72 research outputs found
Novel Space Alters Theta and Gamma Synchrony Across the Longitudinal Axis of the Hippocampus.
Hippocampal theta (6–10 Hz) and gamma (25–50 Hz and 65–100 Hz) local field potentials (LFPs) reflect the dynamic synchronization evoked by inputs impinging upon hippocampal neurons. Novel experience is known to engage hippocampal physiology and promote successful encoding. Does novelty synchronize or desynchronize theta and/or gamma frequency inputs across the septotemporal (long) axis of the hippocampus (HPC)? The present study tested the hypothesis that a novel spatial environment would alter theta power and coherence across the long axis. We compared theta and gamma LFP signals at individual (power) and millimeter distant electrode pairs (coherence) within the dentate gyrus (DG) and CA1 region while rats navigated a runway (1) in a familiar environment, (2) with a modified path in the same environment and (3) in a novel space. Locomotion in novel space was related to increases in theta and gamma power at most CA1 and DG sites. The increase in theta and gamma power was concurrent with an increase in theta and gamma coherence across the long axis of CA1; however, there was a significant decrease in theta coherence across the long axis of the DG. These findings illustrate significant shifts in the synchrony of entorhinal, CA3 and/or neuromodulatory afferents conveying novel spatial information to the dendritic fields of CA1 and DG targets across the long axis of the HPC. This shift suggests that the entire theta/gamma-related input to the CA1 network, and likely output, receives and conveys a more coherent message in response to novel sensory experience. Such may contribute to the successful encoding of novel sensory experience
Chemo-dynamics of outer halo dwarf stars, including \textit{Gaia}-Sausage and \textit{Gaia}-Sequoia candidates
The low-metallicity, kinematically interesting dwarf stars studied by
Stephens \& Boesgaard (2002, SB02) are re-examined using Gaia DR2 astrometry,
and updated model atmospheres and atomic line data. New stellar parameters are
determined based on the Gaia DR2 parallactic distances and Dartmouth Stellar
Evolution Database isochrones. These are in excellent agreement with
spectroscopically determined stellar parameters for stars with [Fe/H];
however, large disagreements are found for stars with [Fe/H], with
offsets as large as T K and
log\,. A subset of six stars (test cases) are analysed ab
initio using high resolution spectra with Keck HIRES and Gemini GRACES. This
sub-sample is found to include two -challenged dwarf stars, suggestive
of origins in a low mass, accreted dwarf galaxy. The orbital parameters for the
entire SB02 sample are re-determined using \textit{Gaia} DR2 data. We find 11
stars that are dynamically coincident with the \textit{Gaia}-Sausage accretion
event and another 17 with the \textit{Gaia}-Sequoia event in action space. Both
associations include low-mass, metal-poor stars with isochrone ages older than
10 Gyr. Two dynamical subsets are identified within \textit{Gaia}-Sequoia. When
these subsets are examined separately, a common knee in [/Fe] is found
for the \textit{Gaia}-Sausage and high energy \textit{Gaia}-Sequoia stars. A
lower metallicity knee is tentatively identified in the \textit{Gaia}-Sequoia
low energy stars. If the metal-poor dwarf stars in these samples are true
members of the \textit{Gaia}-Sausage and \textit{Gaia}-Sequoia events, then
they present a unique opportunity to probe the earlier, more pristine, star
formation histories of these systems.Comment: 20-21 pages, 20 figures, accepted for publication in MNRA
Peeking beneath the precision floor I: metallicity spreads and multiple elemental dispersions in the globular clusters NGC 288 and NGC 362
The view of globular clusters (GCs) as simple systems continues to unravel,
revealing complex objects hosting multiple chemical peculiarities. Using
differential abundance analysis, we probe the chemistry of the Type I GC, NGC
288 and the Type II GC, NGC 362 at the 2\% level for the first time. We measure
20 elements and find differential measurement uncertainties on the order
0.01-0.02 dex in both clusters. The smallest uncertainties are measured for Fe
I in both clusters, with an average uncertainty of 0.013 dex. Dispersion
in the abundances of Na, Al, Ti I, Ni, Fe I, Y, Zr, Ba and Nd are recovered in
NGC 288, none of which can be explained by a spread in He. This is the first
time, to our knowledge, a statistically significant spread in -process
elements and a potential spread in metallicity has been detected in NGC 288. In
NGC 362, we find significant dispersion in the same elements as NGC 288, with
the addition of Co, Cu, Zn, Sr, La, Ce, and Eu. Two distinct groups are
recovered in NGC 362, separated by 0.3 dex in average differential -process
abundances. Given strong correlations between Al and several -process
elements, and a significant correlation between Mg and Si, we propose that the
-process rich group is younger. This agrees with asymptotic giant branch
star (AGB) enrichment between generations, if there is overlap between low- and
intermediate-mass AGBs. In our scenario, the older population is dominated by
the -process with a ratio of
. We propose that the -process dominance and dispersion found
in NGC 362 are primordial.Comment: First paper in a series. Accepted for publication in MNRA
The GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) II: Characterization of 47 Tuc with Bayesian Statistics
We present a photometric analysis of globular cluster 47 Tuc (NGC\,104),
using near-IR imaging data from the GeMS/GSAOI Galactic Globular Cluster Survey
(G4CS) which is in operation at Gemini-South telescope.~Our survey is designed
to obtain AO-assisted deep imaging with near diffraction-limited spatial
resolution of the central fields of Milky Way globular clusters.~The G4CS
near-IR photometry was combined with an optical photometry catalog obtained
from Hubble Space Telescope survey data to produce a high-quality
color-magnitude diagram that reaches down to K 21 Vega mag.~We used
the software suite BASE-9, which uses an adaptive Metropolis sampling algorithm
to perform a Markov chain Monte Carlo (MCMC) Bayesian analysis, and obtained
probability distributions and precise estimates for the age, distance and
extinction cluster parameters.~Our best estimate for the age of 47 Tuc is
12.42 0.08 Gyr, and our true distance modulus estimate
is (mM)=13.250 0.028 mag, in tight agreement
with previous studies using Gaia DR2 parallax and detached eclipsing binaries.Comment: 25 pages, 15 figures. Accepted for publication in The Astrophysical
Journa
Pyxis: A ground-based demonstrator for formation-flying optical interferometry
In the past few years, there has been a resurgence in studies towards
space-based optical/infrared interferometry, particularly with the vision to
use the technique to discover and characterise temperate Earth-like exoplanets
around solar analogues. One of the key technological leaps needed to make such
a mission feasible is demonstrating that formation flying precision at the
level needed for interferometry is possible. Here, we present ,
a ground-based demonstrator for a future small satellite mission with the aim
to demonstrate the precision metrology needed for space-based interferometry.
We describe the science potential of such a ground-based instrument, and detail
the various subsystems: three six-axis robots, a multi-stage metrology system,
an integrated optics beam combiner and the control systems required for the
necessary precision and stability. We end by looking towards the next stage of
: a collection of small satellites in Earth orbit.Comment: 27 Pages, 14 Figures, submitted to JATI
Chasing the impact of the Gaia-Sausage-Enceladus merger on the formation of the Milky Way thick disc
We employ our Bayesian Machine Learning framework BINGO (Bayesian INference
for Galactic archaeOlogy) to obtain high-quality stellar age estimates for
68,360 red giant and red clump stars present in the 17th data release of the
Sloan Digital Sky Survey, the APOGEE-2 high-resolution spectroscopic survey. By
examining the denoised age-metallicity relationship of the Galactic disc stars,
we identify a drop in metallicity with an increase in [Mg/Fe] at an early
epoch, followed by a chemical enrichment episode with increasing [Fe/H] and
decreasing [Mg/Fe]. This result is congruent with the chemical evolution
induced by an early-epoch gas-rich merger identified in the Milky Way-like
zoom-in cosmological simulation Auriga. In the initial phase of the merger of
Auriga 18 there is a drop in metallicity due to the merger diluting the metal
content and an increase in the [Mg/Fe] of the primary galaxy. Our findings
suggest that the last massive merger of our Galaxy, the Gaia-Sausage-Enceladus,
was likely a significant gas-rich merger and induced a starburst, contributing
to the chemical enrichment and building of the metal-rich part of the thick
disc at an early epoch.Comment: 5 pages, 5 figures, submitted to MNRAS Letters (comments are welcome
Peeking beneath the precision floor -- II. Probing the chemo-dynamical histories of the potential globular cluster siblings, NGC 288 and NGC 362
The assembly history of the Milky Way (MW) is a rapidly evolving subject,
with numerous small accretion events and at least one major merger proposed in
the MW's history. Accreted alongside these dwarf galaxies are globular clusters
(GCs), which act as spatially coherent remnants of these past events. Using
high precision differential abundance measurements from our recently published
study, we investigate the likelihood that the MW clusters NGC 362 and NGC 288
are galactic siblings, accreted as part of the Gaia-Sausage-Enceladus (GSE)
merger. To do this, we compare the two GCs at the 0.01 dex level for 20+
elements for the first time. Strong similarities are found, with the two
showing chemical similarity on the same order as those seen between the three
LMC GCs, NGC 1786, NGC 2210 and NGC 2257. However, when comparing GC abundances
directly to GSE stars, marked differences are observed. NGC 362 shows good
agreement with GSE stars in the ratio of Eu to Mg and Si, as well as a clear
dominance in the r- compared to the s-process, while NGC 288 exhibits only a
slight r-process dominance. When fitting the two GC abundances with a GSE-like
galactic chemical evolution model, NGC 362 shows agreement with both the model
predictions and GSE abundance ratios (considering Si, Ni, Ba and Eu) at the
same metallicity. This is not the case for NGC 288. We propose that the two are
either not galactic siblings, or GSE was chemically inhomogeneous enough to
birth two similar, but not identical clusters with distinct chemistry relative
to constituent stars.Comment: Second paper in a series. Accepted for publication by MNRAS, 17
pages, 11 figure
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