697 research outputs found
The Shape, Multiplicity, and Evolution of Superclusters in LambdaCDM Cosmology
We determine the shape, multiplicity, size, and radial structure of
superclusters in the LambdaCDM concordance cosmology from z = 0 to z = 2.
Superclusters are defined as clusters of clusters in our large-scale
cosmological simulation. We find that superclusters are triaxial in shape; many
have flattened since early times to become nearly two-dimensional structures at
present, with a small fraction of filamentary systems. The size and
multiplicity functions are presented at different redshifts. Supercluster sizes
extend to scales of ~ 100 - 200 Mpc/h. The supercluster multiplicity (richness)
increases linearly with supercluster size. The density profile in superclusters
is approximately isothermal (~ R^{-2}) and steepens on larger scales. These
results can be used as a new test of the current cosmology when compared with
upcoming observations of large-scale surveys.Comment: 33 pages, 15 figures, accepted to ApJ; minor content changes, some
figures removed to shorten pape
The small scatter in BH-host correlations and the case for self-regulated BH growth
Supermassive black holes (BHs) obey tight scaling relations between their mass and host galaxy properties such as total stellar mass, velocity dispersion and potential well depth. This has led to the development of self-regulated models for BH growth, in which feedback from the central BH halts its own growth upon reaching a critical threshold. However, models have also been proposed in which feedback plays no role: so long as a fixed fraction of the host gas supply is accreted, relations like those observed can be reproduced. Here, we argue that the scatter in the observed BHâhost correlations presents a demanding constraint on any model for these correlations, and that it favours self-regulated models of BH growth. We show that the scatter in the stellar mass fraction within a radius R in observed ellipticals and spheroids increases strongly at small R. At a fixed total stellar mass (or host velocity dispersion), on very small scales near the BH radius of influence, there is an order-of-magnitude scatter in the amount of gas that must have entered and formed stars. In short, the BH appears to âknow moreâ about the global host galaxy potential on large scales than the stars and gas supply on small scales. This is predicted in self-regulated models; however, models where there is no feedback would generically predict order-of-magnitude scatter in the BHâhost correlations. Likewise, models in which the BH feedback in the âbrightâ mode does not regulate the growth of the BH itself, but sets the stellar mass of the galaxy by inducing star formation or blowing out a mass in gas much larger than the galaxy stellar mass, are difficult to reconcile with the scatter on small scales
Relation of carbon dioxide in flue gases to the efficiency of a boiler
This thesis was undertaken to demonstrate the practical utility of measuring the efficiency of a Heine Boiler, installed in the Power Plant at the Missouri School of Mines, by the amount of COâ in the flue gases. The tests were made on a new boiler which had recently been installed in the Power Plant. The boiler and setting being new, the tests should show a high percentage of COâ in the flue gases and a high evaporative efficiency by the boiler --page 3
The creation and persistence of a misaligned gas disc in a simulated early-type galaxy
Massive early-type galaxies commonly have gas discs which are kinematically
misaligned with the stellar component. These discs feel a torque from the stars
and the angular momentum vectors are expected to align quickly. We present
results on the evolution of a misaligned gas disc in a cosmological simulation
of a massive early-type galaxy from the Feedback In Realistic Environments
project. This galaxy experiences a merger which, together with a strong
galactic wind, removes most of the original gas disc. The galaxy subsequently
reforms a gas disc through accretion of cold gas, but it is initially 120
degrees misaligned with the stellar rotation axis. This misalignment persists
for about 2 Gyr before the gas-star misalignment angle drops below 20 degrees.
The time it takes for the gaseous and stellar components to align is much
longer than previously thought, because the gas disc is accreting a significant
amount of mass for about 1.5 Gyr after the merger, during which the angular
momentum change induced by accreted gas dominates over that induced by stellar
torques. Once the gas accretion rate has decreased sufficiently, the gas disc
decouples from the surrounding halo gas and realigns with the stellar component
in about 6 dynamical times. During the late evolution of the misaligned gas
disc, the centre aligns faster than the outskirts, resulting in a warped disc.
We discuss the observational consequences of the long survival of our
misaligned gas disc and how our results can be used to calibrate merger rate
estimates from observed gas misalignments.Comment: 10 pages, 7 figures. Accepted for publication in MNRAS. Revised
version: minor changes. A movie of the evolution of the gas disc can be
viewed at http://astro.berkeley.edu/~freeke/misalign.htm
How Much Mass do Supermassive Black Holes Eat in their Old Age?
We consider the distribution of local supermassive black hole Eddington
ratios and accretion rates, accounting for the dependence of radiative
efficiency and bolometric corrections on the accretion rate. We find that black
hole mass growth, both of the integrated mass density and the masses of most
individual objects, must be dominated by an earlier, radiatively efficient,
high accretion rate stage, and not by the radiatively inefficient low accretion
rate phase in which most local supermassive black holes are currently observed.
This conclusion is particularly true of supermassive black holes in elliptical
host galaxies, as expected if they have undergone merger activity in the past
which would fuel quasar activity and rapid growth. We discuss models of the
time evolution of accretion rates and show that they all predict significant
mass growth in a prior radiatively efficient state. The only way to avoid this
conclusion is through careful fine-tuning of the accretion/quasar timescale to
a value that is inconsistent with observations. Our results agree with a wide
range of observational inferences drawn from the quasar luminosity function and
X-ray background synthesis models, but our approach has the virtue of being
independent of the modeling of source populations. Models in which black holes
spend the great majority of their time in low accretion rate phases are thus
completely consistent both with observations implying mass gain in relatively
short, high accretion rate phases and with the local distribution of accretion
rates.Comment: 11 pages, 4 figures, matches version accepted to Ap
A Physical Model for the Origin of Quasar Lifetimes
We propose a model of quasar lifetimes in which observational quasar
lifetimes and an intrinsic lifetime of rapid accretion are strongly
distinguished by the physics of obscuration by surrounding gas and dust.
Quasars are powered by gas funneled to galaxy centers, but for a large part of
the accretion lifetime are heavily obscured by the large gas densities powering
accretion. In this phase, starbursts and black hole growth are fueled but the
quasar is buried. Eventually, feedback from accretion energy disperses
surrounding gas, creating a window in which the black hole is observable
optically as a quasar, until accretion rates drop below those required to
maintain a quasar luminosity. We model this process and measure the unobscured
and intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxy
merger. The source luminosity is determined from the black hole accretion rate,
calculated from local gas properties. We calculate the column density of
hydrogen to the source along multiple lines of sight and use these column
densities and gas metallicities to determine B-band attenuation of the source.
Defining the observable quasar lifetime as the total time with an observed
B-band luminosity above some limit L_B,min, we find lifetimes ~10-20 Myr for
L_B,min=10^11 L_sun (M_B=-23), in good agreement with observationally
determined quasar lifetimes. This is significantly smaller than the intrinsic
lifetime ~100 Myr obtained if attenuation is neglected. The ratio of observed
to intrinsic lifetime is also strong function of both the limiting luminosity
and the observed frequency.Comment: 5 pages, 4 figures, submitted to ApJ Letter
A Theoretical Interpretation of the Black Hole Fundamental Plane
We examine the origin and evolution of correlations between properties of
supermassive black holes (BHs) and their host galaxies using simulations of
major galaxy mergers, including the effects of gas dissipation, cooling, star
formation, and BH accretion and feedback. We demonstrate that the simulations
predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH
sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH
M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found
observationally. The simulations indicate that the BHFP can be understood
roughly as a tilted intrinsic correlation between BH mass and spheroid binding
energy, or the condition for feedback coupling to power a pressure-driven
outflow. While changes in halo circular velocity, merger orbital parameters,
progenitor disk redshifts and gas fractions, ISM gas pressurization, and other
parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore
changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given
the empirical trend of decreasing R_e for a given M_bulge at high redshift, the
BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement
with recent observations. This evolution in the structural properties of merger
remnants, to smaller R_e and larger sigma (and therefore larger M_BH,
conserving the BHFP) at a given M_bulge, is driven by the fact that bulge
progenitors have characteristically larger gas fractions at high redshifts.
Adopting the observed evolution of disk gas fractions with redshift, our
simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ.
Companion paper to arXiv:0707.400
Live Fast, Die Young: GMC lifetimes in the FIRE cosmological simulations of Milky Way-mass galaxies
We present the first measurement of the lifetimes of giant molecular clouds (GMCs) in cosmological simulations at z = 0, using the Latte suite of FIRE-2 simulations of Milky Way (MW) mass galaxies. We track GMCs with total gas mass âł10â” Mâ at high spatial (âŒ1 pc), mass (7100 Mâ), and temporal (1 Myr) resolution. Our simulated GMCs are consistent with the distribution of masses for massive GMCs in the MW and nearby galaxies. We find GMC lifetimes of 5â7 Myr, or 1â2 freefall times, on average, with less than 2 perâcent of clouds living longer than 20 Myr. We find decreasing GMC lifetimes with increasing virial parameter, and weakly increasing GMC lifetimes with galactocentric radius, implying that environment affects the evolutionary cycle of GMCs. However, our GMC lifetimes show no systematic dependence on GMC mass or amount of star formation. These results are broadly consistent with inferences from the literature and provide an initial investigation into ultimately understanding the physical processes that govern GMC lifetimes in a cosmological setting
A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies: II. Formation of Red Ellipticals
(Abridged) We develop and test a model for the cosmological role of mergers
in the formation and quenching of red, early-type galaxies. Making the ansatz
that star formation is quenched after a gas-rich, spheroid-forming major
merger, we demonstrate that this naturally predicts the turnover in the
efficiency of star formation at ~L_star, as well as the observed mass
functions/density of red galaxies as a function of redshift, the formation
times of spheroids as a function of mass, and the fraction of quenched galaxies
as a function of galaxy and halo mass, environment, and redshift. Comparing to
a variety of semi-analytic models in which quenching is primarily driven by
halo mass considerations or secular/disk instabilities, we demonstrate that our
model and different broad classes of models make unique and robust qualitative
predictions for a number of observables, including the red fraction as a
function of galaxy and halo mass, the density of passive galaxies and evolution
of the color-morphology-density relations at high z, and the fraction of
disky/boxy spheroids as a function of mass. In each case, the observations
favor a model in which galaxies quench after a major merger builds a massive
spheroid, and disfavor quenching via secular or pure halo processes. We discuss
a variety of physical possibilities for this quenching, and propose a mixed
scenario in which traditional quenching in hot, massive halos is supplemented
by the feedback associated with star formation and quasar activity in a major
merger, which temporarily suppress cooling and establish the conditions of a
dynamically hot halo in the central regions of the host, even in low mass
halos.Comment: 29 pages, 21 figures, submitted to ApJ. Replacement fixes comparison
of models in Figures 6 &
Blood pressure and cardiac autonomic adaptations to isometric exercise training: A randomized shamâcontrolled study
Isometric exercise training (IET) is increasingly cited for its role in reducing resting blood pressure (BP). Despite this, few studies have investigated a potential sham effect attributing to the success of IET, thus dictating the aim of the present study. Thirty physically inactive males (n = 15) and females (n = 15) were randomly assigned into three groups. The IET group completed a wall squat intervention at 95% peak heart rate (HR) using a prescribed knee joint angle. The sham group performed a parallel intervention, but at an intensity (<75% peak HR) previously identified to be inefficacious over a 4-week training period. No-intervention controls maintained their normal daily activities. Pre- and post-measures were taken for resting and continuous blood pressure and cardiac autonomic modulation. Resting clinic and continuous beat-to-beat systolic (â15.2 ± 9.2 and â7.3 ± 5.6 mmHg), diastolic (â4.6 ± 5 and â4.5 ± 5.1), and mean (â7 ± 4.2 and â7.5 ± 5.3) BP, respectively, all significantly decreased in the IET group compared to sham and no-intervention control. The IET group observed a significant decrease in low-frequency normalized units of heart rate variability concurrent with a significant increase in high-frequency normalized units of heart rate variability compared to both the sham and no-intervention control groups. The findings of the present study reject a nonspecific effect and further support the role of IET as an effective antihypertensive intervention.
Clinical Trials ID: NCT05025202
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