39,237 research outputs found
Collisional Grooming Models of the Kuiper Belt Dust Cloud
We modeled the 3-D structure of the Kuiper Belt dust cloud at four different
dust production rates, incorporating both planet-dust interactions and
grain-grain collisions using the collisional grooming algorithm. Simulated
images of a model with a face-on optical depth of ~10^-4 primarily show an
azimuthally-symmetric ring at 40-47 AU in submillimeter and infrared
wavelengths; this ring is associated with the cold classical Kuiper Belt. For
models with lower optical depths (10^-6 and 10^-7), synthetic infrared images
show that the ring widens and a gap opens in the ring at the location of of
Neptune; this feature is caused by trapping of dust grains in Neptune's mean
motion resonances. At low optical depths, a secondary ring also appears
associated with the hole cleared in the center of the disk by Saturn. Our
simulations, which incorporate 25 different grain sizes, illustrate that
grain-grain collisions are important in sculpting today's Kuiper Belt dust, and
probably other aspects of the Solar System dust complex; collisions erase all
signs of azimuthal asymmetry from the submillimeter image of the disk at every
dust level we considered. The model images switch from being dominated by
resonantly-trapped small grains ("transport dominated") to being dominated by
the birth ring ("collision dominated") when the optical depth reaches a
critical value of tau ~ v/c, where v is the local Keplerian speed.Comment: 31 pages, including 9 figure
Astrometry with the Wide-Field InfraRed Space Telescope
The Wide-Field InfraRed Space Telescope (WFIRST) will be capable of
delivering precise astrometry for faint sources over the enormous field of view
of its main camera, the Wide-Field Imager (WFI). This unprecedented combination
will be transformative for the many scientific questions that require precise
positions, distances, and velocities of stars. We describe the expectations for
the astrometric precision of the WFIRST WFI in different scenarios, illustrate
how a broad range of science cases will see significant advances with such
data, and identify aspects of WFIRST's design where small adjustments could
greatly improve its power as an astrometric instrument.Comment: version accepted to JATI
The Detailed Star Formation History in the Spheroid, Outer Disk, and Tidal Stream of the Andromeda Galaxy
Using the Advanced Camera for Surveys on the Hubble Space Telescope, we have
obtained deep optical images reaching stars well below the oldest main sequence
turnoff in the spheroid, tidal stream, and outer disk of the Andromeda Galaxy.
We have reconstructed the star formation history in these fields by comparing
their color-magnitude diagrams to a grid of isochrones calibrated to Galactic
globular clusters observed in the same bands. Each field exhibits an extended
star formation history, with many stars younger than 10 Gyr but few younger
than 4 Gyr. Considered together, the star counts, kinematics, and population
characteristics of the spheroid argue against some explanations for its
intermediate-age, metal-rich population, such as a significant contribution
from stars residing in the disk or a chance intersection with the stream's
orbit. Instead, it is likely that this population is intrinsic to the inner
spheroid, whose highly-disturbed structure is clearly distinct from the
pressure-supported metal-poor halo that dominates farther from the galaxy's
center. The stream and spheroid populations are similar, but not identical,
with the stream's mean age being ~1 Gyr younger; this similarity suggests that
the inner spheroid is largely polluted by material stripped from either the
stream's progenitor or similar objects. The disk population is considerably
younger and more metal-rich than the stream and spheroid populations, but not
as young as the thin disk population of the solar neighborhood; instead, the
outer disk of Andromeda is dominated by stars of age 4 - 8 Gyr, resembling the
Milky Way's thick disk. The disk data are inconsistent with a population
dominated by ages older than 10 Gyr, and in fact do not require any stars older
than 10 Gyr.Comment: Accepted for publication in The Astrophysical Journal. 29 pages, 23
figures (including 9 in color), latex. Updated for minor edits and additional
references. Images and CMDs are significantly smoothed and degraded in this
version; a version with high-quality figures is available at
http://www.stsci.edu/~tbrown/m31sfh/preprint.pd
Allometric trajectories of body and head morphology in three sympatric Arctic charr (Salvelinus alpinus (L.)) morphs
A study of body and head development in three sympatric reproductively isolated Arctic charr (Salvelinus alpinus (L.)) morphs from a subarctic lake (Skogsfjordvatn, northern Norway) revealed allometric trajectories that resulted in morphological differences. The three morphs were ecologically assigned to a littoral omnivore, a profundal benthivore and a profundal piscivore, and this was confirmed by genetic analyses (microsatellites). Principal component analysis was used to identify the variables responsible for most of the morphological variation of the body and head shape. The littoral omnivore and the profundal piscivore morph had convergent allometric trajectories for the most important head shape variables, developing bigger mouths and relatively smaller eyes with increasing head size. The two profundal morphs shared common trajectories for the variables explaining most of the body and head shape variation, namely head size relative to body size, placement of the dorsal and pelvic fins, eye size and mouth size. In contrast, the littoral omnivore and the profundal benthivore morphs were not on common allometric trajectories for any of the examined variables. The findings suggest that different selective pressures could have been working on traits related to their trophic niche such as habitat and diet utilization of the three morphs, with the two profundal morphs experiencing almost identical environmental conditions
Model-Free Multi-Probe Lensing Reconstruction of Cluster Mass Profiles
Lens magnification by galaxy clusters induces characteristic spatial
variations in the number counts of background sources, amplifying their
observed fluxes and expanding the area of sky, the net effect of which, known
as magnification bias, depends on the intrinsic faint-end slope of the source
luminosity function. The bias is strongly negative for red galaxies, dominated
by the geometric area distortion, whereas it is mildly positive for blue
galaxies, enhancing the blue counts toward the cluster center. We generalize
the Bayesian approach of Umetsu et al. for reconstructing projected cluster
mass profiles, by incorporating multiple populations of background sources for
magnification bias measurements and combining them with complementary lens
distortion measurements, effectively breaking the mass-sheet degeneracy and
improving the statistical precision of cluster mass measurements. The approach
can be further extended to include strong-lensing projected mass estimates,
thus allowing for non-parametric absolute mass determinations in both the weak
and strong regimes. We apply this method to our recent CLASH lensing
measurements of MACS J1206.2-0847, and demonstrate how combining multi-probe
lensing constraints can improve the reconstruction of cluster mass profiles.
This method will also be useful for a stacked lensing analysis, combining all
lensing-related effects in the cluster regime, for a definitive determination
of the averaged mass profile.Comment: 13 pages, 2 figures; Typo corrections (Appendix A.2.) to match the
published version in Ap
A panchromatic view of the bulge globular cluster NGC 6569
We used high-resolution optical HST/WFC3 and multi-conjugate adaptive optics
assisted GEMINI GeMS/GSAOI observations in the near-infrared to investigate the
physical properties of the globular cluster NGC 6569 in the Galactic bulge. We
have obtained the deepest purely NIR color-magnitude diagram published so far
for this cluster using ground-based observations, reaching
21.0 mag (two magnitudes below the main-sequence turn-off point). By combining
the two datasets secured at two different epochs, we determined relative proper
motions for a large sample of individual stars in the center of NGC 6569,
allowing a robust selection of cluster member stars. Our proper motion analysis
solidly demonstrates that, despite its relatively high metal content, NGC 6569
hosts some blue horizontal branch stars. A differential reddening map has been
derived in the direction of the system, revealing a maximum color excess
variation of about 0.12 mag in the available field of
view. The absolute age of NGC 6569 has been determined for the first time. In
agreement with the other few bulge globular clusters with available age
estimates, NGC 6569 turns out to be old, with an age of about 12.8 Gyr, and a
typical uncertainty of 0.8-1.0 Gyr.Comment: 25 pages, 16 Figures, 1 Table. Accepted for publication in Ap
The nature of late-type spiral galaxies: structural parameters, optical and near-infrared colour profiles, and dust extinction
We analyse V and H-band surface photometry of a sample of 18 Sb-Sd galaxies.
Combining high resolution HST images with ground-based NIR observations, we
extract photometric profiles, which cover the whole disk and provide the
highest possible resolution. This is the first photometric study of late-type
spirals for which the stellar kinematics have been measured. For 10 out of the
18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for
those, we present colour maps and radial colour profiles at the resolution of
the Hubble Space Telescope.
Colours vary significantly from galaxy to galaxy, but tend to be highly
homogeneous within each galaxy, with smooth and flat colour profiles. Some of
the colour maps show jumps in the inner regions, likely due to dust. We
determine extinction-maps in an almost model-independent way using the V-H
colour map and the SAURON Mg b absorption line map of Ganda et al. (2007). The
maps show that A_V ranges from 0 to 2 mag, in the center from 0 to 1.5 mag, in
agreement with the models of Tuffs et al. (2004).
We describe the surface brightness profiles as the superposition of an
exponential disk and a Sersic bulge. The bulges are small (0.1-2.5 kpc), and
show a shape parameter n ranging from ~ 0.7 to 3, with a mean value smaller
than two: well below the value for the 'classical' de Vaucouleurs bulges. Most
galaxies (16 out of 18) show a central light excess above the Sersic fit to the
bulge, which can be interpreted as a nuclear cluster, as shown by previous
studies. We provide zero-order estimates for the magnitude of these components.
We discuss the correlations among the structural galaxy parameters and with
other relevant quantities (abridged).Comment: 28 pages, 12 figures and 7 tables. Accepted for publication in MNRAS.
Higher resolution version available at
http://www.astro.rug.nl/~peletier/ganda2009.pd
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