2,170 research outputs found
The ACS Nearby Galaxy Survey Treasury VI. The Ancient Star Forming disk of NGC 404
We present HST/WFPC2 observations across the disk of the nearby isolated
dwarf S0 galaxy NGC 404, which hosts an extended gas disk. Our deepest field
reaches the red clump and main-sequence stars with ages <500 Myr. Although we
detect trace amounts of star formation at times more recent than 10 Gyr for all
fields, the proportion of red giant stars to asymptotic giants and main
sequence stars suggests that the disk is dominated by an ancient (>10 Gyr)
population. Detailed modeling of the color-magnitude diagram suggests that ~70%
of the stellar mass in the NGC 404 disk formed by z~2 (10 Gyr ago) and at least
~90% formed prior to z~1 (8 Gyr ago). These results indicate that the stellar
populations of the NGC 404 disk are on average significantly older than those
of other nearby disk galaxies, suggesting that early and late type disks may
have different long-term evolutionary histories, not simply differences in
their recent star formation rates. Comparisons of the spatial distribution of
the young stellar mass and FUV emission in GALEX images show that the brightest
FUV regions contain the youngest stars, but that some young stars (<160 Myr)
lie outside of these regions. FUV luminosity appears to be strongly affected by
both age and stellar mass within individual regions. Finally, we use our
measurements to infer the relationship between the star formation rate and the
gas density of the disk at previous epochs. We find that most of the history of
the NGC 404 disk is consistent with star formation that has decreased with the
gas density according to the Schmidt law. However, 0.5-1 Gyr ago, the star
formation rate was unusually low for the inferred gas density, consistent with
the possibility that there was a gas accretion event that reignited star
formation ~0.5 Gyr ago. Such an event could explain why this S0 galaxy hosts an
extended gas disk.Comment: 28 pages, 20 figures, 1 table, accepted for publication in Ap
Complexity of multi-dimensional spontaneous EEG decreases during propofol induced general anaesthesia
Emerging neural theories of consciousness suggest a correlation between a specific type of neural dynamical complexity and the level of consciousness: When awake and aware, causal interactions between brain regions are both integrated (all regions are to a certain extent connected) and differentiated (there is inhomogeneity and variety in the interactions). In support of this, recent work by Casali et al (2013) has shown that Lempel-Ziv complexity correlates strongly with conscious level, when computed on the EEG response to transcranial magnetic stimulation. Here we investigated complexity of spontaneous high-density EEG data during propofol-induced general anaesthesia. We consider three distinct measures: (i) Lempel-Ziv complexity, which is derived from how compressible the data are; (ii) amplitude coalition entropy, which measures the variability in the constitution of the set of active channels; and (iii) the novel synchrony coalition entropy (SCE), which measures the variability in the constitution of the set of synchronous channels. After some simulations on Kuramoto oscillator models which demonstrate that these measures capture distinct ‘flavours’ of complexity, we show that there is a robustly measurable decrease in the complexity of spontaneous EEG during general anaesthesia
The ACS Nearby Galaxy Survey Treasury
The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to
establish a legacy of uniform multi-color photometry of resolved stars for a
volume-limited sample of nearby galaxies (D<4 Mpc). The survey volume
encompasses 69 galaxies in diverse environments, including close pairs, small &
large groups, filaments, and truly isolated regions. The galaxies include a
nearly complete range of morphological types spanning a factor of ~10^4 in
luminosity and star formation rate. The survey data consists of images taken
with ACS on HST, supplemented with archival data and new WFPC2 imaging taken
after the failure of ACS. Survey images include wide field tilings covering the
full radial extent of each galaxy, and single deep pointings in uncrowded
regions of the most massive galaxies in the volume. The new wide field imaging
in ANGST reaches median 50% completenesses of m_F475W=28.0 mag, m_F606W=27.3
mag, and m_F814W=27.3 mag, several magnitudes below the tip of the red giant
branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the
structure in the red clump. The resulting photometric catalogs are publicly
accessible and contain over 34 million photometric measurements of >14 million
stars. In this paper we present the details of the sample selection, imaging,
data reduction, and the resulting photometric catalogs, along with an analysis
of the photometric uncertainties (systematic and random), for both the ACS and
WFPC2 imaging. We also present uniformly derived relative distances measured
from the apparent magnitude of the TRGB.Comment: 54 pages, including 24 pages of figures and 16 pages of tables.
Project website and data available at http://www.nearbygalaxies.org/ . Data
is also available through MAST. Scheduled to appear in the Astrophysical
Journal Supplements. (Replaced to fix several figures that were damaged
during compression
The ACS Nearby Galaxy Survey Treasury IV. The Star Formation History of NGC 2976
We present resolved stellar photometry of NGC 2976 obtained with the Advanced
Camera for Surveys (ACS) as part of the ACS Nearby Galaxy Survey Treasury
(ANGST) program. The data cover the radial extent of the major axis of the disk
out to 6 kpc, or ~6 scale lengths. The outer disk was imaged to a depth of
M_F606W ~ 1, and an inner field was imaged to the crowding limit at a depth of
M_F606W ~ -1. Through detailed analysis and modeling of these CMDs we have
reconstructed the star formation history of the stellar populations currently
residing in these portions of the galaxy, finding similar ancient populations
at all radii but significantly different young populations at increasing radii.
In particular, outside of the well-measured break in the disk surface
brightness profile, the age of the youngest population increases with distance
from the galaxy center, suggesting that star formation is shutting down from
the outside-in. We use our measured star formation history, along with H I
surface density measurements, to reconstruct the surface density profile of the
disk during previous epochs. Comparisons between the recovered star formation
rates and reconstructed gas densities at previous epochs are consistent with
star formation following the Schmidt law during the past 0.5 Gyrs, but with a
drop in star formation efficiency at low gas densities, as seen in local
galaxies at the present day. The current rate and gas density suggest that
rapid star formation in NGC 2976 is currently in the process of ceasing from
the outside-in due to gas depletion. This process of outer disk gas depletion
and inner disk star formation was likely triggered by an interaction with the
core of the M81 group >~1 Gyr ago that stripped the gas from the galaxy halo
and/or triggered gas inflow from the outer disk toward the galaxy center.Comment: 22 pages, 14 figures, 2 tables, accepted for publication by Ap
Modulating T Cell Responses via Autophagy: The Intrinsic Influence Controlling the Function of Both Antigen-Presenting Cells and T Cells
Autophagy is a homeostatic and inducible process affecting multiple aspects of the immune system. This intrinsic cellular process is involved in MHC-antigen (Ag) presentation, inflammatory signaling, cytokine regulation, and cellular metabolism. In the context of T cell responses, autophagy has an influential hand in dictating responses to self and non-self by controlling extrinsic factors (e.g., MHC-Ag, cytokine production) in antigen-presenting cells (APC) and intrinsic factors (e.g., cell signaling, survival, cytokine production, and metabolism) in T cells. These attributes make autophagy an attractive therapeutic target to modulate T cell responses. In this review, we examine the impact autophagy has on T cell responses by modulating multiple aspects of APC function; the importance of autophagy in the activation, differentiation and homeostasis of T cells; and discuss how the modulation of autophagy could influence T cell responses
Increased spontaneous MEG signal diversity for psychoactive doses of ketamine, LSD and psilocybin
What is the level of consciousness of the psychedelic state? Empirically, measures of neural signal diversity such as entropy and Lempel-Ziv (LZ) complexity score higher for wakeful rest than for states with lower conscious level like propofol-induced anesthesia. Here we compute these measures for spontaneous magnetoencephalographic (MEG) signals from humans during altered states of consciousness induced by three psychedelic substances: psilocybin, ketamine and LSD. For all three, we find reliably higher spontaneous signal diversity, even when controlling for spectral changes. This increase is most pronounced for the single-channel LZ complexity measure, and hence for temporal, as opposed to spatial, signal diversity. We also uncover selective correlations between changes in signal diversity and phenomenological reports of the intensity of psychedelic experience. This is the first time that these measures have been applied to the psychedelic state and, crucially, that they have yielded values exceeding those of normal waking consciousness. These findings suggest that the sustained occurrence of psychedelic phenomenology constitutes an elevated level of consciousness - as measured by neural signal diversity
The ACS Nearby Galaxy Survey Treasury I. The Star Formation History of the M81 Outer Disk
The ACS Nearby Galaxy Survey Treasury (ANGST) is a large Hubble Space
Telescope (HST) Advanced Camera for Surveys (ACS) treasury program to obtain
resolved stellar photometry for a volume-limited sample of galaxies out to 4
Mpc. As part of this program, we have obtained deep ACS imaging of a field in
the outer disk of the large spiral galaxy M81. The field contains the outskirts
of a spiral arm as well as an area containing no current star formation. Our
imaging results in a color-magnitude diagram (CMD) reaching to F814W = 28.8 and
F606W = 29.5, one magnitude fainter than the red clump. Through detailed
modeling of the full CMD, we quantify the age and metallicity distribution of
the stellar populations contained in the field. The mean metallicity in the
field is -1<[M/H]<0 and only a small fraction of stars have ages <~1 Gyr. The
results show that most of the stars in this outer disk field were formed by z~1
and that the arm structure at this radius has a lifetime of >~100 Myr. We
discuss the measured evolution of the M81 disk in the context of surveys of
high-redshift disk galaxies and deep stellar photometry of other nearby
galaxies. All of these indicate that massive spiral disks are mostly formed by
z~1 and that they have experienced rapid metal enrichment.Comment: 26 pages, 2 tables, 18 figures, accepted for publication in A
Dissipation and Extra Light in Galactic Nuclei: II. 'Cusp' Ellipticals
We study the origin and properties of 'extra' or 'excess' central light in
the surface brightness profiles of cusp or power-law ellipticals. Dissipational
mergers give rise to two-component profiles: an outer profile established by
violent relaxation acting on stars present in the progenitors prior to the
final merger, and an inner stellar population comprising the extra light,
formed in a compact starburst. Combining a large set of hydrodynamical
simulations with data that span a broad range of profiles and masses, we show
that this picture is borne out -- cusp ellipticals are indeed 'extra light'
ellipticals -- and examine how the properties of this component scale with
global galaxy properties. We show how to robustly separate the 'extra' light,
and demonstrate that observed cusps are reliable tracers of the degree of
dissipation in the spheroid-forming merger. We show that the typical degree of
dissipation is a strong function of stellar mass, tracing observed disk gas
fractions at each mass. We demonstrate a correlation between extra light
content and effective radius at fixed mass: systems with more dissipation are
more compact. The outer shape of the light profile does not depend on mass,
with a mean outer Sersic index ~2.5. We explore how this relates to shapes,
kinematics, and stellar population gradients. Simulations with the gas content
needed to match observed profiles also reproduce observed age, metallicity, and
color gradients, and we show how these can be used as tracers of the degree of
dissipation in spheroid formation.Comment: 40 pages, 32 figures, accepted to ApJ (revised to match accepted
version
Nonlinear Network Dynamics on Earthquake Fault Systems
Earthquake faults occur in networks that have dynamical modes not displayed
by single isolated faults. Using simulations of the network of strike-slip
faults in southern California, we find that the physics depends critically on
both the interactions among the faults, which are determined by the geometry of
the fault network, as well as on the stress dissipation properties of the
nonlinear frictional physics, similar to the dynamics of integrate-and-fire
neural networks.Comment: 12 pages, 4 figure
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