2,337 research outputs found
The Panchromatic Hubble Andromeda Treasury
The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST
Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6
filters, from the UV to the NIR. The full survey will resolve the galaxy into
more than 100 million stars with projected radii from 0-20 kpc over a
contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W
and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and
F160W with WFC3/IR. The resulting wavelength coverage gives excellent
constraints on stellar temperature, bolometric luminosity, and extinction for
most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9,
F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the
uncrowded outer disk; however, the optical and NIR data are crowding limited,
and the deepest reliable magnitudes are up to 5 magnitudes brighter in the
inner bulge. All pointings are dithered and produce Nyquist-sampled images in
F475W, F814W, and F160W. We describe the observing strategy, photometry,
astrometry, and data products, along with extensive tests of photometric
stability, crowding errors, spatially-dependent photometric biases, and
telescope pointing control. We report on initial fits to the structure of M31's
disk, derived from the density of RGB stars, in a way that is independent of
the assumed M/L and is robust to variations in dust extinction. These fits also
show that the 10 kpc ring is not just a region of enhanced recent star
formation, but is instead a dynamical structure containing a significant
overdensity of stars with ages >1 Gyr. (Abridged)Comment: 48 pages including 22 pages of figures. Accepted to the Astrophysical
Journal Supplements. Some figures slightly degraded to reduce submission siz
Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations
We use three-dimensional simulations to study the atmospheric circulation on
the first Earth-sized exoplanet discovered in the habitable zone of an M star.
We treat Gliese 581g as a scaled-up version of Earth by considering increased
values for the exoplanetary radius and surface gravity, while retaining
terrestrial values for parameters which are unconstrained by current
observations. We examine the long-term, global temperature and wind maps near
the surface of the exoplanet --- the climate. The specific locations for
habitability on Gliese 581g depend on whether the exoplanet is tidally-locked
and how fast radiative cooling occurs on a global scale. Independent of whether
the existence of Gliese 581g is confirmed, our study highlights the use of
general circulation models to quantify the atmospheric circulation on
potentially habitable, Earth-sized exoplanets, which will be the prime targets
of exoplanet discovery and characterization campaigns in the next decade.Comment: Accepted by MNRAS. 15 pages, 13 figures. Sample movies of simulations
are available at http://www.phys.ethz.ch/~kheng/fms
Interstellar Sonic and Alfv\'enic Mach Numbers and the Tsallis Distribution
In an effort to characterize the Mach numbers of ISM magnetohydrodynamic
(MHD) turbulence, we study the probability distribution functions (PDFs) of
patial increments of density, velocity, and magnetic field for fourteen ideal
isothermal MHD simulations at resolution 512^3. In particular, we fit the PDFs
using the Tsallis function and study the dependency of fit parameters on the
compressibility and magnetization of the gas. We find that the Tsallis function
fits PDFs of MHD turbulence well, with fit parameters showing sensitivities to
the sonic and Alfven Mach numbers. For 3D density, column density, and
position-position-velocity (PPV) data we find that the amplitude and width of
the PDFs shows a dependency on the sonic Mach number. We also find the width of
the PDF is sensitive to global Alfvenic Mach number especially in cases where
the sonic number is high. These dependencies are also found for mock
observational cases, where cloud-like boundary conditions, smoothing, and noise
are introduced. The ability of Tsallis statistics to characterize sonic and
Alfvenic Mach numbers of simulated ISM turbulence point to it being a useful
tool in the analysis of the observed ISM, especially when used simultaneously
with other statistical techniques.Comment: 20 pages, 16 figures, ApJ submitte
Constraining the Mass Profiles of Stellar Systems: Schwarzschild Modeling of Discrete Velocity Datasets
(ABRIDGED) We present a new Schwarzschild orbit-superposition code designed
to model discrete datasets composed of velocities of individual kinematic
tracers in a dynamical system. This constitutes an extension of previous
implementations that can only address continuous data in the form of (the
moments of) velocity distributions, thus avoiding potentially important losses
of information due to data binning. Furthermore, the code can handle any
combination of available velocity components, i.e., only line-of-sight
velocities, only proper motions, or a combination of both. It can also handle a
combination of discrete and continuous data. The code finds the distribution
function (DF, a function of the three integrals of motion E, Lz, and I3) that
best reproduces the available kinematic and photometric observations in a given
axisymmetric gravitational potential. The fully numerical approach ensures
considerable freedom on the form of the DF f(E,Lz,I3). This allows a very
general modeling of the orbital structure, thus avoiding restrictive
assumptions about the degree of (an)isotropy of the orbits. We describe the
implementation of the discrete code and present a series of tests of its
performance based on the modeling of simulated datasets generated from a known
DF. We find that the discrete Schwarzschild code recovers the original orbital
structure, M/L ratios, and inclination of the input datasets to satisfactory
accuracy, as quantified by various statistics. The code will be valuable, e.g.,
for modeling stellar motions in Galactic globular clusters, and those of
individual stars, planetary nebulae, or globular clusters in nearby galaxies.
This can shed new light on the total mass distributions of these systems, with
central black holes and dark matter halos being of particular interest.Comment: ApJ, in press; 51 pages, 11 figures; manuscript revised following
comments by refere
Future Challenges and Unsolved Problems in Multi-field Visualization
Evaluation, solved and unsolved problems, and future directions are popular themes pervading the visualization community over the last decade. The top unsolved problem in both scientific and information visualization was the subject of an IEEE Visualization Conference panel in 2004. The future of graphics hardware was another important topic of discussion the same year. The subject of how to evaluate visualization returned a few years later. Chris Johnson published a list of 10 top problems in scientific visualization research. This was followed up by report of both past achievements and future challenges in visualization research as well as financial support recommendations to the National Science Foundation (NSF) and National Institute of Health (NIH). Chen recently published the first list of top unsolved information visualization problems. Future research directions of topology-based visualization was also a major theme of a workshop on topology-based methods. Laramee and Kosara published a list of top future challenges in human-centered visualization
Dwarf galaxy formation with H2-regulated star formation
We describe cosmological galaxy formation simulations with the adaptive mesh
refinement code Enzo that incorporate a star formation prescription regulated
by the local abundance of molecular hydrogen. We show that this H2-regulated
prescription leads to a suppression of star formation in low mass halos (M_h <
~10^10 M_sun) at z>4, alleviating some of the dwarf galaxy problems faced by
theoretical galaxy formation models. H2 regulation modifies the efficiency of
star formation of cold gas directly, rather than indirectly reducing the cold
gas content with "supernova feedback". We determine the local H2 abundance in
our most refined grid cells (76 proper parsec in size at z=4) by applying the
model of Krumholz, McKee, & Tumlinson, which is based on idealized 1D radiative
transfer calculations of H2 formation-dissociation balance in ~100 pc
atomic--molecular complexes. Our H2-regulated simulations are able to reproduce
the empirical (albeit lower z) Kennicutt-Schmidt relation, including the low
Sigma_gas cutoff due to the transition from atomic to molecular phase and the
metallicity dependence thereof, without the use of an explicit density
threshold in our star formation prescription. We compare the evolution of the
luminosity function, stellar mass density, and star formation rate density from
our simulations to recent observational determinations of the same at z=4-8 and
find reasonable agreement between the two.Comment: replaced with version published in Ap
Steps toward the power spectrum of matter. II. The biasing correction with sigma_8 normalization
A new method to determine the bias parameter of galaxies relative to matter
is suggested. The method is based on the assumption that gravity is the
dominating force which determines the formation of the structure in the
Universe. Due to gravitational instability the galaxy formation is a threshold
process: in low-density environments galaxies do not form and matter remains in
primordial form. We investigate the influence of the presence of void and
clustered populations to the power spectrum of matter and galaxies. The power
spectrum of galaxies is similar to the power spectrum of matter; the fraction
of total matter in the clustered population determines the difference between
amplitudes of fluctuations of matter and galaxies, i.e. the bias factor. To
determine the fraction of matter in voids and clustered population we perform
numerical simulations. The fraction of matter in galaxies at the present epoch
is found using a calibration through the sigma_8 parameter.Comment: LaTex (sty files added), 31 pages, 4 PostScript figures embedded,
Astrophysical Journal (accepted
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