520 research outputs found
Noncompact sigma-models: Large N expansion and thermodynamic limit
Noncompact SO(1,N) sigma-models are studied in terms of their large N
expansion in a lattice formulation in dimensions d \geq 2. Explicit results for
the spin and current two-point functions as well as for the Binder cumulant are
presented to next to leading order on a finite lattice. The dynamically
generated gap is negative and serves as a coupling-dependent infrared regulator
which vanishes in the limit of infinite lattice size. The cancellation of
infrared divergences in invariant correlation functions in this limit is
nontrivial and is in d=2 demonstrated by explicit computation for the above
quantities. For the Binder cumulant the thermodynamic limit is finite and is
given by 2/(N+1) in the order considered. Monte Carlo simulations suggest that
the remainder is small or zero. The potential implications for ``criticality''
and ``triviality'' of the theories in the SO(1,N) invariant sector are
discussed.Comment: 46 pages, 2 figure
The M81 Group Dwarf Irregular Galaxy DDO 165. II. Connecting Recent Star Formation with ISM Structures and Kinematics
We compare the stellar populations and complex neutral gas dynamics of the
M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I
identified two kinematically distinct HI components, multiple localized high
velocity gas features, and eight HI holes and shells (the largest of which
spans ~2.2x1.1 kpc). Using the spatial and temporal information from the
stellar populations in DDO 165, we compare the patterns of star formation over
the past 500 Myr with the HI dynamics. We extract localized star formation
histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other
regions that sample a range of stellar densities and neutral gas properties.
From population synthesis modeling, we derive the energy outputs (from stellar
winds and supernovae) of the stellar populations within these regions over the
last 100 Myr, and compare with refined estimates of the energies required to
create the HI holes. In all cases, we find that "feedback" is energetically
capable of creating the observed structures in the ISM. Numerous regions with
significant energy inputs from feedback lack coherent HI structures but show
prominent localized high velocity gas features; this feedback signature is a
natural product of temporally and spatially distributed star formation. In DDO
165, the extended period of heightened star formation activity (lasting more
than 1 Gyr) is energetically capable of creating the observed holes and high
velocity gas features in the neutral ISM.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
The M81 Group Dwarf Irregular Galaxy DDO 165. I. High Velocity Neutral Gas in a Post-Starburst System
We present new multi-configuration VLA HI spectral line observations of the
M81 group dIrr post-starburst galaxy DDO 165. The HI morphology is complex,
with multiple column density peaks surrounding a large region of very low HI
surface density that is offset from the center of the stellar distribution. The
bulk of the neutral gas is associated with the southern section of the galaxy;
a secondary peak in the north contains ~15% of the total HI mass. These
components appear to be kinematically distinct, suggesting that either tidal
processes or large-scale blowout have recently shaped the ISM of DDO 165. Using
spatially-resolved position-velocity maps, we find multiple localized
high-velocity gas features. Cross-correlating with radius-velocity analyses, we
identify eight shell/hole structures in the ISM with a range of sizes (~400-900
pc) and expansion velocities (~7-11 km/s). These structures are compared with
narrow- and broad-band imaging from KPNO and HST. Using the latter data, recent
works have shown that DDO 165's previous "burst" phase was extended temporally
(>1 Gyr). We thus interpret the high-velocity gas features, HI holes, and
kinematically distinct components of the galaxy in the context of the immediate
effects of "feedback" from recent star formation. In addition to creating HI
holes and shells, extended star formation events are capable of creating
localized high velocity motion of the surrounding interstellar material. A
companion paper connects the energetics from the HI and HST data.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
Discovery and Characterization of 3000+ Main-Sequence Binaries from APOGEE Spectra
We develop a data-driven spectral model for identifying and characterizing
spatially unresolved multiple-star systems and apply it to APOGEE DR13 spectra
of main-sequence stars. Binaries and triples are identified as targets whose
spectra can be significantly better fit by a superposition of two or three
model spectra, drawn from the same isochrone, than any single-star model. From
an initial sample of 20,000 main-sequence targets, we identify
2,500 binaries in which both the primary and secondary star contribute
detectably to the spectrum, simultaneously fitting for the velocities and
stellar parameters of both components. We additionally identify and fit
200 triple systems, as well as 700 velocity-variable systems in
which the secondary does not contribute detectably to the spectrum. Our model
simplifies the process of simultaneously fitting single- or multi-epoch spectra
with composite models and does not depend on a velocity offset between the two
components of a binary, making it sensitive to traditionally undetectable
systems with periods of hundreds or thousands of years. In agreement with
conventional expectations, almost all the spectrally-identified binaries with
measured parallaxes fall above the main sequence in the color-magnitude
diagram. We find excellent agreement between spectrally and dynamically
inferred mass ratios for the 600 binaries in which a dynamical mass ratio
can be measured from multi-epoch radial velocities. We obtain full orbital
solutions for 64 systems, including 14 close binaries within hierarchical
triples. We make available catalogs of stellar parameters, abundances, mass
ratios, and orbital parameters.Comment: Accepted to MNRAS with minor revisions since v1. 19 pages, 12
figures, plus Appendice
When threat matters: Self-regulation, threat salience, and stereotyping
The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.jesp.2015.03.003 © 2015. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Four experiments examined whether information implying imminent threat to safety would interact with regulatory focus (Higgins, 1997) to affect the utilization of threat-relevant stereotypes. Because information suggesting imminent danger is more relevant to the safety goals of prevention-focused individuals than the advancement goals of promotion-focused individuals, utilization of threat-relevant stereotypes was expected to increase under such conditions only under prevention focus. Support for this prediction was obtained in four distinct and socially important domains. Using scenarios describing a violent crime committed by an African-American male (Experiment 1) or a petty crime committed by an undocumented immigrant (Experiment 2), prevention-focused individuals made judgments consistent with stereotypes when threat was perceived to be high rather than low. In studies that manipulated the stereotypicality of the target in a terrorism scenario (Experiments 3 & 4), prevention-focused individuals were more likely to endorse scrutinizing a stereotypical compared with a non-stereotypical target when terrorism was described as an increasing problem. Implications for models of stereotyping, self-regulation, and responding to threat are discussed.NSF Grant [1147779
Temporal Integration Windows in Neural Processing and Perception Aligned to Saccadic Eye Movements
SummaryWhen processing dynamic input, the brain balances the opposing needs of temporal integration and sensitivity to change. We hypothesized that the visual system might resolve this challenge by aligning integration windows to the onset of newly arriving sensory samples. In a series of experiments, human participants observed the same sequence of two displays separated by a brief blank delay when performing either an integration or segregation task. First, using magneto-encephalography (MEG), we found a shift in the stimulus-evoked time courses by a 150-ms time window between task signals. After stimulus onset, multivariate pattern analysis (MVPA) decoding of task in occipital-parietal sources remained above chance for almost 1 s, and the task-decoding pattern interacted with task outcome. In the pre-stimulus period, the oscillatory phase in the theta frequency band was informative about both task processing and behavioral outcome for each task separately, suggesting that the post-stimulus effects were caused by a theta-band phase shift. Second, when aligning stimulus presentation to the onset of eye fixations, there was a similar phase shift in behavioral performance according to task demands. In both MEG and behavioral measures, task processing was optimal first for segregation and then integration, with opposite phase in the theta frequency range (3â5Â Hz). The best fit to neurophysiological and behavioral data was given by a dampened 3-Hz oscillation from stimulus or eye fixation onset. The alignment of temporal integration windows to input changes found here may serve to actively organize the temporal processing of continuous sensory input
The ACS Nearby Galaxy Survey Treasury. X. Quantifying the Star Cluster Formation Efficiency of Nearby Dwarf Galaxies
We study the relationship between the field star formation and cluster
formation properties in a large sample of nearby dwarf galaxies. We use optical
data from the Hubble Space Telescope and from ground-based telescopes to derive
the ages and masses of the young (t_age < 100Myr) cluster sample. Our data
provides the first constraints on two proposed relationships between the star
formation rate of galaxies and the properties of their cluster systems in the
low star formation rate regime. The data show broad agreement with these
relationships, but significant galaxy-to-galaxy scatter exists. In part, this
scatter can be accounted for by simulating the small number of clusters
detected from stochastically sampling the cluster mass function. However, this
stochasticity does not fully account for the observed scatter in our data
suggesting there may be true variations in the fraction of stars formed in
clusters in dwarf galaxies. Comparison of the cluster formation and the
brightest cluster in our sample galaxies also provide constraints on cluster
destruction models.Comment: 16 pages, 9 figures, Accepted to Ap
Strong Outflows and Inefficient Star Formation in the Reionization-era Ultra-faint Dwarf Galaxy Eridanus II
We present novel constraints on the underlying galaxy formation physics
(e.g., mass loading factor, star formation history, metal retention) at
for the low-mass ( M) Local Group ultra-faint
dwarf galaxy (UFD) Eridanus {\sc II} (Eri II). Using a hierarchical Bayesian
framework, we apply a one-zone chemical evolution model to Eri II's CaHK-based
photometric metallicity distribution function (MDF; [Fe/H]) and find that the
evolution of Eri II is well-characterized by a short, exponentially declining
star-formation history ( Gyr), a low
star-formation efficiency ( Gyr), and
a large mass-loading factor (). Our results are
consistent with Eri II forming the majority of its stars before the end of
reionization. The large mass-loading factor implies strong outflows in the
early history of Eri II and is in good agreement with theoretical predictions
for the mass-scaling of galactic winds. It also results in the ejection of
90\% of the metals produced in Eri II. We make predictions for the
distribution of [Mg/Fe]-[Fe/H] in Eri II as well as the prevalence of ultra
metal-poor stars, both of which can be tested by future chemical abundance
measurements. Spectroscopic follow-up of the highest metallicity stars in Eri
II () will greatly improve model constraints. Our new
framework can readily be applied to all UFDs throughout the Local Group,
providing new insights into the underlying physics governing the evolution of
the faintest galaxies in the reionization era.Comment: 20 pages; 12 figures, submitted to MNRA
The Nature of Starbursts : II. The Duration of Starbursts in Dwarf Galaxies
The starburst phenomenon can shape the evolution of the host galaxy and the
surrounding intergalactic medium. The extent of the evolutionary impact is
partly determined by the duration of the starburst, which has a direct
correlation with both the amount of stellar feedback and the development of
galactic winds, particularly for smaller mass dwarf systems. We measure the
duration of starbursts in twenty nearby, ongoing, and "fossil" starbursts in
dwarf galaxies based on the recent star formation histories derived from
resolved stellar population data obtained with the Hubble Space Telescope.
Contrary to the shorter times of 3-10 Myr often cited, the starburst durations
we measure range from 450 - 650 Myr in fifteen of the dwarf galaxies and up to
1.3 Gyr in four galaxies; these longer durations are comparable to or longer
than the dynamical timescales for each system. The same feedback from massive
stars that may quench the flickering SF does not disrupt the overall burst
event in our sample of galaxies. While five galaxies present fossil bursts,
fifteen galaxies show ongoing bursts and thus the final durations may be longer
than we report here for these systems. One galaxy shows a burst that has been
ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in
this system. Using the duration of the starbursts, we calculate that the bursts
deposited 10^(53.9)-10^(57.2) ergs of energy into the interstellar medium
through stellar winds and supernovae and produced 3%-26% of the host galaxy's
mass.Comment: 28 pages, 4 figure
Self-Avoiding Gonihedric Srting and Spin Systems
We classify different theories of self-intersecting random surfaces assigning
special weights to intersections. When self-intersection coupling constant
tends to zero, then the surface can freely inetrsect and it is
completely self-avoiding when tends to infinity. Equivalent spin
systems for this general case were constructed. In two-dimension the system
with is in complete disorder as it is in the case of 2D gauge
Ising system.Comment: Preprint CRETE-TH-21, October 1993,8 pages,Late
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