2,613 research outputs found
The Major Sources of the Cosmic Reionizing Background at z ~ 6
In this paper, we address which sources contributed most of the reionizing
photons. Our argument assumes that the reionization ended around z ~ 6 and that
it was a relatively quick process, i.e., that there was a non-negligible
fraction of neutral hydrogen in the Universe at somewhat earlier epochs.
Starting from our earlier estimate of the luminosity function (LF) of galaxies
at z ~ 6, we quantitatively show that the major sources of reionization are
most likely galaxies with L < L*. Our approach allows us to put stronger
constraints to the LF of galaxies at z ~ 6. To have the Universe completely
ionized at this redshift, the faint-end slope of the LF should be steeper than
, which is the value measured at lower redshifts (z ~ 3), unless
either the normalization (Phi*) of the LF or the clumping factor of the ionized
hydrogen has been significantly underestimated. If Phi* is actually lower than
what we assumed by a factor of two, a steep slope close to is
required. Our LF predicts a total of 50 -- 80 z ~ 6 galaxies in the HST Ultra
Deep Field (UDF) to a depth of AB=28.4 mag, which can be used to constraint
both Phi* and . We conclude that the least luminous galaxies existing
at this redshift should reach as low as some critical luminosity in order to
accumulate the entire reionizing photon budget. On the other hand, the
existence of significant amounts of neutral hydrogen at slightly earlier
epochs, e.g. z ~ 7, requires that the least luminous galaxies should not be
fainter than another critical value (i.e., the LF should cut-off at this
point).Comment: ApJL in press (Jan 1, 2004 issue
The Nearby and Extremely Metal-Poor Galaxy CGCG 269-049
We present Hubble Space Telescope (HST) and Spitzer Space Telescope images
and photometry of the extremely metal-poor (Z = 0.03 Z_sol) blue dwarf galaxy
CGCG 269-049. The HST images reveal a large population of red giant and
asymptotic giant branch stars, ruling out the possibility that the galaxy has
recently formed. From the magnitude of the tip of the red giant branch, we
measure a distance to CGCG 269-049 of only 4.9 +/- 0.4 Mpc. The spectral energy
distribution of the galaxy between ~3.6 - 70 microns is also best fitted by
emission from predominantly ~10 Gyr old stars, with a component of thermal dust
emission having a temperature of 52 +/- 10 K. The HST and Spitzer photometry
indicate that more than 60% of CGCG 269-049's stellar mass consists of stars
~10 Gyr old, similar to other local blue dwarf galaxies. Our HST H-alpha image
shows no evidence of a supernova-driven outflow that could be removing metals
from the galaxy, nor do we find evidence that such outflows occurred in the
past. Taken together with CGCG 269-049's large ratio of neutral hydrogen mass
to stellar mass (~10), these results are consistent with recent simulations in
which the metal deficiency of local dwarf galaxies results mainly from
inefficient star formation, rather than youth or the escape of supernova
ejecta.Comment: 35 Pages, 7 Figures, accepted for publication in ApJ; new version
corrects errors in Table 1, Figure 3, and related calculations in tex
Geometric quantization of Hamiltonian actions of Lie algebroids and Lie groupoids
We construct Hermitian representations of Lie algebroids and associated
unitary representations of Lie groupoids by a geometric quantization procedure.
For this purpose we introduce a new notion of Hamiltonian Lie algebroid
actions. The first step of our procedure consists of the construction of a
prequantization line bundle. Next, we discuss a version of K\"{a}hler
quantization suitable for this setting. We proceed by defining a
Marsden-Weinstein quotient for our setting and prove a ``quantization commutes
with reduction'' theorem. We explain how our geometric quantization procedure
relates to a possible orbit method for Lie groupoids. Our theory encompasses
the geometric quantization of symplectic manifolds, Hamiltonian Lie algebra
actions, actions of families of Lie groups, foliations, as well as some general
constructions from differential geometry.Comment: 40 pages, corrected version 11-01-200
MODISTools - downloading and processing MODIS remotely sensed data in R
Remotely sensed data – available at medium to high resolution across global spatial and temporal scales – are a valuable resource for ecologists. In particular, products from NASA's MODerate-resolution Imaging Spectroradiometer (MODIS), providing twice-daily global coverage, have been widely used for ecological applications. We present MODISTools, an R package designed to improve the accessing, downloading, and processing of remotely sensed MODIS data. MODISTools automates the process of data downloading and processing from any number of locations, time periods, and MODIS products. This automation reduces the risk of human error, and the researcher effort required compared to manual per-location downloads. The package will be particularly useful for ecological studies that include multiple sites, such as meta-analyses, observation networks, and globally distributed experiments. We give examples of the simple, reproducible workflow that MODISTools provides and of the checks that are carried out in the process. The end product is in a format that is amenable to statistical modeling. We analyzed the relationship between species richness across multiple higher taxa observed at 526 sites in temperate forests and vegetation indices, measures of aboveground net primary productivity. We downloaded MODIS derived vegetation index time series for each location where the species richness had been sampled, and summarized the data into three measures: maximum time-series value, temporal mean, and temporal variability. On average, species richness covaried positively with our vegetation index measures. Different higher taxa show different positive relationships with vegetation indices. Models had high R2 values, suggesting higher taxon identity and a gradient of vegetation index together explain most of the variation in species richness in our data. MODISTools can be used on Windows, Mac, and Linux platforms, and is available from CRAN and GitHub (https://github.com/seantuck12/MODISTools)
Enteral Vancomycin to Eliminate MRSA Carriership of the Digestive Tract in Critically Ill Patients
Background: Carriership with methicillin resistant Staphylococcus aureus (MRSA) is a risk for the development of secondary infections in critically ill patients. Previous studies suggest that enteral vancomycin is able to eliminate enteral carriership with MRSA. Data on individual effects of this treatment are lacking. Methods: Retrospective analysis of a database containing 15 year data of consecutive patients from a mixed medical-(cardio)surgical 18 bedded intensive care unit was conducted. All consecutive critically ill patients with enteral MRSA carriership detected in throat and/or rectal samples were collected. We analyzed those with follow-up cultures to determine the success rate of enteral vancomycin. Topical application of 2% vancomycin in a sticky oral paste was performed combined with a vancomycin solution of 500 mg four times daily in the nasogastric tube. This treatment was added to a regimen of selective digestive tract decontamination (SDD) to prevent ICU acquired infection. Results: Thirteen patients were included. The mean age was 65 years and the median APACHE II score was 21. MRSA was present in the throat in 8 patients and in both throat and rectum in 5 patients. In all patients MRSA was successfully eliminated from both throat and rectum, which took 2–11 days with a median duration until decontamination of 4 days. Secondary infections with MRSA did not occur. Conclusions: Topical treatment with vancomycin in a 2% sticky oral paste four times daily in the nasogastric tube was effective in all patients in the elimination of MRSA and prevented secondary MRSA infections
The Kuiper Belt and Olbers Paradox
We investigate the constraints that Olbers Paradox, applied to the Zodiacal
Background as measured from space, sets on outer solar system objects. If
extended to very faint limits, R = 40--50 mag, the steep optical luminosity
function (LF) of Kuiper Belt objects (KBOs) at R less than 26 mag implies an
infinitely bright night sky. Small KBOs with radii of 1 mm to 1 km must have a
size distribution n(r) proportional to r^{-a}, with a = 3.5 or smaller to
satisfy the known limits on the sky-surface brightness at optical and
far-infrared wavelengths. Improved limits on the measured KBO surface
brightness can yield direct estimates of the albedo, temperature, and size
distribution for small KBOs in the outer solar system.Comment: 4 pages and 2 figures; submitted to Astrophysical Journal Letter
Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images
Medium-band imaging with HST/WFPC2 in the F410M filter has previously
revealed a population of compact Lyman-alpha emission objects around the radio
galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in
random, high-latitude HST parallel observations of three additional fields,
lending support to the idea that they constitute a widespread population at
these redshifts. The three new fields contain 18 Lyman-alpha candidates, in
contrast to the 17 detected in the deeper exposure of the single WFPC2 field
around 53W002. We find substantial differences in the number of candidates from
field to field, suggesting that significant large-scale structure is already
present in the galaxy distribution at this cosmic epoch. The likely existence
of z~2.4 sub-galactic clumps in several random fields shows that these objects
may have been common in the early universe and strengthens the argument that
such objects may be responsible for the formation of a fraction of the luminous
present-day galaxies through hierarchical merging.Comment: Uses slightly modified AASTeX preprint style file (included).
Contains 22 pages, including 5 figures and 2 tables. Accepted for the
December issue of the Astronomical Journa
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