3,520 research outputs found
Absence of reflection as a function of the coupling constant
We consider solutions of the one-dimensional equation where is locally integrable, is integrable with supp, and
is a coupling constant. Given a family of solutions
which satisfy for all , we prove that the zeros of , the Wronskian of and , form a discrete set
unless . Setting , one sees that a particular
consequence of this result may be stated as: if the fixed energy scattering
experiment gives rise to a reflection coefficient
which vanishes on a set of couplings with an accumulation point, then .Comment: To appear in Journal of Mathematical Physic
Dipole anisotropies of IRAS galaxies and the contribution of a large-scale local void
Recent observations of dipole anisotropies show that the velocity of the
Local Group (\Vec v_{\rm G}) induced by the clustering of IRAS galax ies has
an amplitude and direction similar to those of the velocity of Cosmic Microwave
Background dipole anisotropy (\Vec v_{\rm CMB}), but the difference | \Vec
v_{\rm G} - \Vec v_{\rm CMB} | is still km/s, which is about 28% of
|\Vec v_{\rm CMB} |. Here we consider the possibility that the origin of this
difference comes from a hypothetical large-scale local void, with which we can
account for the accelerating behavior of type Ia supernovae due to the spatial
inhomogeneity of the Hubble constant without dark energies and derive the
constraint to the model parameters of the local void. It is found as a result
that the distance between the Local Group and the center of the void must be
Mpc, whose accurate value depends on the background model
parameters.Comment: 13 pages, 1 figure, to be published in ApJ 584, No.2 (2003
The star-formation history of the universe - an infrared perspective
A simple and versatile parameterized approach to the star formation history
allows a quantitative investigation of the constraints from far infrared and
submillimetre counts and background intensity measurements.
The models include four spectral components: infrared cirrus (emission from
interstellar dust), an M82-like starburst, an Arp220-like starburst and an AGN
dust torus. The 60 m luminosity function is determined for each chosen
rate of evolution using the PSCz redshift data for 15000 galaxies. The
proportions of each spectral type as a function of 60 m luminosity are
chosen for consistency with IRAS and SCUBA colour-luminosity relations, and
with the fraction of AGN as a function of luminosity found in 12 m
samples. The luminosity function for each component at any wavelength can then
be calculated from the assumed spectral energy distributions. With assumptions
about the optical seds corresponding to each component and, for the AGN
component, the optical and near infrared counts can be accurately modelled.
A good fit to the observed counts at 0.44, 2.2, 15, 60, 90, 175 and 850
m can be found with pure luminosity evolution in all 3 cosmological models
investigated: = 1, = 0.3 ( = 0), and
= 0.3, = 0.7.
All 3 models also give an acceptable fit to the integrated background
spectrum. Selected predictions of the models, for example redshift
distributions for each component at selected wavelengths and fluxes, are shown.
The total mass-density of stars generated is consistent with that observed,
in all 3 cosmological models.Comment: 20 pages, 25 figures. Accepted for publication in ApJ. Full details
of models can be found at http://astro.ic.ac.uk/~mrr/countmodel
Biomarker Discovery in Animal Health and Disease: The Application of Post-Genomic Technologies
The causes of many important diseases in animals are complex and multifactorial, which present unique challenges. Biomarkers indicate the presence or extent of a biological process, which is directly linked to the clinical manifestations and outcome of a particular disease. Identifying biomarkers or biomarker profiles will be an important step towards disease characterization and management of disease in animals. The emergence of post-genomic technologies has led to the development of strategies aimed at identifying specific and sensitive biomarkers from the thousands of molecules present in a tissue or biological fluid. This review will summarize the current developments in biomarker discovery and will focus on the role of transcriptomics, proteomics and metabolomics in biomarker discovery for animal health and disease
Velocity-selective direct frequency-comb spectroscopy of atomic vapors
We present an experimental and theoretical investigation of two-photon direct
frequency-comb spectroscopy performed through velocity-selective excitation. In
particular, we explore the effect of repetition rate on the
two-photon transitions
excited in a rubidium atomic vapor cell. The transitions occur via step-wise
excitation through the states by use of the direct
output of an optical frequency comb. Experiments were performed with two
different frequency combs, one with a repetition rate of MHz and
one with a repetition rate of MHz. The experimental spectra are
compared to each other and to a theoretical model.Comment: 10 pages, 7 figure
A Survey for HO Megamasers in Active Galactic Nuclei -- II. A Comparison of Detected and Undetected Galaxies
A survey for H2O megamaser emission from 354 active galaxies has resulted in
the detection of 10 new sources, making 16 known altogether. The galaxies
surveyed include a distance-limited sample (covering Seyferts and LINERs with
recession velocities < 7000 km s^{-1}) and a magnitude-limited sample (covering
Seyferts and LINERs with m(B) <= 14.5). In order to determine whether the
H2O-detected galaxies are "typical" AGN or have special properties which
facilitate the production of powerful masers, we have accumulated a database of
physical, morphological, and spectroscopic properties of the observed galaxies.
The most significant finding is that H2O megamasers are detected only in
Seyfert 2 and LINER galaxies, not Seyfert 1s. Implications for this finding are
discussed. LINERs are detected at a similar rate to Seyfert 2s, constituting a
strong argument that at least some nuclear LINERs are AGN rather than
starbursts, since starbursts have not been detected as H2O megamasers. We
preferentially detect H2O emission from the nearer galaxies and from those
which are apparently brighter at mid- and far-infrared and centimeter radio
wavelengths. There is also a possible trend for the H2O-detected galaxies to be
more intrinsically luminous in nuclear 6 cm radio emission than the undetected
ones, though these data are incomplete. We find evidence that Seyfert 2s with
very high (N(H) > 10^{24} cm^{-2}) X-ray absorbing columns of gas are more
often detected as H2O maser emitters than Seyfert 2s with lower columns. It may
be that the probability of detecting H2O maser emission in Seyfert galaxies
increases with increasing column of cool gas to the nucleus, from Seyfert 1s
through NLXGs to Seyfert 2s.Comment: 19 pages, 35 figures, 3 of the tables are contained in separate LaTeX
files: Table 1-a, 1-b, and 3. Also, figure captions are contained in a
separate file which must be compiled with plain TeX, not LaTe
Optical Spectroscopy of the IRAS 1-Jy Sample of Ultraluminous Infrared Galaxies
This paper discusses the optical spectroscopic properties of the IRAS 1-Jy
sample of ultraluminous infrared galaxies (ULIGs). One hundred and eight of the
118 1-Jy ULIGs have been observed at dlambda = 8.3 AA resolution over the
wavelength range ~4500 A -- 8900 A. These data are combined with large,
previously published sets of optical spectroscopic data of lower luminosity
infrared galaxies to look for systematic trends with infrared luminosity over
the luminosity range L_ir ~ 10^{10.5}-10^{13} L_sun. As found in previous
studies, the fraction of Seyfert galaxies among luminous infrared galaxies
increases abruptly above L_ir ~ 10^{12.3} L_sun --- about 50% of the galaxies
with L_ir > 10^{12.3} L_sun present Seyfert characteristics. Many of the
optical and infrared spectroscopic properties of the Seyfert galaxies are
consistent with the presence of a genuine active galactic nucleus (AGN). About
30% of these galaxies are Seyfert 1s with broad-line regions similar to those
of optical quasars. The percentage of Seyfert 1 ULIGs increases with infrared
luminosity, contrary to the predictions of the standard unification model for
Seyfert galaxies. Comparisons of the broad-line luminosities of optical and
obscured Seyfert 1 ULIGs with those of optically selected quasars of comparable
bolometric luminosity suggest that the dominant energy source in most of these
ULIGs is the same as in optical quasars, namely mass accretion onto a
supermassive black hole, rather than a starburst. These results are consistent
with recently published ISO, ASCA, and VLBI data. (abridged)Comment: Text and 23 figures (45 pages), Tables 1 - 6 (16 pages
Thermal noise in half infinite mirrors with non-uniform loss: a slab of excess loss in a half infinite mirror
We calculate the thermal noise in half-infinite mirrors containing a layer of
arbitrary thickness and depth made of excessively lossy material but with the
same elastic material properties as the substrate. For the special case of a
thin lossy layer on the surface of the mirror, the excess noise scales as the
ratio of the coating loss to the substrate loss and as the ratio of the coating
thickness to the laser beam spot size. Assuming a silica substrate with a loss
function of 3x10-8 the coating loss must be less than 3x10-5 for a 6 cm spot
size and a 7 micrometers thick coating to avoid increasing the spectral density
of displacement noise by more than 10%. A similar number is obtained for
sapphire test masses.Comment: Passed LSC (internal) review. Submitted to Phys. Rev. D. (5/2001)
Replacement: Minor typo in Eq. 17 correcte
Morphological studies of the Spitzer Wide-Area Infrared Extragalactic survey galaxy population in the UGC 10214 Hubble space telescope/advanced camera for surveys field
We present the results of a morphological analysis of a small subset of the Spitzer Wide-Area Infrared Extragalactic survey (SWIRE) galaxy population. The analysis is based on public Advanced Camera for Surveys (ACS) data taken inside the SWIRE N1 field, which are the deepest optical high-resolution imaging available within the SWIRE fields as of today. Our reference sample includes 156 galaxies detected by both ACS and SWIRE. Among the various galaxy morphologies, we disentangle two main classes, spheroids (or bulge-dominated galaxies) and disc-dominated ones, for which we compute the number counts as a function of flux. We then limit our sample to objects with Infrared Array Camera (IRAC) fluxes brighter than 10 μJy, estimated ~90 per cent completeness limit of the SWIRE catalogues, and compare the observed counts to model predictions. We find that the observed counts of the spheroidal population agree with the expectations of a hierarchical model while a monolithic scenario predicts steeper counts. Both scenarios, however, underpredict the number of late-type galaxies. These observations show that the large majority (close to 80 per cent) of the 3.6- and 4.5-μm galaxy population, even at these moderately faint fluxes, is dominated by spiral and irregular galaxies or mergers
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