2,484 research outputs found
A cortical potential reflecting cardiac function
Emotional trauma and psychological stress can precipitate cardiac arrhythmia and sudden death through arrhythmogenic effects of efferent sympathetic drive. Patients with preexisting heart disease are particularly at risk. Moreover, generation of proarrhythmic activity patterns within cerebral autonomic centers may be amplified by afferent feedback from a dysfunctional myocardium. An electrocortical potential reflecting afferent cardiac information has been described, reflecting individual differences in interoceptive sensitivity (awareness of one's own heartbeats). To inform our understanding of mechanisms underlying arrhythmogenesis, we extended this approach, identifying electrocortical potentials corresponding to the cortical expression of afferent information about the integrity of myocardial function during stress. We measured changes in cardiac response simultaneously with electroencephalography in patients with established ventricular dysfunction. Experimentally induced mental stress enhanced cardiovascular indices of sympathetic activity (systolic blood pressure, heart rate, ventricular ejection fraction, and skin conductance) across all patients. However, the functional response of the myocardium varied; some patients increased, whereas others decreased, cardiac output during stress. Across patients, heartbeat-evoked potential amplitude at left temporal and lateral frontal electrode locations correlated with stress-induced changes in cardiac output, consistent with an afferent cortical representation of myocardial function during stress. Moreover, the amplitude of the heartbeat-evoked potential in the left temporal region reflected the proarrhythmic status of the heart (inhomogeneity of left ventricular repolarization). These observations delineate a cortical representation of cardiac function predictive of proarrhythmic abnormalities in cardiac repolarization. Our findings highlight the dynamic interaction of heart and brain in stress-induced cardiovascular morbidity
Parametrising Star Formation Histories
We examine the star formation histories (SFHs) of galaxies in smoothed
particle hydrodynamics (SPH) simulations, compare them to parametric models
that are commonly used in fitting observed galaxy spectral energy
distributions, and examine the efficacy of these parametric models as practical
tools for recovering the physical parameters of galaxies. The commonly used
tau-model, with SFR ~ exp(-t/tau), provides a poor match to the SFH of our SPH
galaxies, with a mismatch between early and late star formation that leads to
systematic errors in predicting colours and stellar mass-to-light ratios. A
one-parameter lin-exp model, with SFR ~ t*exp(-t/tau), is much more successful
on average, but it fails to match the late-time behavior of the bluest, most
actively star-forming galaxies and the passive, "red and dead" galaxies. We
introduce a 4-parameter model, which transitions from lin-exp to a linear ramp
after a transition time, which describes our simulated galaxies very well. We
test the ability of these parametrised models to recover (at z=0, 0.5, and 1)
the stellar mass-to-light ratios, specific star formation rates, and stellar
population ages from the galaxy colours, computed from the full SPH star
formation histories using the FSPS code of Conroy et al. (2009). Fits with
tau-models systematically overestimate M/L by ~ 0.2 dex, overestimate
population ages by ~ 1-2 Gyr, and underestimate sSFR by ~ 0.05 dex. Fits with
lin-exp are less biased on average, but the 4-parameter model yields the best
results for the full range of galaxies. Marginalizing over the free parameters
of the 4-parameter model leads to slightly larger statistical errors than
1-parameter fits but essentially removes all systematic biases, so this is our
recommended procedure for fitting real galaxies.Comment: 28 pages, 18 figure
Intergalactic Dust Extinction in Hydrodynamic Cosmological Simulations
Recently Menard et al. detected a subtle but systematic change in the mean
color of quasars as a function of their projected separation from foreground
galaxies, extending to comoving separations of ~10Mpc/h, which they interpret
as a signature of reddening by intergalactic dust. We present theoretical
models of this remarkable observation, using SPH cosmological simulations of a
(50Mpc/h)^3 volume. Our primary model uses a simulation with galactic winds and
assumes that dust traces the intergalactic metals. The predicted galaxy-dust
correlation function is similar in form to the galaxy-mass correlation
function, and reproducing the MSFR data requires a dust-to-metal mass ratio of
0.24, about half the value in the Galactic ISM. Roughly half of the reddening
arises in dust that is more than 100Kpc/h from the nearest massive galaxy. We
also examine a simulation with no galactic winds, which predicts a much smaller
fraction of intergalactic metals (3% vs. 35%) and therefore requires an
unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both
models, the signal is dominated by sightlines with E(g-i)=0.001-0.1. The
no-wind simulation can be reconciled with the data if we also allow reddening
to arise in galaxies up to several x 10^10 Msun. The wind model predicts a mean
visual extinction of A_V ~0.0133 mag out to z=0.5, with a
sightline-to-sightline dispersion similar to the mean, which could be
significant for future supernova cosmology studies. Reproducing the MSFR
results in these simulations requires that a large fraction of ISM dust survive
its expulsion from galaxies and its residence in the intergalactic medium.
Future observational studies that provide higher precision and measure the
dependence on galaxy type and environment will allow detailed tests for models
of enriched galactic outflows and the survival of IG dust.Comment: Matches version accepted by MNRA
A Budget and Accounting of Metals at z~0: Results from the COS-Halos Survey
We present a budget and accounting of metals in and around star-forming
galaxies at . We combine empirically derived star formation histories
with updated supernova and AGB yields and rates to estimate the total mass of
metals produced by galaxies with present-day stellar mass of
--. On the accounting side of the ledger, we
show that a surprisingly constant 20--25% mass fraction of produced metals
remain in galaxies' stars, interstellar gas and interstellar dust, with little
dependence of this fraction on the galaxy stellar mass (omitting those metals
immediately locked up in remnants). Thus, the bulk of metals are outside of
galaxies, produced in the progenitors of today's galaxies. The COS-Halos
survey is uniquely able to measure the mass of metals in the circumgalactic
medium (to impact parameters of kpc) of low-redshift
galaxies. Using these data, we map the distribution of CGM metals as traced by
both the highly ionized OVI ion and a suite of low-ionization species; combined
with constraints on circumgalactic dust and hotter X-ray emitting gas out to
similar impact parameters, we show that % of metals produced by
galaxies can be easily accounted for out to
150 kpc. With the current data, we cannot rule out a constant mass of metals
within this fixed physical radius. This census provides a crucial boundary
condition for the eventual fate of metals in galaxy evolution models.Comment: 19 pages, 12 figures, 2 tables. ApJ, in pres
The formation of black holes in spherically symmetric gravitational collapse
We consider the spherically symmetric, asymptotically flat Einstein-Vlasov
system. We find explicit conditions on the initial data, with ADM mass M, such
that the resulting spacetime has the following properties: there is a family of
radially outgoing null geodesics where the area radius r along each geodesic is
bounded by 2M, the timelike lines are incomplete, and for r>2M
the metric converges asymptotically to the Schwarzschild metric with mass M.
The initial data that we construct guarantee the formation of a black hole in
the evolution. We also give examples of such initial data with the additional
property that the solutions exist for all and all Schwarzschild time,
i.e., we obtain global existence in Schwarzschild coordinates in situations
where the initial data are not small. Some of our results are also established
for the Einstein equations coupled to a general matter model characterized by
conditions on the matter quantities.Comment: 36 pages. A corollary on global existence in Schwarzschild
coordinates for data which are not small is added together with minor
modification
On the Role of Initial Data in the Gravitational Collapse of Inhomogeneous Dust
We consider here the gravitational collapse of a spherically symmetric
inhomogeneous dust cloud described by the Tolman-Bondi models. By studying a
general class of these models, we find that the end state of the collapse is
either a black hole or a naked singularity, depending on the parameters of the
initial density distribution, which are , the initial central density
of the massive body, and , the initial boundary. The collapse ends in a
black hole if the dimensionless quantity constructed out of this
initial data is greater than 0.0113, and it ends in a naked singularity if
is less than this number. A simple interpretation of this result can be
given in terms of the strength of the gravitational potential at the starting
epoch of the collapse.Comment: Original title changed, numerical range of naked singularity
corrected. Plain Tex File. 14 pages. To appear in Physical Review
Hydrogen and Metal Line Absorption Around Low-Redshift Galaxies in Cosmological Hydrodynamic Simulations
We study the physical conditions of the circum-galactic medium (CGM) around
z=0.25 galaxies as traced by HI and metal line absorption, using cosmological
hydrodynamic simulations that include galactic outflows. Using lines of sight
targeted at impact parameters from 10 kpc to 1 Mpc around galaxies with halo
masses from 10^11-10^13 M_solar, we study the physical conditions and their
variation with impact parameter b and line-of-sight velocity delta v in the CGM
as traced by HI, MgII, SiIV, CIV, OVI, and NeVIII absorbers. All ions show a
strong excess of absorption near galaxies compared to random lines of sight.
The excess continues beyond 1 Mpc, reflecting the correlation of metal
absorption with large-scale structure. Absorption is particularly enhanced
within about v<300 km/sec and roughly 300 kpc of galaxies (with distances
somewhat larger for the highest ion), approximately delineating the CGM; this
range contains the majority of global metal absorption. Low ions like MgII and
SiIV predominantly arise in denser gas closer to galaxies and drop more rapidly
with b, while high ions OVI and NeVIII trace more diffusely distributed gas
with a comparatively flat radial profile; CIV is intermediate. All ions
predominantly trace T~10^4-4.5 K photo-ionised gas at all b, but when hot CGM
gas is present (mostly in larger halos), we see strong collisionally-ionised
OVI and NeVIII at b <= 100 kpc. Larger halo masses generally produce more
absorption, though overall the trends are not as strong as that with impact
parameter. These findings arise using our favoured outflow scalings as expected
for momentum-driven winds; with no winds, the CGM gas remains mostly
unenriched, while our outflow model with a constant velocity and mass loading
factor produce hotter, more widely dispersed metals.Comment: 26 pages, 15 figures, published in MNRAS. Updates to citations from
previous versio
The neutral hydrogen content of galaxies in cosmological hydrodynamic simulations
We examine the global HI properties of galaxies in quarter billion particle cosmological
simulations using GADGET-2, focusing on howgalactic outflows impactHI content.We consider
four outflow models, including a new one (ezw) motivated by recent interstellar medium
simulations in which the wind speed and mass loading factor scale as expected for momentumdriven
outflows for larger galaxies and energy-driven outflows for dwarfs (Ï <75 km sâ1). To
obtain predicted HI masses, we employ a simple but effective local correction for particle selfshielding
and an observationally constrained transition from neutral to molecular hydrogen.
Our ezw simulation produces an HI mass function whose faint-end slope of â1.3 agrees well
with observations from the Arecibo Fast Legacy ALFA survey; other models agree less well.
Satellite galaxies have a bimodal distribution in HI fraction versus halo mass, with smaller
satellites and/or those in larger haloes more often being HI deficient. At a given stellar mass,
HI content correlates with the star formation rate and inversely correlates with metallicity,
as expected if driven by stochasticity in the accretion rate. To higher redshifts, massive HI
galaxies disappear and the mass function steepens. The global cosmic HI density conspires
to remain fairly constant from z ⌠5â0, but the relative contribution from smaller galaxies
increases with redshift.Department of HE and Training approved lis
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