265 research outputs found
The 1928 salary schedule of the City of Alameda and its effect upon the improvement of teachers in service
The improvement of the teaching staff is a problem that constantly confronts every school department. Writers of leading books on school administration consider this problem so important that they almost universally devote one or two chapters to its discussion. Dr. Ellwood F. Cubberley suggests that the addition of a few young, well-trained teachers to the staff each year is one way of improving the work of a whole department. In recent years, however, many school departments have been unable to add new strength to their school systems by the addition of new teachers, for the reason that there were no new positions to fill. Teacher Tenure laws on the one hand and a dropping off in school enrollments on the other have forced many departments to rely upon an almost static teaching force. In order to improve the teaching staff in such a system a second way must be found and that is by improving the teachers who are already in the system.
The salary schedule is one of the most potent instruments to bring about a desirable situation in regard to the training of teachers in service. Over fifty percent of the more than two billion dollars spent annual in the United States for education is devoted to teachers\u27 salaries. How this amount of money is to be spent and how the salary schedule is to operate becomes a very vital and significant educational problem
Partisanship and ideology are likely to shape how women will react to Hillary Clinton and Carly Fiorina’s candidacies
In Congress, the representation of women currently stands at around 20 percent – far lower than it should be. But how can we encourage more women to run for office? Past research shows that in the 1980s and early 90s, women running for national office inspired other women to get involved in politics, but this did not occur in 2008, despite Hillary Clinton and Sarah Palin’s presidential and vice-presidential runs. In new research which measures young women’s interest in political involvement, A. Lanethea Mathews-Schultz, Bryan W. Marshall, and Mack D. Mariani find that the extent to which young women see themselves as likely to participate in politics is now much more tied to partisanship and ideology
Dispersal of Galactic Magnetic Fields into Intracluster Space
Little is known about the origin and basic properties of magnetic fields in
clusters of galaxies. High conductivity in magnetized interstellar plasma
suggests that galactic magnetic fields are (at least partly) ejected into
intracluster (IC) space by the same processes that enrich IC gas with metals.
We explore the dispersal of galactic fields by hydrodynamical simulations with
our new {\em Enzo-Galcon} code, which is capable of tracking a large number
galaxies during cluster assembly, and modeling the processes that disperse
their interstellar media. Doing so we are able to describe the evolution of the
mean strength of the field and its profile across the cluster. With the known
density profile of dispersed gas and an estimated range of coherence scales, we
predict the spatial distribution of Faraday rotation measure and find it to be
consistent with observational data
A Census of Baryons and Dark Matter in an Isolated, Milky Way-sized Elliptical Galaxy
We present a study of the dark and luminous matter in the isolated elliptical
galaxy NGC720, based on deep X-ray observations made with Chandra and Suzaku.
The gas is reliably measured to ~R2500, allowing us to place good constraints
on the enclosed mass and baryon fraction (fb) within this radius
(M2500=1.6e12+/-0.2e12 Msun, fb(2500)=0.10+/-0.01; systematic errors are
<~20%). The data indicate that the hot gas is close to hydrostatic, which is
supported by good agreement with a kinematical analysis of the dwarf satellite
galaxies. We confirm a dark matter (DM) halo at ~20-sigma. Assuming an NFW DM
profile, our physical model for the gas distribution enables us to obtain
meaningful constraints at scales larger than R2500, revealing that most of the
baryons are in the hot gas. We find that fb within Rvir is consistent with the
Cosmological value, confirming theoretical predictions that a ~Milky Way-mass
(Mvir=3.1e12+/-0.4e12 Msun) galaxy can sustain a massive, quasi-hydrostatic gas
halo. While fb is higher than the cold baryon fraction typically measured in
similar-mass spiral galaxies, both the gas fraction (fg) and fb in NGC720 are
consistent with an extrapolation of the trends with mass seen in massive galaxy
groups and clusters. After correcting for fg, the entropy profile is close to
the self-similar prediction of gravitational structure formation simulations,
as observed in galaxy clusters. Finally, we find a strong heavy metal abundance
gradient in the ISM similar to those observed in massive galaxy groups.Comment: 23 pages, 13 figures, 4 tables. Accepted for publication in the
Astrophysical Journal. Minor modifications to match accepted version.
Conclusions unchange
The ELIXR Galaxy Survey. II: Baryons and Dark Matter in an Isolated Elliptical Galaxy
The Elliptical Isolated X-ray (ElIXr) Galaxy Survey is a volume-limited
(<110Mpc) study of optically selected, isolated, Lstar elliptical galaxies, to
provide an X-ray census of galaxy-scale (virial mass, Mvir < 1e13 Msun)
objects, and identify candidates for detailed hydrostatic mass modelling. In
this paper, we present a Chandra and XMM study of one such candidate, NGC1521,
and constrain its distribution of dark and baryonic matter. We find a
morphologically relaxed hot gas halo, extending almost to R500, that is well
described by hydrostatic models similar to the benchmark, baryonically closed,
Milky Way-mass elliptical galaxy NGC720. We obtain good constraints on the
enclosed gravitating mass (M500=3.8e12+/-1e12 Msun, slightly higher than
NGC\thin 720), and baryon fraction (fb500=0.13+/-0.03). We confirm at 8.2-sigma
the presence of a dark matter (DM) halo consistent with LCDM. Assuming a
Navarro-Frenk-White DM profile, our self-consistent, physical model enables
meaningful constraints beyond R500, revealing that most of the baryons are in
the hot gas. Within the virial radius, fb is consistent with the Cosmic mean,
suggesting that the predicted massive, quasi-hydrostatic gas halos may be more
common than previously thought. We confirm that the DM and stars conspire to
produce an approximately powerlaw total mass profile (rho \propto r^-alpha)
that follows the recently discovered scaling relation between alpha and optical
effective radius. Our conclusions are insensitive to modest, observationally
motivated, deviations from hydrostatic equilibrium. Finally, after correcting
for the enclosed gas fraction, the entropy profile is close to the self-similar
prediction of gravitational structure formation simulations, as observed in
massive galaxy clusters.Comment: Accepted for publication in the Astrophysical Journal. Minor
modifications to match accepted version. Conclusions unchanged. 18 pages, 11
figures and 3 table
Self-similar scaling and evolution in the galaxy cluster X-ray Luminosity-Temperature relation
We investigate the form and evolution of the X-ray luminosity-temperature
(LT) relation of a sample of 114 galaxy clusters observed with Chandra at
0.1<z<1.3. The clusters were divided into subsamples based on their X-ray
morphology or whether they host strong cool cores. We find that when the core
regions are excluded, the most relaxed clusters (or those with the strongest
cool cores) follow an LT relation with a slope that agrees well with simple
self-similar expectations. This is supported by an analysis of the gas density
profiles of the systems, which shows self-similar behaviour of the gas profiles
of the relaxed clusters outside the core regions. By comparing our data with
clusters in the REXCESS sample, which extends to lower masses, we find evidence
that the self-similar behaviour of even the most relaxed clusters breaks at
around 3.5keV. By contrast, the LT slopes of the subsamples of unrelaxed
systems (or those without strong cool cores) are significantly steeper than the
self-similar model, with lower mass systems appearing less luminous and higher
mass systems appearing more luminous than the self-similar relation. We argue
that these results are consistent with a model of non-gravitational energy
input in clusters that combines central heating with entropy enhancements from
merger shocks. Such enhancements could extend the impact of central energy
input to larger radii in unrelaxed clusters, as suggested by our data. We also
examine the evolution of the LT relation, and find that while the data appear
inconsistent with simple self-similar evolution, the differences can be
plausibly explained by selection bias, and thus we find no reason to rule out
self-similar evolution. We show that the fraction of cool core clusters in our
(non-representative) sample decreases at z>0.5 and discuss the effect of this
on measurements of the evolution in the LT relation.Comment: 21 pages, 15 figures. Submitted to MNRAS. Comments welcom
2-D Magnetohydrodynamic Simulations of Induced Plasma Dynamics in the Near-Core Region of a Galaxy Cluster
We present results from numerical simulations of the cooling-core cluster
A2199 produced by the two-dimensional (2-D) resistive magnetohydrodynamics
(MHD) code MACH2. In our simulations we explore the effect of anisotropic
thermal conduction on the energy balance of the system. The results from
idealized cases in 2-D axisymmetric geometry underscore the importance of the
initial plasma density in ICM simulations, especially the near-core values
since the radiation cooling rate is proportional to . Heat conduction
is found to be non-effective in preventing catastrophic cooling in this
cluster. In addition we performed 2-D planar MHD simulations starting from
initial conditions deliberately violating both thermal balance and hydrostatic
equilibrium in the ICM, to assess contributions of the convective terms in the
energy balance of the system against anisotropic thermal conduction. We find
that in this case work done by the pressure on the plasma can dominate the
early evolution of the internal energy over anisotropic thermal conduction in
the presence of subsonic flows, thereby reducing the impact of the magnetic
field. Deviations from hydrostatic equilibrium near the cluster core may be
associated with transient activity of a central active galactic nucleus and/or
remnant dynamical activity in the ICM and warrant further study in three
dimensions.Comment: 16 pages, 13 figures, accepted for publication in MNRA
The gas distribution in the outer regions of galaxy clusters
We present the analysis of a local (z = 0.04 - 0.2) sample of 31 galaxy
clusters with the aim of measuring the density of the X-ray emitting gas in
cluster outskirts. We compare our results with numerical simulations to set
constraints on the azimuthal symmetry and gas clumping in the outer regions of
galaxy clusters. We exploit the large field-of-view and low instrumental
background of ROSAT/PSPC to trace the density of the intracluster gas out to
the virial radius. We perform a stacking of the density profiles to detect a
signal beyond r200 and measure the typical density and scatter in cluster
outskirts. We also compute the azimuthal scatter of the profiles with respect
to the mean value to look for deviations from spherical symmetry. Finally, we
compare our average density and scatter profiles with the results of numerical
simulations. As opposed to some recent Suzaku results, and confirming previous
evidence from ROSAT and Chandra, we observe a steepening of the density
profiles beyond \sim r500. Comparing our density profiles with simulations, we
find that non-radiative runs predict too steep density profiles, whereas runs
including additional physics and/or treating gas clumping are in better
agreement with the observed gas distribution. We report for the first time the
high-confidence detection of a systematic difference between cool-core and
non-cool core clusters beyond \sim 0.3r200, which we explain by a different
distribution of the gas in the two classes. Beyond \sim r500, galaxy clusters
deviate significantly from spherical symmetry, with only little differences
between relaxed and disturbed systems. We find good agreement between the
observed and predicted scatter profiles, but only when the 1% densest clumps
are filtered out in the simulations. [Abridged]Comment: The data for the average profiles and individual clusters can be
downloaded at:
http://www.isdc.unige.ch/~deckert/newsite/The_Planck_ROSAT_project.htm
Gravitational Quenching in Massive Galaxies and Clusters by Clumpy Accretion
We consider a simple gravitational-heating mechanism for the long-term
quenching of cooling flows and star formation in massive dark-matter haloes
hosting ellipticals and clusters. The virial shock heating in haloes >10^12 Mo
triggers quenching in 10^12-13 Mo haloes (Birnboim, Dekel & Neistein 2007). We
show that the long-term quenching in haloes >Mmin~7x10^12 Mo could be due to
the gravitational energy of cosmological accretion delivered to the inner-halo
hot gas by cold flows via ram-pressure drag and local shocks. Mmin is obtained
by comparing the gravitational power of infall into the potential well with the
overall radiative cooling rate. The heating wins if the gas inner density cusp
is not steeper than r^-0.5 and if the masses in the cold and hot phases are
comparable. The effect is stronger at higher redshifts, making the maintenance
easier also at later times. Clumps >10^5 Mo penetrate to the inner halo with
sufficient kinetic energy before they disintegrate, but they have to be <10^8
Mo for the drag to do enough work in a Hubble time. Pressure confined ~10^4K
clumps are stable against their own gravity and remain gaseous once below the
Bonnor-Ebert mass ~10^8 Mo. They are also immune to tidal disruption. Clumps in
the desired mass range could emerge by thermal instability in the outer halo if
the conductivity is not too high. Alternatively, such clumps may be embedded in
dark-matter subhaloes if the ionizing flux is ineffective, but they separate
from their subhaloes by ram pressure before entering the inner halo. Heating by
dynamical friction becomes dominant for massive satellites, which can
contribute up to one third of the total gravitational heating. We conclude that
gravitational heating by cosmological accretion is a viable alternative to AGN
feedback as a long-term quenching mechanism.Comment: 24 pages, 20 figures, some improvements, MNRAS accepted versio
Feedback from galactic stellar bulges and hot gaseous haloes of galaxies
We demonstrate that the feedback from stellar bulges can play an essential
role in shaping the halo gas of galaxies with substantial bulge components by
conducting 1-D hydrodynamical simulations. The feedback model we consider
consists of two distinct phases: 1) an early starburst during the bulge
formation and 2) a subsequent long-lasting mass and energy injection from
stellar winds of low-mass stars and Type Ia SNe. An energetic outward blastwave
is initiated by the starburst and is maintained and enhanced by the
long-lasting stellar feedback. For a MW-like galactic bulge, this blastwave
sweeps up the halo gas in the proto-galaxy and heats up the surrounding medium
to a scale much beyond the virial radius of the halo, thus the accretion of the
halo hot gas can be completely stopped. In addition, the long-lasting feedback
in the later phase powers a galactic bulge wind that is reverse-shocked at a
large radius in the presence of surrounding intergalactic medium and hence
maintains a hot gaseous halo. As the mass and energy injection decreases with
time, the feedback evolves to a subsonic and quasi-stable outflow, which is
enough to prevent halo gas from cooling. The two phases of the feedback thus
re-enforce each-other's impact on the gas dynamics. The simulation results
demonstrate that the stellar bulge feedback may provide a plausible solution to
the long-standing problems in understanding the MW type galaxies, such as the
"missing stellar feedback" problem and the "over-cooling" problem. The
simulations also show that the properties of the hot gas in the subsonic
outflow state depend sensitively on the environment and the formation history
of the bulge. This dependence and variance may explain the large dispersion in
the X-ray to B-band luminosity ratio of the low Es.Comment: v2, discussions added, accepted for publication in MNRA
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