1,608 research outputs found
An investigation of rooftop STOL port aerodynamics
An investigation into aerodynamic problems associated with large building rooftop STOLports was performed. Initially, a qualitative flow visualization study indicated two essential problems: (1) the establishment of smooth, steady, attached flow over the rooftop, and (2) the generation of acceptable crosswind profile once (1) has been achieved. This study indicated that (1) could be achieved by attaching circular-arc rounded edge extensions to the upper edges of the building and that crosswind profiles could be modified by the addition of porous vertical fences to the lateral edges of the rooftop. Important fence parameters associated with crosswind alteration were found to be solidity, fence element number and spacing. Large scale building induced velocity fluctuations were discovered for most configurations tested and a possible explanation for their occurrence was postulated. Finally, a simple equation relating fence solidity to the resulting velocity profile was developed and tested for non-uniform single element fences with 30 percent maximum solidity
The Millennium Galaxy Catalogue: the space density and surface brightness distribution(s) of galaxies
We recover the joint and individual space density and surface brightness
distribution(s) of galaxies from the Millennium Galaxy Catalogue. The MGC is a
local survey spanning 30.9 sq deg and probing approximately one--two mag/sq
arcsec deeper than either the Two-Degree Field Galaxy Redshift Survey (2dFGRS)
or the Sloan Digital Sky Survey (SDSS). The MGC contains 10,095 galaxies to
B_mgc < 20 mag with 96 per cent spectroscopic completeness. We implement a
joint luminosity-surface brightness step-wise maximum likelihood method to
recover the bivariate brightness distribution (BBD) inclusive of most selection
effects. Integrating the BBD over surface brightness we recover the following
Schechter function parameters: phi* = (0.0177 +/- 0.0015) h^3 Mpc^{-3}, M_{B}*
- 5 log h = (-19.60 +/- 0.04) mag and alpha =-1.13 +/- 0.02. Compared to the
2dFGRS (Norberg et al 2002) we find a consistent M* value but a slightly
flatter faint-end slope and a higher normalisation, resulting in a final
luminosity density j_{b_J} = (1.99 +/- 0.17) x 10^8 h L_{odot} Mpc^{-3}. The
MGC surface brightness distribution is a well bounded Gaussian at the M* point
with phi* = (3.5 +/- 0.1) x 10^{-2} h^3 Mpc^{-3}, mu^{e*} = (21.90 +/- 0.01)
mag/sq arcsec and sigma_{ln R_e} = 0.35 +/- 0.01. The characteristic surface
brightness for luminous systems is invariant to M_{B} - 5 log h ~ -19 mag
faintwards of which it moves to lower surface brightness. Higher resolution
(FWHM 26 mag/sq arcsec in the B-band)
observations of the local universe are now essential to probe to lower
luminosity and lower surface brightness levels. [abridged]Comment: Accepted for publication in MNRAS, 26 pages with 21 figures (some
degraded). A full pdf version, along with MGC data release, is available from
the MGC website at, http://www.eso.org/~jliske/mg
Time Evolution of Galaxy Formation and Bias in Cosmological Simulations
The clustering of galaxies relative to the mass distribution declines with
time because: first, nonlinear peaks become less rare events; second, the
densest regions stop forming new galaxies because gas there becomes too hot to
cool and collapse; third, after galaxies form, they are gravitationally
``debiased'' because their velocity field is the same as the dark matter. To
show these effects, we perform a hydrodynamic cosmological simulation and
examine the density field of recently formed galaxies as a function of
redshift. We find the bias b_* of recently formed galaxies (the ratio of the
rms fluctuations of these galaxies and mass), evolves from 4.5 at z=3 to around
1 at z=0, on 8 h^{-1} Mpc comoving scales. The correlation coefficient r_*
between recently formed galaxies and mass evolves from 0.9 at z=3 to 0.25 at
z=0. As gas in the universe heats up and prevents star formation, star-forming
galaxies become poorer tracers of the mass density field. After galaxies form,
the linear continuity equation is a good approximation to the gravitational
debiasing, even on nonlinear scales. The most interesting observational
consequence of the simulations is that the linear regression of the
star-formation density field on the galaxy density field evolves from about 0.9
at z=1 to 0.35 at z=0. These effects also provide a possible explanation for
the Butcher-Oemler effect, the excess of blue galaxies in clusters at redshift
z ~ 0.5. Finally, we examine cluster mass-to-light ratio estimates of Omega,
finding that while Omega(z) increases with z, one's estimate Omega_est(z)
decreases. (Abridged)Comment: 31 pages of text and figures; submitted to Ap
Cold fronts and multi-temperature structures in the core of Abell 2052
The physics of the coolest phases in the hot Intra-Cluster Medium (ICM) of
clusters of galaxies is yet to be fully unveiled. X-ray cavities blown by the
central Active Galactic Nucleus (AGN) contain enough energy to heat the
surrounding gas and stop cooling, but locally blobs or filaments of gas appear
to be able to cool to low temperatures of 10^4 K. In X-rays, however, gas with
temperatures lower than 0.5 keV is not observed. Using a deep XMM-Newton
observation of the cluster of galaxies Abell 2052, we derive 2D maps of the
temperature, entropy, and iron abundance in the core region. About 130 kpc
South-West of the central galaxy, we discover a discontinuity in the surface
brightness of the hot gas which is consistent with a cold front. Interestingly,
the iron abundance jumps from ~0.75 to ~0.5 across the front. In a smaller
region to the North-West of the central galaxy we find a relatively high
contribution of cool 0.5 keV gas, but no X-ray emitting gas is detected below
that temperature. However, the region appears to be associated with much cooler
H-alpha filaments in the optical waveband. The elliptical shape of the cold
front in the SW of the cluster suggests that the front is caused by sloshing of
the hot gas in the clusters gravitational potential. This effect is probably an
important mechanism to transport metals from the core region to the outer parts
of the cluster. The smooth temperature profile across the sharp jump in the
metalicity indicates the presence of heat conduction and the lack of mixing
across the discontinuity. The cool blob of gas NW of the central galaxy was
probably pushed away from the core and squeezed by the adjacent bubble, where
it can cool efficiently and relatively undisturbed by the AGN. Shock induced
mixing between the two phases may cause the 0.5 keV gas to cool non-radiatively
and explain our non-detection of gas below 0.5 keV.Comment: 11 pages, 9 figures, A&A, in pres
A List of Galaxies for Gravitational Wave Searches
We present a list of galaxies within 100 Mpc, which we call the Gravitational
Wave Galaxy Catalogue (GWGC), that is currently being used in follow-up
searches of electromagnetic counterparts from gravitational wave searches. Due
to the time constraints of rapid follow-up, a locally available catalogue of
reduced, homogenized data is required. To achieve this we used four existing
catalogues: an updated version of the Tully Nearby Galaxy Catalog, the Catalog
of Neighboring Galaxies, the V8k catalogue and HyperLEDA. The GWGC contains
information on sky position, distance, blue magnitude, major and minor
diameters, position angle, and galaxy type for 53,255 galaxies. Errors on these
quantities are either taken directly from the literature or estimated based on
our understanding of the uncertainties associated with the measurement method.
By using the PGC numbering system developed for HyperLEDA, the catalogue has a
reduced level of degeneracies compared to catalogues with a similar purpose and
is easily updated. We also include 150 Milky Way globular clusters. Finally, we
compare the GWGC to previously used catalogues, and find the GWGC to be more
complete within 100 Mpc due to our use of more up-to-date input catalogues and
the fact that we have not made a blue luminosity cut.Comment: Accepted for publication in Classical and Quantum Gravity, 13 pages,
7 figure
Variability of Surface Pigment Concentrations in the South Atlantic Bight
A 1âyear time sequence (November 1978 through October 1979) of surface pigment images from the South Atlantic Bight (SAB) was derived from the Nimbus 7 coastal zone color scanner. This data set is augmented with in situ observations of hydrographic parameters, freshwater discharge, sea level, coastal winds, and currents for the purpose of examining the coupling between physical processes and the spatial and temporal variability of the surface pigment fields. The SAB is divided into three regions: the east Florida shelf, the GeorgiaâSouth Carolina shelf and the Carolina Capes. Sixâmonth seasonal mean pigment fields and time series of mean values within subregions were generated. While the seasonal mean isopleths were closely oriented along isobaths, significant differences between seasons in each region were found to exist. These differences are explained by correlating the pigment time series with physical parameters and processes known to be important in the SAB. Specifically, summertime concentrations between Cape Romain and Cape Canaveral were greater than those in winter, but the opposite was true north of Cape Romain. It is suggested that during the abnormally high freshwater discharge in the winterâspring of 1979, Cape Romain and Cape Fear were the major sites of crossâshelf transport, while the crossâshelf exchange during the fall of 1979 occurred just north of Cape Canaveral. Finally, the alongshore band of high pigment concentrations increased in width throughout the year in the vicinity of Charleston, but near Jacksonville it exhibited a minimum width in the summer and a maximum width in the fall of 1979
Astrophysically Motivated Bulge-Disk Decompositions of SDSS Galaxies
We present a set of bulge-disk decompositions for a sample of 71,825 SDSS
main-sample galaxies in the redshift range 0.003<z<0.05. We have fit each
galaxy with either a de Vaucouleurs ('classical') or an exponential ('pseudo-')
bulge and an exponential disk. Two dimensional Sersic fits are performed when
the 2-component fits are not statistically significant or when the fits are
poor, even in the presence of high signal-to-noise. We study the robustness of
our 2-component fits by studying a bright subsample of galaxies and we study
the systematics of these fits with decreasing resolution and S/N. Only 30% of
our sample have been fit with two-component fits in which both components are
non-zero. The g-r and g-i colours of each component for the two-component
models are determined using linear templates derived from the r-band model. We
attempt a physical classification of types of fits into disk galaxies,
pseudo-bulges, classical bulges, and ellipticals. Our classification of
galaxies agrees well with previous large B+D decomposed samples. Using our
galaxy classifications, we find that Petrosian concentration is a good
indicator of B/T, while overall Sersic index is not. Additionally, we find that
the majority of green valley galaxies are bulge+disk galaxies. Furthermore, in
the transition from green to red B+D galaxies, the total galaxy colour is most
strongly correlated with the disk colour.Comment: 28 pages, 34 figures, MNRAS accepte
Hydrographic Variability of Southeastern United States Shelf and Slope Waters During the Genesis of Atlantic Lows Experiment: Winter 1986
Continental shelf waters are particularly responsive to winter storm events mainly because of their shallow depths. Those of the southeastern United States (the South Atlantic Bight (SAB)) are especially responsive because they are broad and shallow. Also, the Gulf Stream serves as a continual source of warm water at the outer boundary. Thus the SAB receives strong meteorological (wind stress and heat loss) and oceanographic (advective) forcing. During the Genesis of Atlantic Lows Experiment (GALE) the response of shelf waters to winter storm events and Gulf Stream forcing was observed. The mean conditions showed a mixed water column with areas of stratification near the coast and at the shelf break. The nearshore area was stratified only during weak offshore winds, and the shelf break area was stratified during southward winds with accompanying onshore Ekman flow. On the inner shelf, advective buoyancy flux was similar in value to heat flux buoyancy and the buoyancy equivalent of wind mixing. Over the shelf break the advective buoyancy flux was 4 times the other forms of buoyancy flux and controlled the observed potential energy variability. A simple box model heat budget used to separate the effect of Gulf Stream eddies and meanders, and Ekman flow and airâsea heat exchange on the shelf heat content showed that the observed heat content variability was caused by intrusion of Gulf Stream water. The intrusions may be caused either by onshore Ekman flow during southward winds or Gulf Stream meander events
Galaxy distribution and extreme value statistics
We consider the conditional galaxy density around each galaxy, and study its
fluctuations in the newest samples of the Sloan Digital Sky Survey Data Release
7. Over a large range of scales, both the average conditional density and its
variance show a nontrivial scaling behavior, which resembles to criticality.
The density depends, for 10 < r < 80 Mpc/h, only weakly (logarithmically) on
the system size. Correspondingly, we find that the density fluctuations follow
the Gumbel distribution of extreme value statistics. This distribution is
clearly distinguishable from a Gaussian distribution, which would arise for a
homogeneous spatial galaxy configuration. We also point out similarities
between the galaxy distribution and critical systems of statistical physics
- âŠ