49,827 research outputs found
Collapse of an initially spherical vapour cavity in the neighbourhood of a solid boundary
Vapour bubble collapse problems lacking spherical symmetry are solved here using a numerical method designed especially for these problems. Viscosity and compressibility in the liquid are neglected. Two specific cases of initially spherical bubbles collapsing near a plane solid wall were simulated: a bubble initially in contact with the wall, and a bubble initially half its radius from the wall at the closest point. It is shown that the bubble develops a jet directed towards the wall rather early in the collapse history. Free surface shapes and velocities are presented at various stages in the collapse. Velocities are scaled like (Δp/ρ)^½ where ρ is the density of the liquid and Δp is the constant difference between the ambient liquid pressure and the pressure in the cavity. For Δp/ρ=10^6cm^2/sec^2 ≈ 1 atm/density of water
the jet had a speed of about 130m/sec in the first case and 170m/sec in the second when it struck the opposite side of the bubble. Such jet velocities are of a magnitude which can explain cavitation damage. The jet develops so early in the bubble collapse history that compressibility effects in the liquid and the vapour are not important
Collapse of an initially spherical vapor cavity in the neighborhood of a solid boundary
Vapor bubble collapse problems lacking spherical symmetry are solved here using a numerical method designed especially for these problems. Viscosity and compressibility in the liquid are neglected. The method uses finite time steps and features an iterative technique for applying the boundary conditions at infinity directly to the liquid at a finite distance from the free surface. Two specific cases of initially spherical bubbles collapsing near a plane solid wall were simulated: a bubble initially in contact with the wall, and a bubble initially half its radius from the wall at the closest point. It is shown that the bubble develops a jet directed towards the wall rather early in the collapse history. Free surface shapes and velocities are presented at various stages in the collapse. Velocities are scaled like (Δp/ρ)^1/2 where ρ is the density of the liquid and Δp is the constant difference between the ambient liquid pressure and the pressure in the cavity. For Δp/ρ = 10^6 (cm/sec)^2 ~ 1 atm./density of water the jet had a speed of about 130 m/sec in the first case and 170 m/sec in the second when it struck the opposite side of the bubble. Such jet velocities are of a magnitude which can explain cavitation damage. The jet develops so early in the bubble collapse history that compressibility effects in the liquid and the vapor are not important
Demographics by depth: spatially explicit life-history dynamics of a protogynous reef fish
Distribution and demographics of the hogfish (Lachnolaimus
maximus) were investigated by using a combined approach of in situ observations and life history analyses. Presence, density, size, age, and size and age at sex change all varied with depth in the eastern Gulf of Mexico. Hogfish (64–774 mm fork length and 0–19 years old) were observed year-round and were most common over complex, natural hard bottom habitat. As depth increased, the presence and density of hogfish decreased, but mean size and age increased. Size at age was smaller nearshore (<30 m). Length and age at sex change of nearshore hogfish were half those of offshore hogfish and were coincident with the minimum legal size limit. Fishing pressure is presumably
greater nearshore and presents a confounding source of increased mortality; however, a strong red tide occurred the year before this study began and likely also affected nearshore demographics. Nevertheless, these data indicate ontogenetic migration and escapement of fast-growing fish to offshore habitat, both of which should reduce the likelihood of fishing-induced evolution. Data regarding
the hogfish fishery are limited and regionally dependent, which has confounded previous stock assessments; however, the spatially explicit vital rates reported herein can be applied to future monitoring efforts
Union Maids: Unions and the Female Workforce
How have women fared in unions in recent years? The major findings of this paper are that unions have been more beneficial for women in the public sector than in the private sector, and that unionism for women is primarily a public sector wriite collar phenomenon distinguished from that of males. According to our analysis:(1) Women have come to be an increasingly large proportion of the unionized work force, and are critical in the one area in which unions have recently succeeded --the public sector.(2) In the public sector and in white collar occupations where women unionists are concentrated, unions raise women's wages more than they raise the wages of men.(3) In the private sector unions have essentially the same effect on women in wages, turnover, employment and so forth, and do not deter affirmative action programs to raise female employment. (4) Comparable worth presents a rare confluence of interests of unions in search of members, particularly in the public sector,and women in search of higher wages, and will likely continue to be used by both especially within the confines of collective bargaining.
Velocity Dispersions and Dynamical Masses for a Large Sample of Quiescent Galaxies at z > 1: Improved Measures of the Growth in Mass and Size
We present Keck LRIS spectroscopy for a sample of 103 massive galaxies with
redshifts 0.9 < z < 1.6. Of these, 56 are quiescent with high signal-to-noise
absorption line spectra, enabling us to determine robust stellar velocity
dispersions for the largest sample yet available beyond a redshift of 1.
Together with effective radii measured from deep Hubble Space Telescope images,
we calculate dynamical masses and address key questions relating to the
puzzling size growth of quiescent galaxies over 0 < z < 2. We examine the
relationship between stellar and dynamical masses at high redshift, finding
that it closely follows that determined locally. We also confirm the utility of
the locally-established empirical calibration which enables high-redshift
velocity dispersions to be estimated photometrically, and we determine its
accuracy to be 35%. To address recent suggestions that progenitor bias - the
continued arrival of recently-quenched larger galaxies - can largely explain
the size evolution of quiescent galaxies, we examine the growth at fixed
velocity dispersion assuming this quantity is largely unaffected by the merger
history. We demonstrate that significant size and mass growth have clearly
occurred in individual systems. Parameterizing the relation between mass and
size growth over 0 < z < 1.6 as R \propto M^alpha, we find alpha = 1.6 +- 0.3,
in agreement with theoretical expectations from simulations of minor mergers.
Relaxing the assumption that the velocity dispersion is unchanging, we examine
growth assuming a constant ranking in galaxy velocity dispersion. This approach
is applicable only to the large-dispersion tail of the distribution, but yields
a consistent growth rate of alpha = 1.4 +- 0.2. Both methods confirm that
progenitor bias alone is insufficient to explain our new observations and that
quiescent galaxies have grown in both size and stellar mass over 0 < z < 1.6.Comment: Updated to match the published versio
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. II - Star Formation Histories and Galaxy Quenching
We investigate the stellar populations for a sample of 24 quiescent galaxies
at 1.5 < z < 2.5 using deep rest-frame optical spectra obtained with Keck
MOSFIRE. By fitting templates simultaneously to the spectroscopic and
photometric data, and exploring a variety of star formation histories, we
obtain robust measurements of median stellar ages and residual levels of star
formation. After subtracting the stellar templates, the stacked spectrum
reveals the Halpha and [NII] emission lines, providing an upper limit on the
ongoing star formation rate of 0.9 +/- 0.1 Msun/yr. By combining the MOSFIRE
data to our sample of Keck LRIS spectra at lower redshift, we analyze in a
consistent manner the quiescent population at 1 < z < 2.5. We find a tight
relation (with a scatter of 0.13 dex) between the stellar age and the
rest-frame U-V and V-J colors, which can be used to estimate the age of
quiescent galaxies given their colors. Applying this age--color relation to
large, photometric samples, we are able to model the number density evolution
for quiescent galaxies of various ages. We find evidence for two distinct
quenching paths: a fast quenching that produces compact post-starburst systems,
and a slow quenching of larger galaxies. Fast quenching accounts for about a
fifth of the growth of the red sequence at z~1.4, and half at z~2.2. We
conclude that fast quenching is triggered by dramatic events such as gas-rich
mergers, while slow quenching is likely caused by a different physical
mechanism.Comment: 28 pages, 15 figures, accepted in Ap
MOSFIRE Spectroscopy of Quiescent Galaxies at 1.5 < z < 2.5. I - Evolution of Structural and Dynamical Properties
We present deep near-infrared spectra for a sample of 24 quiescent galaxies
in the redshift range 1.5 < z < 2.5 obtained with the MOSFIRE spectrograph at
the W. M. Keck Observatory. In conjunction with a similar dataset we obtained
in the range 1 < z < 1.5 with the LRIS spectrograph, we analyze the kinematic
and structural properties for 80 quiescent galaxies, the largest
homogeneously-selected sample to date spanning 3 Gyr of early cosmic history.
Analysis of our Keck spectra together with measurements derived from associated
HST images reveals increasingly larger stellar velocity dispersions and smaller
sizes to redshifts beyond z~2. By classifying our sample according to Sersic
indices, we find that among disk-like systems the flatter ones show a higher
dynamical to stellar mass ratio compared to their rounder counterparts which we
interpret as evidence for a significant contribution of rotational motion. For
this subset of disk-like systems, we estimate that V/sigma, the ratio of the
circular velocity to the intrinsic velocity dispersion, is a factor of two
larger than for present-day disky quiescent galaxies. We use the velocity
dispersion measurements also to explore the redshift evolution of the dynamical
to stellar mass ratio, and to measure for the first time the physical size
growth rate of individual systems over two distinct redshift ranges, finding a
faster evolution at earlier times. We discuss the physical origin of this
time-dependent growth in size in the context of the associated reduction of the
systematic rotation.Comment: Updated to match the published versio
Discovery of a Strongly Lensed Massive Quiescent Galaxy at z=2.636: Spatially Resolved Spectroscopy and Indications of Rotation
We report the discovery of RG1M0150, a massive, recently quenched galaxy at
z=2.636 that is multiply imaged by the cluster MACSJ0150.3-1005. We derive a
stellar mass of log M_*=11.49+0.10-0.16 and a half-light radius of R_e,maj
=1.8+-0.4 kpc. Taking advantage of the lensing magnification, we are able to
spatially resolve a remarkably massive yet compact quiescent galaxy at z>2 in
ground-based near-infrared spectroscopic observations using Magellan/FIRE and
Keck/MOSFIRE. We find no gradient in the strength of the Balmer absorption
lines over 0.6 R_e - 1.6 R_e, which are consistent with an age of 760 Myr. Gas
emission in [NII] broadly traces the spatial distribution of the stars and is
coupled with weak Halpha emission (log [NII]/Halpha = 0.6+-0.2), indicating
that OB stars are not the primary ionizing source. The velocity dispersion
within the effective radius is sigma_e = 271+-41 km/s. We detect rotation in
the stellar absorption lines for the first time beyond z~1. Using a
two-integral Jeans model that accounts for observational effects, we measure a
dynamical mass of log M_dyn =11.24+-0.14 and V/sigma=0.70+-0.21. This is a high
degree of rotation considering the modest observed ellipticity of 0.12+-0.08,
but it is consistent with predictions from dissipational merger simulations
that produce compact remnants. The mass of RG1M0150 implies that it is likely
to become a slowly rotating elliptical. If it is typical, this suggests that
the progenitors of massive ellipticals retain significant net angular momentum
after quenching which later declines, perhaps through accretion of satellites.Comment: Accepted to ApJ Letters; updated to include revisions from the
referee process, including an improved Fig.
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