11,478 research outputs found
Autophagy and phagocytosis converge for better vision
The retinal pigment epithelium (RPE) is a single layer of nonregenerating cells essential to homeostasis in the retina and the preservation of vision. While the RPE perform a number of important functions, 2 essential processes are phagocytosis, which removes the most distal tips of the photoreceptors to support disk renewal, and the visual cycle, which maintains the supply of chromophore for regeneration of photo-bleached visual pigments. We recently reported that these processes are linked by a noncanonical form of autophagy termed LC3-associated phagocytosis (LAP) in which components of the autophagy pathway are co-opted by phagocytosis to recover vitamin A in support of optimal vision. Here we summarize these findings
Far-UV Emission from Elliptical Galaxies at z=0.55
The restframe UV-to-optical flux ratio, characterizing the ``UV upturn''
phenomenon, is potentially the most sensitive tracer of age in elliptical
galaxies; models predict that it may change by orders of magnitude over the
course of a few Gyr. In order to trace the evolution of the UV upturn as a
function of redshift, we have used the far-UV camera on the Space Telescope
Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our
25''x25'' field includes four bright elliptical galaxies, spectroscopically
confirmed to be passively evolving cluster members. The weak UV emission from
the galaxies in our image demonstrates that the UV upturn is weaker at a
lookback time 5.6 Gyr earlier than our own, as compared to measurements of the
UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the
first with sufficient depth to demonstrate the fading of the UV upturn expected
at moderate redshifts. We discuss these observations and the implications for
the formation history of galaxies.Comment: 4 pages, Latex. 2 figures. Uses corrected version of emulateapj.sty
and apjfonts.sty (included). Accepted for publication in ApJ Letter
Color--Luminosity Relations for the Resolved Hot Stellar Populations in the Centers of M 31 and M 32
We present Faint Object Camera (FOC) ultraviolet images of the central
14x14'' of Messier 31 and Messier 32. The hot stellar population detected in
the composite UV spectra of these nearby galaxies is partially resolved into
individual stars, and their individual colors and apparent magnitudes are
measured. We detect 433 stars in M 31 and 138 stars in M 32, down to detection
limits of m_F275W = 25.5 mag and m_F175W = 24.5 mag. We investigate the
luminosity functions of the sources, their spatial distribution, their
color-magnitude diagrams, and their total integrated far-UV flux. Although M 32
has a weaker UV upturn than M 31, the luminosity functions and color-magnitude
diagrams of M 31 and M 32 are surprisingly similar, and are inconsistent with a
majority contribution from any of the following: PAGB stars more massive than
0.56 Msun, main sequence stars, or blue stragglers. Both the the luminosity
functions and color-magnitude diagrams are consistent with a dominant
population of stars that have evolved from the extreme horizontal branch (EHB)
along tracks with masses between 0.47 and 0.53 Msun. These stars are well below
the detection limits of our images while on the zero-age EHB, but become
detectable while in the more luminous (but shorter) AGB-Manque' and post-early
asymptotic giant branch (PEAGB) phases. The FOC observations require that only
a only a very small fraction of the main sequence population (2% in M 31 and
0.5% in M 32) in these two galaxies evolve though the EHB and post-EHB phases,
with the remainder evolving through bright PAGB evolution that is so rapid that
few if any stars are expected in the small field of view covered by the FOC.Comment: 35 pages, Latex. 19 figures. To appear in ApJ. Uses emulateapj.sty
and apjfonts.sty (included). Color plates distributed seperatedly: fig1.jpg
and fig2.jp
Validating a Vegetative Filter Strip Performance Model
Vegetative filter strips (VFS) reduce losses of nutrients, solids, and other materials from land area treated with fertilizers and manures. A number of models are available that simulate nutrient and sediment transport in VFS. While VFS effectiveness is considered to depend on lengths of pollutant source area and VFS areas, few published studies have tried to validate these models using variable pollutant source area and VFS area. The objective of this study was to validate an event-based nutrient transport model (Chaubey et al., 1995) that simulates soluble nutrient transport in VFS. This model links three sub-models: modified Green-Ampt infiltration, non-linear kinematic wave overland flow routing, and a nutrient transport component. The nutrient transport component considers infiltration as the only mechanism of pollutant removal from runoff. Data from a field plot experiment were used to validate the model. The model was executed using an uncalibrated runoff component, a calibrated runoff component, and measured runoff. The concentrations of parameters entering the VFS from three different poultry litter application lengths (6.1, 12.2, and 18.3 m) were not significantly different. However, predicted concentrations at subsequent lengths were different for all the three poultry litter application lengths. This finding was consistent with the observed data. Model execution with the uncalibrated runoff component, calibrated runoff component, and measured runoff underpredicted concentrations and mass transport at various locations along the length of the VFS. Underprediction of concentration was judged to be the reason for underprediction of mass transport. The agreement between the observed and predicted concentrations and mass transport, however, improved when runoff predictions from the calibrated runoff component and measured runoff were used. This suggests that accurate prediction of infiltration and runoff is critical for accurate prediction of concentration mass transport. Furthermore, since concentration was underpredicted even when measured runoff was used, this study suggests that the nutrient transport component might be improved, possibly by including nutrient removal mechanisms other than infiltration
Age Constraints for an M31 Globular Cluster from Main Sequence Photometry
We present a color-magnitude diagram (CMD) of the globular cluster SKHB-312
in the Andromeda galaxy (M31), obtained with the Advanced Camera for Surveys on
the Hubble Space Telescope. The cluster was included in deep observations taken
to measure the star formation history of the M31 halo. Overcoming a very
crowded field, our photometry of SKHB-312 reaches V ~ 30.5 mag, more than 1 mag
below the main sequence turnoff. These are the first observations to allow a
direct age estimate from the turnoff in an old M31 cluster. We analyze its CMD
and luminosity function using a finely-spaced grid of isochrones that have been
calibrated using observations of Galactic clusters taken with the same camera
and filters. The luminosity difference between the subgiant and horizontal
branches is ~0.2 mag smaller in SKHB-312 than in the Galactic clusters 47 Tuc
and NGC 5927, implying SKHB-312 is 2-3 Gyr younger. A quantitative comparison
to isochrones yields an age of 10 +2.5/-1 Gyr
RR Lyrae Stars in the Andromeda Halo from Deep Imaging with the Advanced Camera for Surveys
We present a complete census of RR Lyrae stars in a halo field of the
Andromeda galaxy. These deep observations, taken as part of a program to
measure the star formation history in the halo, spanned a period of 41 days
with sampling on a variety of time scales, enabling the identification of short
and long period variables. Although the long period variables cannot be fully
characterized within the time span of this program, the enormous advance in
sensitivity provided by the Advanced Camera for Surveys on the Hubble Space
Telescope allows accurate characterization of the RR Lyrae population in this
field. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc stars
with a mean period of 0.316 days, and 1 RRd star with a fundamental period of
0.473 days and a first overtone period of 0.353 days. These 55 RR Lyrae stars
imply a specific frequency S_RR=5.6, which is large given the high mean
metallicity of the halo, but not surprising given that these stars arise from
the old, metal-poor tail of the distribution. This old population in the
Andromeda halo cannot be clearly placed into one of the Oosterhoff types: the
ratio of RRc/RRabc stars is within the range seen in Oosterhoff II globular
clusters, the mean RRab period is in the gap between Oosterhoff types, and the
mean RRc period is in the range seen in Oosterhoff I globular clusters. The
periods of these RR Lyraes suggest a mean metallicity of [Fe/H]=-1.6, while
their brightness implies a distance modulus to Andromeda of 24.5+/-0.1, in good
agreement with the Cepheid distance.Comment: 15 pages, latex. Accepted for publication in The Astronomical Journa
Measurements of the Diffuse Ultraviolet Background and the Terrestrial Airglow with the Space Telescope Imaging Spectrograph
Far-UV observations in and near the Hubble Deep Fields demonstrate that the
Space Telescope Imaging Spectrograph (STIS) can potentially obtain unique and
precise measurements of the diffuse far-ultraviolet background. Although STIS
is not the ideal instrument for such measurements, high-resolution images allow
Galactic and extragalactic objects to be masked to very faint magnitudes, thus
ensuring a measurement of the truly diffuse UV signal. The programs we have
analyzed were not designed for this scientific purpose, but would be sufficient
to obtain a very sensitive measurement if it were not for a weak but
larger-than-expected signal from airglow in the STIS 1450-1900 A bandpass. Our
analysis shows that STIS far-UV crystal quartz observations taken near the limb
during orbital day can detect a faint airglow signal, most likely from NI\1493,
that is comparable to the dark rate and inseparable from the far-UV background.
Discarding all but the night data from these datasets gives a diffuse
far-ultraviolet background measurement of 501 +/- 103 ph/cm2/sec/ster/A, along
a line of sight with very low Galactic neutral hydrogen column (N_HI = 1.5E20
cm-2) and extinction (E(B-V)=0.01 mag). This result is in good agreement with
earlier measurements of the far-UV background, and should not include any
significant contribution from airglow. We present our findings as a warning to
other groups who may use the STIS far-UV camera to observe faint extended
targets, and to demonstrate how this measurement may be properly obtained with
STIS.Comment: 7 pages, Latex. 4 figures. Uses corrected version of emulateapj.sty
and apjfonts.sty (included). Accepted for publication in A
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