1,970 research outputs found
Ongoing Galactic Accretion: Simulations and Observations of Condensed Gas in Hot Halos
Ongoing accretion onto galactic disks has been recently theorized to progress
via the unstable cooling of the baryonic halo into condensed clouds. These
clouds have been identified as analogous to the High-Velocity Clouds (HVCs)
observed in HI in our Galaxy. Here we compare the distribution of HVCs observed
around our own Galaxy and extra-planar gas around the Andromeda galaxy to these
possible HVC analogs in a simulation of galaxy formation that naturally
generates these condensed clouds. We find a very good correspondence between
these observations and the simulation, in terms of number, angular size,
velocity distribution, overall flux and flux distribution of the clouds. We
show that condensed cloud accretion only accounts for ~ 0.2 M_solar / year of
the current overall Galactic accretion in the simulations. We also find that
the simulated halo clouds accelerate and become more massive as they fall
toward the disk. The parameter space of the simulated clouds is consistent with
all of the observed HVC complexes that have distance constraints, except the
Magellanic Stream which is known to have a different origin. We also find that
nearly half of these simulated halo clouds would be indistinguishable from
lower-velocity gas and that this effect is strongest further from the disk of
the galaxy, thus indicating a possible missing population of HVCs. These
results indicate that the majority of HVCs are consistent with being infalling,
condensed clouds that are a remnant of Galaxy formation.Comment: 10 pages, 6 figures, ApJ Accepted. Some changes to techniqu
A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations
We determine the parallax and proper motion of the flaring, non-thermal radio
star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline
Array observations. Based on the parallax, we measure a distance of 389 +24/-21
parsecs to the source. Our measurement places the Orion Nebula Cluster
considerably closer than the canonical distance of 480 +/- 80 parsecs
determined by Genzel et al. (1981). A change of this magnitude in distance
lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We
briefly discuss two effects of this change--an increase in the age spread of
the pre-main sequence stars and better agreement between the zero-age
main-sequence and the temperatures and luminosities of massive stars.Comment: 10 pages, 4 figures, emulateapj, accepted to Ap
Flying Drosophila stabilize their vision-based velocity controller by sensing wind with their antennae
Flies and other insects use vision to regulate their groundspeed in flight, enabling them to fly in varying wind conditions. Compared with mechanosensory modalities, however, vision requires a long processing delay (~100 ms) that might introduce instability if operated at high gain. Flies also sense air motion with their antennae, but how this is used in flight control is unknown. We manipulated the antennal function of fruit flies by ablating their aristae, forcing them to rely on vision alone to regulate groundspeed. Arista-ablated flies in flight exhibited significantly greater groundspeed variability than intact flies. We then subjected them to a series of controlled impulsive wind gusts delivered by an air piston and experimentally manipulated antennae and visual feedback. The results show that an antenna-mediated response alters wing motion to cause flies to accelerate in the same direction as the gust. This response opposes flying into a headwind, but flies regularly fly upwind. To resolve this discrepancy, we obtained a dynamic model of the flyâs velocity regulator by fitting parameters of candidate models to our experimental data. The model suggests that the groundspeed variability of arista-ablated flies is the result of unstable feedback oscillations caused by the delay and high gain of visual feedback. The antenna response drives active damping with a shorter delay (~20 ms) to stabilize this regulator, in exchange for increasing the effect of rapid wind disturbances. This provides insight into fliesâ multimodal sensory feedback architecture and constitutes a previously unknown role for the antennae
Welfare Performance of Three Foothold Traps for Capturing North American River Otters \u3ci\u3eLontra canadensis\u3c/i\u3e
Foothold traps are effective tools for the live capture and restraint of wildlife for management and research. Successful river otter Lontra canadensis restoration programs throughout North America used them extensively. Restoration programs used a variety of methods and models of foothold traps, but comprehensive efforts to describe and quantify injuries associated with river otter captures have been limited. We evaluated injuries of river otters caught in three commercially available models of foothold traps including the number 11 double long-spring with standard jaws, the number 11 double long-spring with double jaws, and the number 2 coil-spring trap. Based on examinations of 70 captured river otters, we classified 78% of the total inj uries detected as ââmildââ (n=174 injuries) and 17% were classified as ââmoderateââ (n= 37 injuries). We classified less than 3% of the injuries observed as ââmoderately severeââ or ââsevere.ââ We focused only on the animal welfare performance of traps; the three trap types we tested met the animal welfare criteria required for inclusion in the best management practices for trapping river otter. The criteria based on International Standards Organization guidelines used in this assessment of trap performance provides a scientific basis for future evaluations of river otter welfare when foothold traps are used for restoration, research, and population management
Kinetic energy of solid neon by Monte Carlo with improved Trotter- and finite-size extrapolation
The kinetic energy of solid neon is calculated by a path-integral Monte Carlo
approach with a refined Trotter- and finite-size extrapolation. These accurate
data present significant quantum effects up to temperature T=20 K. They confirm
previous simulations and are consistent with recent experiments.Comment: Text and figures revised for minor corrections (4 pages, 3 figures
included by psfig
Head-Tail Clouds: Drops to Probe the Diffuse Galactic Halo
A head-tail high-velocity cloud (HVC) is a neutral hydrogen halo cloud that
appears to be interacting with the diffuse halo medium as evident by its
compressed head trailed by a relatively diffuse tail. This paper presents a
sample of 116 head-tail HVCs across the southern sky (d < 2 deg) from the HI
Parkes All Sky Survey (HIPASS) HVC catalog, which has a spatial resolution of
15.5 arcmin (45 pc at 10 kpc) and a sensitivity of N_HI=2 x 10^(18) cm^(-2) (5
sigma). 35% of the HIPASS compact and semi-compact HVCs (CHVCs and :HVCs) can
be classified as head-tail clouds from their morphology. The clouds have
typical masses of 730 M_sun at 10 kpc (26,000 M_sun at 60 kpc) and the majority
can be associated with larger HVC complexes given their spatial and kinematic
proximity. This proximity, together with their similar properties to CHVCs and
:HVCs without head-tail structure, indicate the head-tail clouds have short
lifetimes, consistent with simulation predictions. Approximately half of the
head-tail clouds can be associated with the Magellanic System, with the
majority in the region of the Leading Arm with position angles pointing in the
general direction of the movement of the Magellanic System. The abundance in
the Leading Arm region is consistent with this feature being closer to the
Galactic disk than the Magellanic Stream and moving through a denser halo
medium. The head-tail clouds will feed the multi-phase halo medium rather than
the Galactic disk directly and provide additional evidence for a diffuse
Galactic halo medium extending to at least the distance of the Magellanic
Clouds.Comment: MNRAS Accepted, 10 figures, 7 in colo
H^+_2$ in a strong magnetic field described via a solvable model
We consider the hydrogen molecular ion in the presence of a strong
homogeneous magnetic field. In this regime, the effective Hamiltonian is almost
one dimensional with a potential energy which looks like a sum of two Dirac
delta functions. This model is solvable, but not close enough to our exact
Hamiltonian for relevant strenght of the magnnetic field. However we show that
the correct values of the equilibrium distance as well as the binding energy of
the ground state of the ion, can be obtained when incorporating perturbative
corrections up to second order. Finally, we show that exists for
sufficiently large magnetic fields
Measurement of a Magnetic Field in a Leading Arm High Velocity Cloud
Using a recent catalogue of extragalactic Faraday rotation derived from the
NRAO VLA Sky Survey we have found an agreement between Faraday rotation
structure and the HI emission structure of a High Velocity Cloud (HVC)
associated with the Leading Arm of the Magellanic System. We suggest that this
morphological agreement is indicative of Faraday rotation through the HVC.
Under this assumption we have used 48 rotation measures through the HVC,
together with estimates of the electron column density from H-\alpha\
measurements and QSO absorption lines to estimate a strength for the
line-of-sight component of the coherent magnetic field in the HVC of >
6 {\rm \mu G}B_{r}/B_{||} < 0.8$, which suggests that the random field does not dominate
over the coherent field as it does in the Magellanic Clouds from which this HVC
likely originates.Comment: 17 pages, 3 figure
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