2,555 research outputs found
Cis-regulatory basis of sister cell type divergence in the vertebrate retina
Multicellular organisms evolved via repeated functional divergence of transcriptionally related sister cell types, but the mechanisms underlying sister cell type divergence are not well understood. Here, we study a canonical pair of sister cell types, retinal photoreceptors and bipolar cells, to identify the ke
Multi-wavelength Observations of Dusty Star Formation at Low and High Redshift
This paper examines what can be learned about high-redshift star formation
from the small fraction of high-redshift galaxies' luminosities that is emitted
at accessible wavelengths. We review and quantify empirical correlations
between bolometric luminosities produced by star formation and the UV, mid-IR,
sub-mm, and radio luminosities of galaxies in the local universe. These
correlations suggest that observations of high-redshift galaxies at any of
these wavelengths should constrain their star-formation rates to within
0.2--0.3 dex. We assemble the limited evidence that high-redshift galaxies obey
these locally calibrated correlations. The characteristic luminosities and dust
obscurations of galaxies at z ~ 0, z ~ 1, and z ~ 3 are reviewed. After
discussing the relationship between the high-redshift populations selected in
surveys at different wavelengths, we calculate the contribution to the 850um
background from each. The available data show that a correlation between
star-formation rate and dust obscuration L_dust/L_UV exists at low and high
redshift. This correlation plays a central role in the major conclusion of this
paper: most star formation at high redshift occurred in galaxies with 1 <
L_dust/L_UV < 100 similar to those that host the majority of star formation in
the local universe and to those that are detected in UV-selected surveys.
(abridged)Comment: Scheduled for publication in ApJ v544 Dec 2000. Significant changes
to section 4. Characteristic UV and dust luminosities of star-forming
galaxies at redshifts z~0, z~1, and z~3 presented. Existence of extremely
obscured galaxies more clearly acknowledged. Original conclusions reinforced
by the observed correlation between bolometric luminosity and dust
obscuration at 0<z<
Spatially resolved spectroscopy of Cassiopeia A with MECS on board BeppoSAX
We have performed the first detailed spatially resolved spectroscopy of Cas A
in the 1.6-10 keV energy range, using data taken with the MECS spectrometer on
board the BeppoSAX Observatory. We performed a spatial deconvolution of the
data and eventually generated a set of spectra, covering a region of about 3
arcmin radius around the centre of Cas A. The results obtained by fitting these
spectra using a non-equilibrium ionisation plasma model and a power law, are:
(i) a single thermal component is sufficient to fit all the spectra; (ii) kT is
rather uniformly distributed with a minimum in the east and a maximum in the
west, and no evidence is found for high kT expected from the interaction of the
main shock with the ISM; (iii) from the distribution of the values of the
ionisation parameter n_et we infer the presence of two distinct components: the
first (a) in the range 1-10 cm^(-3), the second (b) with values ten times
higher; if we associate component a to the CSM and component b to the ejecta,
the mass ratio M(a)/M(b)<= 1/10 indicates a progenitor star that lost only a
small fraction of the envelope during its pre-SN life. In this hypothesis the
distribution of component b across the remnant suggests that the explosion was
not spherically symmetric; (iv) the distribution of abundances indicates that
we are detecting a CSM component with almost solar composition, and an ejecta
component enriched in heavier elements. Abundances found for alpha-elements are
consistent with the current view that Cas A was produced by the explosion of a
massive star.Comment: 16 pages, 9 PostScript figures, 4 tables, Accepted for publication on
Astronomy & Astrophysics (submitted July 12, 2000; accepted December 20,
2000
The SCUBA 8-mJy survey - II: Multiwavelength analysis of bright sub-mm sources
We present the results of a multi-wavelength study of the 19 most significant
sub-mm sources detected in the SCUBA 8-mJy survey. As described in Scott et al.
(2001), this survey covers ~260 arcmin^2 using the sub-millimetre camera SCUBA,
to a limiting source detection limit S(850um) ~ 8 mJy. One advantage of this
relatively bright flux-density limit is that accurate astrometric positions are
potentially achievable for every source using existing radio and/or mm-wave
interferometers. However, an associated advantage is that SED-based redshift
constraints should be more powerful than in fainter sub-mm surveys. Here we
therefore exploit the parallel SCUBA 450um data, in combination with existing
radio and ISO data at longer and shorter wavelengths to set constraints on the
redshift of each source. We also analyse new and existing optical and
near-infrared imaging of our SCUBA survey fields to select potential
identifications consistent with these constraints. Our derived SED-based
redshift constraints, and the lack of statistically significant associations
with even moderately bright galaxies allow us to conclude that all 19 sources
lie at z > 1, and at least half of them apparently lie at z > 2.Comment: 14 pages (including 30 figures and 4 tables) accepted by MNRAS.
Figure 1 and 2 and a higher quality version of the full paper are available
at http://astro.ic.ac.uk/~mfox
Sensitive Radio Observations of High Redshift Dusty QSOs
We present sensitive radio continuum imaging at 1.4 GHz and 4.9 GHz of seven
high redshift QSOs selected for having a 240 GHz continuum detection, which is
thought to be thermal dust emission. We detect radio continuum emission from
four of the sources: BRI 0952-0115, BR 1202-0725, LBQS 1230+1627B, and BRI
1335-0417. The radio source in BR 1202-0725 is resolved into two components,
coincident with the double mm and CO sources. We compare the results at 1.4 GHz
and 240 GHz to empirical and semi-analytic spectral models based on star
forming galaxies at low redshift. The radio-to-submm spectral energy
distribution for BR 1202-0725, LBQS 1230+1627B, and BRI 1335-0417 are
consistent with that expected for a massive starburst galaxy, with implied
massive star formation rates of order 1000 solar masses per year (without
correcting for possible amplification by gravitational lensing). The
radio-to-submm spectral energy distribution for BRI 0952-0115 suggests a
low-luminosity radio jet source driven by the AGN.Comment: 12 pages, Latex emulateapj format, including 1 table and 3 figures.
The Astrophysical Journal, to appear in the January 2000 issu
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Observations of Past Lunar Landing Sites by the D-CIXS X-Ray Spectrometer on SMART-1
D-CIXS initial observations show a first unambiguous remote sensing of calcium in the lunar regolith. Data obtained are broadly consistent with current understanding of mare and highland composition. Ground truth is provided by the returned Apollo and Luna sample sets
On the Nature of X-ray Variability in Ark 564
We use data from a recent long ASCA observation of the Narrow Line Seyfert 1
Ark 564 to investigate in detail its timing properties. We show that a thorough
analysis of the time series, employing techniques not generally applied to AGN
light curves, can provide useful information to characterize the engines of
these powerful sources.We searched for signs of non-stationarity in the data,
but did not find strong evidences for it. We find that the process causing the
variability is very likely nonlinear, suggesting that variability models based
on many active regions, as the shot noise model, may not be applicable to Ark
564. The complex light curve can be viewed, for a limited range of time scales,
as a fractal object with non-trivial fractal dimension and statistical
self-similarity. Finally, using a nonlinear statistic based on the scaling
index as a tool to discriminate time series, we demonstrate that the high and
low count rate states, which are indistinguishable on the basis of their
autocorrelation, structure and probability density functions, are intrinsically
different, with the high state characterized by higher complexity.Comment: 13 pages, 13 figures, accepted for publication in A&
A critical knowledge pathway to low-carbon, sustainable futures: Integrated understanding of urbanization, urban areas, and carbon
The microvascular endothelial glycocalyx:An additional piece of the puzzle in veterinary medicine
The endothelial glycocalyx (eGlx) is a critically important structure lining the luminal surface of endothelial cells. There is increasing evidence, in human patients and animal models, for its crucial role in the maintenance of health. Moreover, its damage is associated with the pathogenesis of multiple disease states. This review provides readers with an overview of the eGlx; summarising its structure, essential functions, and evidence for its role in disease. We highlight the lack of studies regarding the eGlx in cats and dogs, particularly in naturally occurring diseases. Importantly, we discuss techniques to aid its study, which can be applied to veterinary species. Finally, we present targeted therapies aimed at preserving, and in some cases, restoring damaged eGlx
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