2,629 research outputs found
Resonant Relaxation in Stellar Systems
We demonstrate the existence of an enhanced rate of angular momentum
relaxation in nearly Keplerian star clusters, such as those found in the
centers of galactic nuclei containing massive black holes. The enhanced
relaxation arises because the radial and azimuthal orbital frequencies in a
Keplerian potential are equal, and hence may be termed {\em resonant
relaxation}. We explore the dynamics of resonant relaxation using both
numerical simulations and order-of-magnitude analytic calculations. We find
that the resonant angular momentum relaxation time is shorter than the
non-resonant relaxation time by of order , where is the
mass in stars and is the mass of the central object. Resonance does not
enhance the energy relaxation rate. We examine the effect of resonant
relaxation on the rate of tidal disruption of stars by the central mass; we
find that the flux of stars into the loss cone is enhanced when the loss cone
is empty, but that the disruption rate averaged over the entire cluster is not
strongly affected. We show that relativistic precession can disable resonant
relaxation near the main-sequence loss cone for black hole masses comparable to
those in galactic nuclei. Resonant dynamical friction leads to growth or decay
of the eccentricity of the orbit of a massive body, depending on whether the
distribution function of the stars is predominantly radial or tangential. The
accelerated relaxation implies that there are regions in nuclear star clusters
that are relaxed in angular momentum but not in energy; unfortunately, these
regions are not well-resolved in nearby galaxies by the Hubble Space Telescope.Comment: 34 pages (LaTeX2e format), 2 PostScript figures, uses epsf.sty and
aaspp4.sty (AASTeX v4.0), submitted to New Astronom
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Clinical application of brain imaging for the diagnosis of mood disorders: the current state of play
In response to queries about whether brain imaging technology has reached the point where it is useful for making a clinical diagnosis and for helping to guide treatment selection, the American Psychiatric Association (APA) has recently written a position paper on the Clinical Application of Brain Imaging in Psychiatry. The following perspective piece is based on our contribution to this APA position paper, which specifically emphasized the application of neuroimaging in mood disorders. We present an introductory overview of the challenges faced by researchers in developing valid and reliable biomarkers for psychiatric disorders, followed by a synopsis of the extant neuroimaging findings in mood disorders, and an evidence-based review of the current research on brain imaging biomarkers in adult mood disorders. Although there are a number of promising results, by the standards proposed below, we argue that there are currently no brain imaging biomarkers that are clinically useful for establishing diagnosis or predicting treatment outcome in mood disorders
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Individual Differences in Cerebral Blood Flow in Area 17 Predict the Time to Evaluate Visualized Letters
Sixteen subjects closed their eyes and visualized uppercase letters of the alphabet at two sizes, as small as possible or as large as possible while remaining “visible.” Subjects evaluated a shape characteristic of each letter (e.g., whether it has any curved lines), and responded as quickly as possible. Cerebral blood flow was normalized to the same value for each subject, and relative blood flow was computed for a set of regions of interest. The mean response time for each subject in the task was regressed onto the blood flow values. Blood flow in area 17 was negatively correlated with response time (r = -0.65), as was blood flow in area 19 (r = -0.66), whereas blood flow in the inferior parietal lobe was positively correlated with response time (r = 0.54). The first two effects persisted even when variance due to the other correlations was removed. These findings suggest that individual differences in the activation of specific brain loci are directly related to performance of tasks that rely on processing in those loci.Psycholog
A cold metal-poor cloud traced by a weak MgII absorption at z~0.45. First detection of SiI, CaI and FeI in a QSO absorber
We present the observations of a weak MgII absorption system detected at
z~0.452 in the UVES high resolution spectrum of the QSO HE0001-2340. The weaker
of the two MgII components forming the system shows associated absorptions due
to SiI, CaI and FeI observed for the first time in a QSO spectrum. We
investigate the nature of this absorber by comparing its properties with those
of different classes of absorbers (weak MgII, Damped Ly-alpha systems and local
interstellar clouds) and reproducing its ionization conditions with
photoionization models. The observed absorber belongs to the class of weak MgII
systems on the basis of its equivalent width, however the relative strength of
commonly observed transitions deviates significantly from those of the above
mentioned absorbers. A rough estimate of the probability to cross such a system
with a QSO line of sight is P~0.03. The presence of rare neutral transitions
suggests that the cloud is shielded by a large amount of neutral hydrogen. A
detailed comparison of the observed column densities with the average
properties of damped Ly-alpha systems and local interstellar cold clouds shows,
in particular, deficient MgII/MgI and CaII/CaI ratios in our cloud. The results
of photoionization models indicate that the cloud could be ionized by the UV
background. However, a simple model of a single cloud with uniform density
cannot reproduce the observed ionic abundance ratios, suggesting a more complex
density structure for the absorber. Supposing that ionization corrections are
negligible, the most puzzling result is the underabundance of magnesium with
respect to iron which is hard to explain both with nucleosynthesis and with
differential dust depletion. [Abridged]Comment: 7 pages, 9 figures. A&A in pres
Amygdala and fusiform gyrus temporal dynamics: Responses to negative facial expressions
<p>Abstract</p> <p>Background</p> <p>The amygdala habituates in response to repeated human facial expressions; however, it is unclear whether this brain region habituates to schematic faces (i.e., simple line drawings or caricatures of faces). Using an fMRI block design, 16 healthy participants passively viewed repeated presentations of schematic and human neutral and negative facial expressions. Percent signal changes within anatomic regions-of-interest (amygdala and fusiform gyrus) were calculated to examine the temporal dynamics of neural response and any response differences based on face type.</p> <p>Results</p> <p>The amygdala and fusiform gyrus had a within-run "U" response pattern of activity to facial expression blocks. The initial block within each run elicited the greatest activation (relative to baseline) and the final block elicited greater activation than the preceding block. No significant differences between schematic and human faces were detected in the amygdala or fusiform gyrus.</p> <p>Conclusion</p> <p>The "U" pattern of response in the amygdala and fusiform gyrus to facial expressions suggests an initial orienting, habituation, and activation recovery in these regions. Furthermore, this study is the first to directly compare brain responses to schematic and human facial expressions, and the similarity in brain responses suggest that schematic faces may be useful in studying amygdala activation.</p
Spatial Correlations in the Helium-Ionizing Background
After quasars ionize intergalactic HeII at z~3, a large radiation field
builds up above the HeII ionization edge. Unlike the background responsible for
HI ionizations, this field should be highly variable, thanks to the scarcity of
bright quasars and the relatively short attenuation lengths (~50 Mpc) of these
high-energy photons. Recent observations of the HeII and HI Lyman-alpha forests
show that this background does indeed vary strongly, with substantial
fluctuations on scales as small as ~2 Mpc. Here we show that such spatial
fluctuation scales are naturally expected in any model in which the sources are
as rare as bright quasars, so long as the attenuation length is relatively
small. The correlation length itself is comparable to the attenuation length
(~10 Mpc) for the most plausible physical scenarios, but we find
order-of-magnitude fluctuations on all scales smaller than ~6 Mpc. Moreover,
aliasing along the one-dimensional skewers probed by the HeII and HI
Lyman-alpha forests exaggerates these variations, so that order-of-magnitude
fluctuations should be observed on all scales smaller than ~20 Mpc. Complex
radiative transfer is therefore not required to explain the observed
fluctuations, at least at the level of current data.Comment: 6 pages, 3 figures, submitted to Ap
On probing the properties of QSOs through their proximity effects on the intergalactic medium
The proximity effect (PE) of QSOs is believed to be useful in constraining
the QSO lifetime. Observations on the PE so far, however, give apparently
contradictory results -- some are consistent with a long QSO lifetime (>~ a few
10^7 yr), but others appear to be only consistent with a short QSO lifetime <~
10^6 yr. In this paper, we show that this apparent contradiction may be solved
by simultaneously taking into account both the effect due to the density
enhancement in the QSO near zones and that due to the obscuration of the tori
associated with the QSOs, using a large number of Monte-Carlo generated
synthetic Lyman alpha forest spectra. We demonstrate that the QSO properties
and environment can be constrained simultaneously by the transverse PE and the
line of sight PE of bright type 1 QSOs together. The current available
measurements on the PEs of type 1 QSOs suggest that (1) the density is
significantly enhanced in the vicinity of the QSOs; (2) the QSO lifetime is
consistent with being as large as a few 10^7 yr and a substantially shorter
lifetime (e.g., <~10^6 yr) is not required; and (3) the half opening angle of
the tori associated with QSOs is ~60 deg, consistent with some other
independent estimates. Our simulations also show that the TPE of type 2 QSOs
can be significantly different from that of type 1 QSOs, which may be useful to
put further constraints on the QSO properties and the QSO environment.Comment: 14 pages, 11 figures, to appear in Ap
The Keck+Magellan Survey for Lyman Limit Absorption II: A Case Study on Metallicity Variations
We present an absorption line analysis of the Lyman limit system (LLS) at
z=3.55 in our Magellan/MIKE spectrum of PKS2000-330. Our analysis of the Lyman
limit and full HI Lyman series constrains the total HI column density of the
LLS (N_HI = 10^[18.0 +/- 0.25] cm^{-2} for b_HI >= 20 km/s) and also the N_HI
values of the velocity subsystems comprising the absorber. We measure ionic
column densities for metal-line transitions associated with the subsystems and
use these values to constrain the ionization state (>90% ionized) and relative
abundances of the gas. We find an order of magnitude dispersion in the
metallicities of the subsystems, marking the first detailed analysis of
metallicity variations in an optically thick absorber. The results indicate
that metals are not well mixed within the gas surrounding high galaxies.
Assuming a single-phase photoionization model, we also derive an N_H-weighted
metallicity, = -1.66 +/- 0.25, which matches the mean metallicity in
the neutral ISM in high z damped Lya systems (DLAs). Because the line density
of LLSs is ~10 times higher than the DLAs, we propose that the former dominate
the metal mass-density at z~3 and that these metals reside in the galaxy/IGM
interface. Considerations of a multi-phase model do not qualitatively change
these conclusions. Finally, we comment on an anomalously large O^0/Si^+ ratio
in the LLS that suggests an ionizing radiation field dominated by soft UV
sources (e.g. a starburst galaxy). Additional abundance analysis is performed
on the super-LLS systems at z=3.19.Comment: 20 pages, 7 figures (most in color). Accepted to Ap
A Flat Photoionization Rate at 2<z<4.2: Evidence for a Stellar-Dominated UV Background and Against a Decline of Cosmic Star Formation Beyond z~3
We investigate the implications of our measurement of the Lyman-alpha forest
opacity at redshifts 2<z<4.2 from a sample of 86 high-resolution quasar spectra
for the evolution of the cosmic ultraviolet luminosity density and its sources.
The derived hydrogen photoionization rate is remarkably flat over this redshift
range, implying an increasing comoving ionizing emissivity with redshift.
Because the quasar luminosity function is strongly peaked near z~2,
star-forming galaxies likely dominate the ionizing emissivity at z>~3. Our
measurement argues against a star formation rate density declining beyond z~3,
in contrast with existing state-of-the-art determinations of the cosmic star
formation history from direct galaxy counts. Stellar emission from galaxies
therefore likely reionized the Universe.Comment: 5 pages, including 1 figure, published by Ap
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