255 research outputs found
Why hyperbolic theories of dissipation cannot be ignored: Comments on a paper by Kostadt and Liu
Contrary to what is asserted in a recent paper by Kostadt and Liu ("Causality
and stability of the relativistic diffusion equation"), experiments can tell
apart (and in fact do) hyperbolic theories from parabolic theories of
dissipation. It is stressed that the existence of a non--negligible relaxation
time does not imply for the system to be out of the hydrodynamic regime.Comment: 8 pages Latex, to appear in Phys.Rev.
HIFI Spectroscopy of submm Lines in Nuclei of Actively Star Forming Galaxies
We present a systematic survey of multiple velocity-resolved HO spectra
using Herschel/HIFI towards nine nearby actively star forming galaxies. The
ground-state and low-excitation lines (E) show
profiles with emission and absorption blended together, while absorption-free
medium-excitation lines ()
typically display line shapes similar to CO. We analyze the HIFI observation
together with archival SPIRE/PACS HO data using a state-of-the-art 3D
radiative transfer code which includes the interaction between continuum and
line emission. The water excitation models are combined with information on the
dust- and CO spectral line energy distribution to determine the physical
structure of the interstellar medium (ISM). We identify two ISM components that
are common to all galaxies: A warm (),
dense () phase which dominates the
emission of medium-excitation HO lines. This gas phase also dominates the
FIR emission and the CO intensities for . In addition a cold
(), dense () more extended phase is present. It outputs the emission
in the low-excitation HO lines and typically also produces the prominent
line absorption features. For the two ULIRGs in our sample (Arp 220 and Mrk
231) an even hotter and more compact (R pc) region is present
which is possibly linked to AGN activity. We find that collisions dominate the
water excitation in the cold gas and for lines with
and in the warm and hot component, respectively.
Higher energy levels are mainly excited by IR pumping.Comment: Accepted by ApJ, in pres
Detecting the Most Distant (z>7) Objects with ALMA
Detecting and studying objects at the highest redshifts, out to the end of
Cosmic Reionization at z>7, is clearly a key science goal of ALMA. ALMA will in
principle be able to detect objects in this redshift range both from high-J
(J>7) CO transitions and emission from ionized carbon, [CII], which is one of
the main cooling lines of the ISM. ALMA will even be able to resolve this
emission for individual targets, which will be one of the few ways to determine
dynamical masses for systems in the Epoch of Reionization. We discuss some of
the current problems regarding the detection and characterization of objects at
high redshifts and how ALMA will eliminate most (but not all) of them.Comment: to appear in Astrophysics and Space Science, "Science with ALMA: a
new era for Astrophysics", ed. R. Bachille
HIFI spectroscopy of low-level water transitions in M82
We present observations of the rotational ortho-water ground transition, the
two lowest para-water transitions, and the ground transition of ionised
ortho-water in the archetypal starburst galaxy M82, performed with the HIFI
instrument on the Herschel Space Observatory. These observations are the first
detections of the para-H2O(111-000) (1113\,GHz) and ortho-H2O+(111-000)
(1115\,GHz) lines in an extragalactic source. All three water lines show
different spectral line profiles, underlining the need for high spectral
resolution in interpreting line formation processes. Using the line shape of
the para-H2O(111-000) and ortho-H2O+(111-000) absorption profile in conjunction
with high spatial resolution CO observations, we show that the (ionised) water
absorption arises from a ~2000 pc^2 region within the HIFI beam located about
~50 pc east of the dynamical centre of the galaxy. This region does not
coincide with any of the known line emission peaks that have been identified in
other molecular tracers, with the exception of HCO. Our data suggest that water
and ionised water within this region have high (up to 75%) area-covering
factors of the underlying continuum. This indicates that water is not
associated with small, dense cores within the ISM of M82 but arises from a more
widespread diffuse gas component.Comment: 5 pages, 4 figures. Accepted for publication in A&
Molecular gas in high-velocity clouds: revisited scenario
We report a new search for 12CO(1-0) emission in high-velocity clouds (HVCs)
performed with the IRAM 30 m telescope. This search was motivated by the recent
detection of cold dust emission in the HVCs of Complex C. Despite a spatial
resolution which is three times better and sensitivity twice as good compared
to previous studies, no CO emission is detected in the HVCs of Complex C down
to a best 5 sigma limit of 0.16 K km/s at a 22'' resolution. The CO emission
non-detection does not provide any evidence in favor of large amounts of
molecular gas in these HVCs and hence in favor of the infrared findings. We
discuss different configurations which, however, allow us to reconcile the
negative CO result with the presence of molecular gas and cold dust emission.
H2 column densities higher than our detection limit, N(H2) = 3x10^{19} cm^{-2},
are expected to be confined in very small and dense clumps with 20 times
smaller sizes than the 0.5 pc clumps resolved in our observations according to
the results obtained in cirrus clouds, and might thus still be highly diluted.
As a consequence, the inter-clump gas at the 1 pc scale has a volume density
lower than 20 cm^{-3} and already appears as too diffuse to excite the CO
molecules. The observed physical conditions in the HVCs of Complex C also play
an important role against CO emission detection. It has been shown that the
CO-to-H2 conversion factor in low metallicity media is 60 times higher than at
the solar metallicity, leading for a given H2 column density to a 60 times
weaker integrated CO intensity. And the very low dust temperature estimated in
these HVCs implies the possible presence of gas cold enough (< 20 K) to cause
CO condensation onto dust grains under interstellar medium pressure conditions
and thus CO depletion in gas-phase observations.Comment: 9 pages, 4 figures, Accepted for publication in A&
The very red afterglow of GRB 000418 - further evidence for dust extinction in a GRB host galaxy
We report near-infrared and optical follow-up observations of the afterglow
of the Gamma-Ray Burst 000418 starting 2.5 days after the occurrence of the
burst and extending over nearly seven weeks. GRB 000418 represents the second
case for which the afterglow was initially identified by observations in the
near-infrared. During the first 10 days its R-band afterglow was well
characterized by a single power-law decay with a slope of 0.86. However, at
later times the temporal evolution of the afterglow flattens with respect to a
simple power-law decay. Attributing this to an underlying host galaxy we find
its magnitude to be R=23.9 and an intrinsic afterglow decay slope of 1.22. The
afterglow was very red with R-K=4 mag. The observations can be explained by an
adiabatic, spherical fireball solution and a heavy reddening due to dust
extinction in the host galaxy. This supports the picture that (long) bursts are
associated with events in star-forming regions.Comment: Accepted for publication in The Astrophysical Journal. 12 pages;
citations & references updated; minor textual change
Stochastic dynamics of correlations in quantum field theory: From Schwinger-Dyson to Boltzmann-Langevin equation
The aim of this paper is two-fold: in probing the statistical mechanical
properties of interacting quantum fields, and in providing a field theoretical
justification for a stochastic source term in the Boltzmann equation. We start
with the formulation of quantum field theory in terms of the Schwinger - Dyson
equations for the correlation functions, which we describe by a
closed-time-path master () effective action. When the hierarchy
is truncated, one obtains the ordinary closed-system of correlation functions
up to a certain order, and from the nPI effective action, a set of
time-reversal invariant equations of motion. But when the effect of the higher
order correlation functions is included (through e.g., causal factorization--
molecular chaos -- conditions, which we call 'slaving'), in the form of a
correlation noise, the dynamics of the lower order correlations shows
dissipative features, as familiar in the field-theory version of Boltzmann
equation. We show that fluctuation-dissipation relations exist for such
effectively open systems, and use them to show that such a stochastic term,
which explicitly introduces quantum fluctuations on the lower order correlation
functions, necessarily accompanies the dissipative term, thus leading to a
Boltzmann-Langevin equation which depicts both the dissipative and stochastic
dynamics of correlation functions in quantum field theory.Comment: LATEX, 30 pages, no figure
Mapping the cold dust temperatures and masses of nearby Kingfish galaxies with Herschel
Taking advantage of the sensitivity and angular resolution of the Herschel
Space Observatory at far-infrared and submm wavelengths, we aim to characterize
the physical properties of cold dust within nearby galaxies and study the
robustness of the parameters we derive using different modified blackbody
models. For a pilot subsample of the KINGFISH program, we perform 2 temperature
fits of the Spitzer and Herschel photometric data (24 to 500um), with a warm
and a cold component, globally and in each resolution element.At global scales,
we observe ranges of values for beta_c(0.8 to 2.5) and Tc(19.1 to 25.1K).We
compute maps of our parameters with beta fixed or free to test the robustness
of the temperature and dust surface density maps we deduce. When the emissivity
is fixed, we observe temperature gradients as a function of radius.When the
emissivity is fitted as a free parameter, barred galaxies tend to have uniform
fitted emissivities.Gathering resolved elements in a Tc-beta_c diagram
underlines an anti-correlation between the two parameters.It remains difficult
to assess whether the dominant effect is the physics of dust grains, noise, or
mixing along the line of sight and in the beam. We finally observe in both
cases that the dust column density peaks in central regions of galaxies and bar
ends (coinciding with molecular gas density enhancements usually found in these
locations).We also quantify how the total dust mass varies with our assumptions
about the emissivity index as well as the influence of the wavelength coverage
used in the fits. We show that modified blackbody fits using a shallow
emissivity (beta_c < 2.0) lead to significantly lower dust masses compared to
the beta_c < 2.0 case, with dust masses lower by up to 50% if beta_c=1.5 for
instance.The working resolution affects our total dust mass estimates: masses
increase from global fits to spatially-resolved fits.Comment: 26 pages, 12 figures, 4 tables, accepted for publication in MNRAS,
2012 June 2
Optimal designs for rational function regression
We consider optimal non-sequential designs for a large class of (linear and
nonlinear) regression models involving polynomials and rational functions with
heteroscedastic noise also given by a polynomial or rational weight function.
The proposed method treats D-, E-, A-, and -optimal designs in a
unified manner, and generates a polynomial whose zeros are the support points
of the optimal approximate design, generalizing a number of previously known
results of the same flavor. The method is based on a mathematical optimization
model that can incorporate various criteria of optimality and can be solved
efficiently by well established numerical optimization methods. In contrast to
previous optimization-based methods proposed for similar design problems, it
also has theoretical guarantee of its algorithmic efficiency; in fact, the
running times of all numerical examples considered in the paper are negligible.
The stability of the method is demonstrated in an example involving high degree
polynomials. After discussing linear models, applications for finding locally
optimal designs for nonlinear regression models involving rational functions
are presented, then extensions to robust regression designs, and trigonometric
regression are shown. As a corollary, an upper bound on the size of the support
set of the minimally-supported optimal designs is also found. The method is of
considerable practical importance, with the potential for instance to impact
design software development. Further study of the optimality conditions of the
main optimization model might also yield new theoretical insights.Comment: 25 pages. Previous version updated with more details in the theory
and additional example
Stochastic semiclassical gravity
In the first part of this paper, we show that the semiclassical
Einstein-Langevin equation, introduced in the framework of a stochastic
generalization of semiclassical gravity to describe the back reaction of matter
stress-energy fluctuations, can be formally derived from a functional method
based on the influence functional of Feynman and Vernon. In the second part, we
derive a number of results for background solutions of semiclassical gravity
consisting of stationary and conformally stationary spacetimes and scalar
fields in thermal equilibrium states. For these cases, fluctuation-dissipation
relations are derived. We also show that particle creation is related to the
vacuum stress-energy fluctuations and that it is enhanced by the presence of
stochastic metric fluctuations.Comment: 26 pages, RevTeX, no figure
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