1,071 research outputs found
Consumption, protein and energy intake of fallow deer fawns on diets of differing nutritional quality
Patterns of habitat use and activity in British populations of sika deer of contrasting environments
Are Compact High-Velocity Clouds Extragalactic Objects?
Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the
Local Group model and would have HI volume densities of order 0.0003/cm^3.
Clouds with these volume densities and the observed neutral hydrogen column
densities will be largely ionized, even if exposed only to the extragalactic
ionizing radiation field. Here we examine the implications of this process for
models of CHVCs. We have modeled the ionization structure of spherical clouds
(with and without dark matter halos) for a large range of densities and sizes,
appropriate to CHVCs over the range of suggested distances, exposed to the
extragalactic ionizing photon flux. Constant-density cloud models in which the
CHVCs are at Local Group distances have total (ionized plus neutral) gas masses
roughly 20-30 times larger than the neutral gas masses, implying that the gas
mass alone of the observed population of CHVCs is about 40 billion solar
masses. With a realistic (10:1) dark matter to gas mass ratio, the total mass
in such CHVCs is a significant fraction of the dynamical mass of the Local
Group, and their line widths would exceed the observed FWHM. Models with dark
matter halos fare even more poorly; they must lie within approximately 200 kpc
of the Galaxy. We show that exponential neutral hydrogen column density
profiles are a natural consequence of an external source of ionizing photons,
and argue that these profiles cannot be used to derive model-independent
distances to the CHVCs. These results argue strongly that the CHVCs are not
cosmological objects, and are instead associated with the Galactic halo.Comment: 30 pages, 14 figures; to appear in The Astrophysical Journa
Digital particle image velocimetry (DPIV) : systematic error analysis
Digital Particle Image Velocimetry (DPIV) is a flow diagnostic technique that is able to provide velocity measurements within a fluid whilst also offering flow visualisation during analysis. Whole field velocity measurements are calculated by using cross-correlation algorithms to process sequential images of flow tracer particles recorded using a laser-camera system. This technique is capable of calculating velocity fields in both two and three dimensions and is the most widely used whole field measurement technique in flow diagnostics. With the advent of time-resolved DPIV it is now possible to resolve the 3D spatio-temporal dynamics of turbulent and transient flows as they develop over time. Minimising the systematic and random errors associated with the cross-correlation of flow images is essential in providing accurate quantitative results for DPIV. This research has explored a variety of cross-correlation algorithms and techniques developed to increase the accuracy of DPIV measurements. It is shown that these methods are unable to suppress either the inherent errors associated with the random distribution of particle images within each interrogation region or the background noise of an image. This has been achieved through a combination of both theoretical modelling and experimental verification for a uniform particle image displacement. The study demonstrates that normalising the correlation field by the signal strength that contributes to each point of the correlation field suppresses both the mean bias and RMS error. A further enhancement to this routine has lead to the development of a robust cross-correlation algorithm that is able to suppress the systematic errors associated to the random distribution of particle images and background noise.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Gaseous Galaxy Halos
Galactic halo gas traces inflowing star formation fuel and feedback from a
galaxy's disk and is therefore crucial to our understanding of galaxy
evolution. In this review, we summarize the multi-wavelength observational
properties and origin models of Galactic and low redshift spiral galaxy halo
gas. Galactic halos contain multiphase gas flows that are dominated in mass by
the ionized component and extend to large radii. The densest, coldest halo gas
observed in neutral hydrogen (HI) is generally closest to the disk (< 20 kpc),
and absorption line results indicate warm and warm-hot diffuse halo gas is
present throughout a galaxy's halo. The hot halo gas detected is not a
significant fraction of a galaxy's baryons. The disk-halo interface is where
the multiphase flows are integrated into the star forming disk, and there is
evidence for both feedback and fueling at this interface from the temperature
and kinematic gradient of the gas and HI structures. The origin and fate of
halo gas is considered in the context of cosmological and idealized local
simulations. Accretion along cosmic filaments occurs in both a hot (> 10^5.5 K)
and cold mode in simulations, with the compressed material close to the disk
the coldest and densest, in agreement with observations. There is evidence in
halo gas observations for radiative and mechanical feedback mechanisms,
including escaping photons from the disk, supernova-driven winds, and a
galactic fountain. Satellite accretion also leaves behind abundant halo gas.
This satellite gas interacts with the existing halo medium, and much of this
gas will become part of the diffuse halo before it can reach the disk. The
accretion rate from cold and warm halo gas is generally below a galaxy disk's
star formation rate, but gas at the disk-halo interface and stellar feedback
may be important additional fuel sources.Comment: 50 pages, 9 figures (1 in 3D, view with a current version of Adobe),
to appear in ARA&A, 50, 49
The Relationship Between Baryons and Dark Matter in Extended Galaxy Halos
The relationship between gas-rich galaxies and Ly-alpha absorbers is
addressed in this paper in the context of the baryonic content of galaxy halos.
Deep Arecibo HI observations are presented of two gas-rich spiral galaxies
within 125 kpc projected distance of a Ly-alpha absorber at a similar velocity.
The galaxies investigated are close to edge-on and the absorbers lie almost
along their major axes, allowing for a comparison of the Ly-alpha absorber
velocities with galactic rotation. This comparison is used to examine whether
the absorbers are diffuse gas rotating with the galaxies' halos, outflow
material from the galaxies, or intergalactic gas in the low redshift cosmic
web. The results indicate that if the gas resides in the galaxies' halos it is
not rotating with the system and possibly counter-rotating. In addition, simple
geometry indicates the gas was not ejected from the galaxies and there are no
gas-rich satellites detected down to 3.6 - 7.5 x 10^6 Msun, or remnants of
satellites to 5-6 x 10^{18} cm^{-2}. The gas could potentially be infalling
from large radii, but the velocities and distances are rather high compared to
the high velocity clouds around the Milky Way. The most likely explanation is
the galaxies and absorbers are not directly associated, despite the vicinity of
the spiral galaxies to the absorbers (58-77 kpc from the HI edge). The spiral
galaxies reside in a filament of intergalactic gas, and the gas detected by the
absorber has not yet come into equilibrium with the galaxy. These results also
indicate that the massive, extended dark matter halos of spiral galaxies do not
commonly have an associated diffuse baryonic component at large radii.Comment: Accepted by AJ, 33 pages preprint format, see
http://www.astro.lsa.umich.edu/~mputman/putman1.pdf for a higher resolution
versio
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