prominent emission, but no cases with nitrogen emission as strong as in
Q0353-383. The quasar Q0353-383 has long been known to have extremely strong
nitrogen intercombination lines at lambda 1486 and lambda 1750 Angstroms,
implying an anomalously high nitrogen abundance of about 15 times solar. It is
still the only one of its kind known. A preliminary search through the EDR
using the observed property of the weak C IV emission seen in Q0353-383
resulted in a sample of 23 objects with unusual emission or absorption-line
properties, including one very luminous redshift 2.5 star-forming galaxy. We
present descriptions, preliminary emission-line measurements, and spectra for
all the objects discussed here.Comment: 20 pages, 5 figures, submitted to AJ; final refereed versio
How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?
The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone
which is smaller than those around quasars of similar luminosity at z~6. The
spectrum also exhibits evidence for a damping wing extending redward of the
systemic Lya redshift. We use radiative transfer simulations in a cosmological
context to investigate the implications for the ionisation state of the
inhomogeneous IGM surrounding this quasar. Our simulations show that the
transmission profile is consistent with an IGM in the vicinity of the quasar
with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has
been bright for 10^6--10^7 yr. The observed spectrum is also consistent with
smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an
otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is,
however, predicted to occur in only ~5 per cent of our simulated sight-lines
for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a
previous optically obscured phase, the low age inferred for the quasar adds to
the theoretical challenge of forming a 2x10^9 M_sol black hole at this high
redshift.Comment: 5 pages, 4 figures, accepted to MNRAS letter
Closing in on the picture : analyzing interactions in video recordings
This paper provides a detailed account of the processing and analysing of data, obtained through video recording during reflective practitioner research. It sets out five stages in the analysis of video recordings of classroom interactions during a series of educational drama lessons: from decisions relating to the selection of data for close analysis, to the seeking of themes, and finally to the presentation of conclusions. The researcher adapted and synthesised several processes derived from discourse analysis (Wells, 2001; Spiers, 2004; Gee, 2005) to produce a range of instruments for use in transcription and analysis of verbal and non-verbal discourse. These include: a simple transcription key; classifications for verbal and non-verbal discourse; and a template for a transcription and analysis matrix
Possible evidence for an inverted temperature-density relation in the intergalactic medium from the flux distribution of the Lyman-alpha forest
We compare the improved measurement of the Lya forest flux probability
distribution at 1.7<z<3.2 presented by Kim et al. (2007) to a large set of
hydrodynamical simulations of the Lya forest with different cosmological
parameters and thermal histories. The simulations are in good agreement with
the observational data if the temperature-density relation for the low density
intergalactic medium (IGM), T=T_0 Delta^{gamma-1}, is either close to
isothermal or inverted (gamma<1). Our results suggest that the voids in the IGM
may be significantly hotter and the thermal state of the low density IGM may be
substantially more complex than is usually assumed at these redshifts. We
discuss radiative transfer effects which alter the spectral shape of ionising
radiation during the epoch of HeII reionisation as a possible physical
mechanism for achieving an inverted temperature-density relation at z~3.Comment: 16 pages, 6 figures, accepted for publication in MNRAS following
minor revision. The accepted version includes an expanded discussion of the
flux power spectru
The impact of relative position and returns on sacrifice and reciprocity: an experimental study using individual decisions
We present a comprehensive experimental design that makes it possible to characterize other-regarding preferences and their relationship to the decision maker’s relative position. Participants are faced with a large number of decisions involving variations in the trade-offs between own and other’s payoffs, as well as in other potentially important factors like the decision maker’s relative position. We find that: (1) choices are responsive to the cost of helping and hurting others; (2) The weight a decision maker places on others’ monetary payoffs depends on whether the decision maker is in an advantageous or disadvantageous relative position; and (3) We find no evidence of reciprocity of the type linked to menu-dependence. The results of a mixture-model estimation show considerable heterogeneity in subjects’ motivations and confirm the absence of reciprocal motives. Pure selfish behavior is the most frequently observed behavior. Among the subjects exhibiting social preferences, social-welfare maximization is the most frequent, followed by inequality-aversion and by competitiveness
The Interiors of Giant Planets: Models and Outstanding Questions
We know that giant planets played a crucial role in the making of our Solar
System. The discovery of giant planets orbiting other stars is a formidable
opportunity to learn more about these objects, what is their composition, how
various processes influence their structure and evolution, and most importantly
how they form. Jupiter, Saturn, Uranus and Neptune can be studied in detail,
mostly from close spacecraft flybys. We can infer that they are all enriched in
heavy elements compared to the Sun, with the relative global enrichments
increasing with distance to the Sun. We can also infer that they possess dense
cores of varied masses. The intercomparison of presently caracterised
extrasolar giant planets show that they are also mainly made of hydrogen and
helium, but that they either have significantly different amounts of heavy
elements, or have had different orbital evolutions, or both. Hence, many
questions remain and are to be answered for significant progresses on the
origins of planets.Comment: 43 pages, 11 figures, 3 tables. To appear in Annual Review of Earth
and Planetary Sciences, vol 33, (2005
- …