97 research outputs found
Optophysiological characterisation of inner retina responses with high-resolution optical coherence tomography
Low coherence laser interferometry has revolutionised quantitative biomedical
imaging of optically transparent structures at cellular resolutions. We report the first
optical recording of neuronal excitation at cellular resolution in the inner retina by
quantifying optically recorded stimulus-evoked responses from the retinal ganglion
cell layer and comparing them with an electrophysiological standard. We imaged
anaesthetised paralysed tree shrews, gated image acquisition, and used numerical
filters to eliminate noise arising from retinal movements during respiratory and
cardiac cycles. We observed increases in contrast variability in the retinal ganglion
cell layer and nerve fibre layer with flash stimuli and gratings. Regions of interest
were subdivided into three-dimensional patches (up to 5-15ÎĽm in diameter) based on
response similarity. We hypothesise that these patches correspond to individual
cells, or segments of blood vessels within the inner retina. We observed a close
correlation between the patch optical responses and mean electrical activity of
afferent visual neurons. While our data suggest that optical imaging of retinal activity
is possible with high resolution OCT, the technical challenges are not trivial
The effect of women's property rights on HIV: a search for quantitative evidence
In recent years efforts to reduce HIV transmission have begun to incorporate a structural interventions approach, whereby the social, political, and economic environment in which people live is considered an important determinant of individual behaviors. This approach to HIV prevention is reflected in the growing number of programs designed to address insecure or nonexistent property rights for women living in developing countries. Qualitative and anecdotal evidence suggests that property ownership may allow women to mitigate social, economic, and biological effects of HIV for themselves and others through increased food security and income generation. Even so, the relationship between women’s property and inheritance rights (WPIR) and HIV transmission behaviors is not well understood. We explored sources of data that could be used to establish quantitative links between WPIR and HIV. Our search for quantitative evidence included (1) a review of peer-reviewed and “grey” literature reporting on quantitative associations between WPIR and HIV, (2) identification and assessment of existing data sets for their utility in exploring this relationship, and (3) interviews with organizations addressing women’s property rights in Kenya and Uganda about the data they collect. We found no quantitative studies linking insecure WPIR to HIV transmission behaviors. Data sets with relevant variables were scarce, and those with both WPIR and HIV variables could only provide superficial evidence of associations. Organizations addressing WPIR in Kenya and Uganda did not collect data that could shed light on the connection between WPIR and HIV, but two had data and community networks that could provide a good foundation for a future study that would include the collection of additional information. Collaboration between groups addressing WPIR and HIV transmission could provide the quantitative evidence needed to determine whether and how a WPIR structural intervention could decrease HIV transmission
The propagation of uncertainties in stellar population synthesis modeling I: The relevance of uncertain aspects of stellar evolution and the IMF to the derived physical properties of galaxies
The stellar masses, mean ages, metallicities, and star formation histories of
galaxies are now commonly estimated via stellar population synthesis (SPS)
techniques. SPS relies on stellar evolution calculations from the main sequence
to stellar death, stellar spectral libraries, phenomenological dust models, and
stellar initial mass functions (IMFs). The present work is the first in a
series that explores the impact of uncertainties in key phases of stellar
evolution and the IMF on the derived physical properties of galaxies and the
expected luminosity evolution for a passively evolving set of stars. A
Monte-Carlo Markov-Chain approach is taken to fit near-UV through near-IR
photometry of a representative sample of low- and high-redshift galaxies with
this new SPS model. Significant results include the following: 1) including
uncertainties in stellar evolution, stellar masses at z~0 carry errors of ~0.3
dex at 95% CL with little dependence on luminosity or color, while at z~2, the
masses of bright red galaxies are uncertain at the ~0.6 dex level; 2) either
current stellar evolution models, current observational stellar libraries, or
both, do not adequately characterize the metallicity-dependence of the
thermally-pulsating asymptotic giant branch phase; 3) conservative estimates on
the uncertainty of the slope of the IMF in the solar neighborhood imply that
luminosity evolution per unit redshift is uncertain at the ~0.4 mag level in
the K-band, which is a substantial source of uncertainty for interpreting the
evolution of galaxy populations across time; 4) The more plausible assumption
of a distribution of stellar metallicities, rather than a fixed value as is
usually assumed, can have significant effects on the interpretation of colors
blueward of the V-band. (ABRIDGED)Comment: 21 pages, 17 figures, ApJ in pres
Characterizing the Habitable Zones of Exoplanetary Systems with a Large Ultraviolet/Visible/Near-IR Space Observatory
Understanding the surface and atmospheric conditions of Earth-size, rocky
planets in the habitable zones (HZs) of low-mass stars is currently one of the
greatest astronomical endeavors. Knowledge of the planetary effective surface
temperature alone is insufficient to accurately interpret biosignature gases
when they are observed in the coming decades. The UV stellar spectrum drives
and regulates the upper atmospheric heating and chemistry on Earth-like
planets, is critical to the definition and interpretation of biosignature
gases, and may even produce false-positives in our search for biologic
activity. This white paper briefly describes the scientific motivation for
panchromatic observations of exoplanetary systems as a whole (star and planet),
argues that a future NASA UV/Vis/near-IR space observatory is well-suited to
carry out this work, and describes technology development goals that can be
achieved in the next decade to support the development of a UV/Vis/near-IR
flagship mission in the 2020s.Comment: Submitted in response to NASA call for white papers: "Large
Astrophysics Missions to Be Studied by NASA Prior to the 2020 Decadal Survey
Ly-alpha emitting galaxies at redshift z~4.5 in the LALA Cetus field
We present a large sample of Ly-alpha emitting galaxies spectroscopically
confirmed at z~4.5, based on IMACS spectroscopic observations of candidate LAEs
in the LALA Cetus field. We identify 110 z~4.5 LAEs based on single line
detections with no continuum emission blueward of the line. The Ly-alpha
confirmation rate varies from <50% to 76% for candidates selected in different
narrowband filters at slightly different redshifts. We find a drop in the LAE
density at redshift 4.50+-0.03 from redshift 4.39-4.47 by a factor of 66%,
which could be a large scale void in the distribution of star-forming galaxies
(~18Mpc along the line of sight and ~80Mpc across). The sample includes many
objects with equivalent widths >200A. These large EW candidates are
spectroscopically confirmed at the same rate as candidates with more modest
EWs. A composite spectrum of all 110 confirmed LAEs shows the characteristic
asymmetry of the Ly-alpha line. It also places new and stringent upper limits
on the CIV 1549/Ly-alpha and HeII 1640/Ly-alpha line ratios, providing a new
upper limit on the fraction of active galactic nuclei in Ly-alpha selected
galaxy samples, and on the contribution of Pop III populations. Finally, we
calculate the Ly-alpha luminosity function for our z~4.5 sample, which is
consistent with those at other redshifts, showing that there is no evolution in
Ly-alpha luminosity function from z~3.1-6.6.Comment: 30 pages, 10 figures, ApJ submitte
The Stellar Content of Galaxy Halos: A Comparison between LambdaCDM Models and Observations of M31
Recent observations have revealed that high surface-brightness, metal-rich
debris is present over large regions of the Andromeda (M31) stellar halo. We
use a set of numerical models to determine whether extended metal-rich debris
is expected to exist in galaxy halos formed in a hierarchical LambdaCDM
universe. We identify tidal debris in the simulations according to the current
best surface brightness detection limits in M31 and demonstrate that bright
features in the outer halo regions usually originate from single satellites,
with very little contamination from other sources due to the low probability of
tidal streams from two overlapping accretion events. In contrast, high-surface
brightness features in the inner halo often originate from multiple
progenitors. We also compare the age and metallicity distribution of the debris
with the well-mixed stellar halos in which they reside. We find that
high-surface brightness tidal debris is produced almost exclusively by
relatively high mass progenitors (Mstar ~ 10^7- 10^9 Msun) and thus is expected
to be of moderate- to high-metallicity. Similarly, in our models the smooth
inner halo is expected to be metal-rich as this region has been built up mainly
from massive satellites. Our results imply that the stellar populations of
substructure observed around external galaxies with current techniques should
not resemble old and metal-poor dwarf spheroidal satellites, nor the underlying
component of the stellar halo.Comment: 12 pages, 8 figures (accepted to ApJ
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