6,091 research outputs found
The Fluctuating Intergalactic Radiation Field at Redshifts z = 2.3-2.9 from He II and H I Absorption towards HE 2347-4342
We provide an in-depth analysis of the He II and H I absorption in the
intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using
spectra from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Ultraviolet-Visual Echelle Spectrograph (UVES) on the VLT telescope. Following
up on our earlier study (Kriss et al. 2001, Science, 293, 1112), we focus here
on two major topics: (1) small-scale variability (Delta z = 10^-3) in the ratio
eta = N(He II)/N(H I); and (2) an observed correlation of high-eta absorbers
(soft radiation fields) with voids in the (H I) Ly-alpha distribution. These
effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc,
together with radiative transfer through a filamentary IGM whose opacity
variations control the penetration of 1-5 ryd radiation over 30-40 Mpc
distances. Owing to photon statistics and backgrounds, we can measure optical
depths over the ranges 0.1 < tau(HeII) < 2.3 and 0.02 < tau(HI) < 3.9, and
reliably determine values of eta = 4 tau(HeII)/tau(HI) over the range 0.1 to
460. Values of eta = 20-200 are consistent with models of photoionization by
quasars with observed spectral indices alpha_s = 0-3. Values of eta > 200 may
require additional contributions from starburst galaxies, heavily filtered
quasar radiation, or density variations. Regions with eta < 30 may indicate the
presence of local hard sources. We find that eta is higher in "void" regions,
where H I is weak or undetected and 80% of the path length has eta > 100. These
voids may be ionized by soft sources (dwarf starbursts) or by QSO radiation
softened by escape from the AGN cores or transfer through the "cosmic web". The
apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag
between the reionization epochs, z(HI) = 6.2 +/-0.2 and z(HeII) = 2.8 +/-0.2.Comment: 27 pages, 7 figures, to appear in Ap
The QSO evolution derived from the HBQS and other complete QSO surveys
An ESO Key programme dedicated to an Homogeneous Bright QSO Survey (HBQS) has
been completed. 327 QSOs (Mb<-23, 0.3<z<2.2) have been selected over 555 deg^2
with 15<B<18.75. For B<16.4 the QSO surface density turns out to be a factor
2.2 higher than what measured by the PG survey, corresponding to a surface
density of 0.013+/-.006 deg^{-2}. If the Edinburgh QSO Survey is included, an
overdensity of a factor 2.5 is observed, corresponding to a density of
0.016+/-0.005 deg^{-2}. In order to derive the QSO optical luminosity function
(LF) we used Monte Carlo simulations that take into account of the selection
criteria, photometric errors and QSO spectral slope distribution. The LF can be
represented with a Pure Luminosity Evolution (L(z)\propto(1+z)^k) of a two
power law both for q_0=0.5 and q_0=0.1. For q_0=0.5 k=3.26, slower than the
previous Boyle's (1992) estimations of k=3.45. A flatter slope beta=-3.72 of
the bright part of the LF is also required. The observed overdensity of bright
QSOs is concentrated at z<0.6. It results that in the range 0.3<z<0.6 the
luminosity function is flatter than observed at higher redshifts. In this
redshift range, for Mb<-25, 32 QSOs are observed instead of 19 expected from
our best-fit PLE model. This feature requires a luminosity dependent luminosity
evolution in order to satisfactorily represent the data in the whole 0.3<z<2.2
interval.Comment: Invited talk in "Wide Field Spectroscopy" (20-24 May 1996, Athens),
eds. M. Kontizas et al. 6 pages and 3 eps figures, LaTex file, uses epfs.sty
and crckapb.sty (included
Using Adobe Flash Lite on mobile phones for psychological research: reaction time measurement reliability and inter-device variability
Mobile telephones have significant potential for use in psychological research, possessing unique characteristics—not least their ubiquity—that may make them useful tools for psychologists. We examined whether it is possible to measure reaction times (RTs) accurately using Adobe Flash Lite on mobile phones. We ran simple and choice RT experiments on two widely available mobile phones, a Nokia 6110 Navigator and a Sony Ericsson W810i, using a wireless application protocol (WAP) connection to access the Internet from the devices. RTs were compared within subjects with those obtained using a Linux-based millisecond-accurate measurement system. Results show that measured RTs were significantly longer on mobile devices, and that overall RTs and distribution of RTs varied across device
Static Critical Behavior of the Spin-Freezing Transition in the Geometrically Frustrated Pyrochlore Antiferromagnet Y2Mo2O7
Some frustrated pyrochlore antiferromagnets, such as Y2Mo2O7, show a
spin-freezing transition and magnetic irreversibilities below a temperature Tf
similar to what is observed nonlinear magnetization measurements on Y2Mo2O7
that provide strong evidence that there is an underlying thermodynamic phase
transition at Tf, which is characterized by critical exponents \gamma \approx
2.8 and \beta \approx 0.8. These values are typical of those found in random
spin glasses, despite the fact that the level of random disorder in Y2Mo2O7 is
immeasurably small.Comment: Latex file, calls for 4 encapsulated postscript figures (included).
Submitted to Phys. Rev. Letters
High-resolution O VI absorption line observations at 1.2 < z < 1.7 in the bright QSO HE 0515-4414
STIS Echelle observations at a resolution of 10 km/s and UVES/VLT
spectroscopy at a resolution of 7 km/s of the luminous QSO HE 0515-4414 (z_em =
1.73, B = 15.0) reveal four intervening O VI absorption systems in the redshift
range 1.2 < z_abs < 1.7 (1.38503, 1.41601, 1.60175, 1.67359). In addition two
associated systems at z = 1.69707 and z = 1.73585 are present. For the first
time high resolution observations allow to measure radial velocities of H I, C
IV and O VI simultaneously in several absorption systems (1.385, 1.674, 1.697)
with the result that significant velocity differences (up to 18 km/s) are
observed between H I and O VI, while smaller differences (up to 5 km/s) are
seen between C IV and O VI. We tentatively conclude that H I, O VI, and C IV
are not formed in the same volumes and that therefore implications on
ionization mechanisms are not possible from observed column density ratios O
VI/H I or O VI/C IV. The number density of O VI absorbers with W_rest > 25 mA
is dN/dz < 10, roughly a factor of 5 less than what has been found by Tripp at
al. (2000) at low redshift. An estimate of the cosmological mass-density of the
O VI-phase yields Omega_b(O VI) = 0.0003 h^{-1}_{75} for [O/H] = -1 and an
assumed ionization fraction O VI/O = 0.2. This corresponds to an increase by
roughly a factor of 15 between z = 1.5 (this work) and the value found by Tripp
et al. (2000) at z = 0.21, if the same oxygen abundance [O/H] = -1 is assumed.
Agreement with the simulations by Dave et al. (2001) can be obtained, if the
oxygen abundance increases by a factor of 3 over the same redshift interval.Comment: 8 pages, 1 figure, accepted for publication in A&
Reionization of Hydrogen and Helium by Early Stars and Quasars
We compute the reionization histories of hydrogen and helium due to the
ionizing radiation fields produced by stars and quasars. For the quasars we use
a model based on halo-merger rates that reproduces all known properties of the
quasar luminosity function at high redshifts. The less constrained properties
of the ionizing radiation produced by stars are modeled with two free
parameters: (i) a transition redshift, z_tran, above which the stellar
population is dominated by massive, zero-metallicity stars and below which it
is dominated by a Scalo mass function; (ii) the product of the escape fraction
of stellar ionizing photons from their host galaxies and the star-formation
efficiency, f_esc f_*. We constrain the allowed range of these free parameters
at high redshifts based on the lack of the HI Gunn-Peterson trough at z<6 and
the upper limit on the total intergalactic optical depth for electron
scattering, tau_es<0.18, from recent cosmic microwave background (CMB)
experiments. We find that quasars ionize helium by a redshift z~4, but cannot
reionize hydrogen by themselves before z~6. A major fraction of the allowed
combinations of f_esc f_* and z_tran lead to an early peak in the ionized
fraction due to metal-free stars at high redshifts. This sometimes results in
two reionization epochs, namely an early HII or HeIII overlap phase followed by
recombination and a second overlap phase. Even if early overlap is not
achieved, the peak in the visibility function for scattering of the CMB often
coincides with the early ionization phase rather than with the actual
reionization epoch. Consequently, tau_es does not correspond directly to the
reionization redshift. We generically find values of tau_es>7%, that should be
detectable by the MAP satellite.Comment: 33 pages, 10 figures, Accepted for publication in Ap
Aging and memory properties of topologically frustrated magnets
The model 2d kagome system (H3O)Fe3(SO4)2(OH)6 and the 3d pyrochlore Y2Mo2O7
are two well characterized examples of low-disordered frustrated
antiferromagnets which rather then condensing into spin liquid have been found
to undergo a freezing transition with spin glass-like properties. We explore
more deeply the comparison of their properties with those of spin glasses, by
the study of characteristic rejuvenation and memory effects in the
non-stationary susceptibility. While the pyrochlore shows clear evidence for
these non-trivial effects, implying temperature selective aging, that is
characteristic of a wide hierarchical distribution of equilibration processes,
the kagome system does n not show clearly these effects. Rather, it seems to
evolve towards the same final state independently of temperature.Comment: submitted for the proceedings of the 46th MMM conference (Seattle,
2001
Representations of p-brane topological charge algebras
The known extended algebras associated with p-branes are shown to be
generated as topological charge algebras of the standard p-brane actions. A
representation of the charges in terms of superspace forms is constructed. The
charges are shown to be the same in standard/extended superspace formulations
of the action.Comment: 22 pages. Typos fixed, refs added. Minor additions to comments
sectio
PRAESENTIA EX MACHINA: The Millennial Technoculture, Liveness and Theatre
This work examines the modern technoculture, its component parts and the changing role of theatre in Anglo-American culture. Our contemporary society is marked by the ubiquity of digital technologies and has become reliant on their commodities. The emerging Millennial Generation has matured with many of these devices and has embraced the modern technoculture. Many changes in daily life brought on by new technologies are obvious, but fundamental shifts remain obscured by the nature of technology itself. These subtle transformations are cumulative and have caused a technological disorientation with broad reaching implications. Liveness, initially defined as that which is unrecorded, has been perceived in many new technological formats. This phenomenon, combined with pervasive digitization of information and entertainment has minimized the cultural significance of the theatrical arts. Theatre, however, offers a form of liveness that cannot be digitized. In its varied iterations, theatre may be a vital forum for combating technological disorientation and fostering pertinent social discourse as the technoculture develops
- …