24,107 research outputs found
Extreme UV QSOs
We present a sample of spectroscopically confirmed QSOs with FUV-NUV color
(as measured by GALEX photometry) bluer than canonical QSO templates and than
the majority of known QSOs. We analyze their FUV to NIR colors, luminosities
and optical spectra. The sample includes a group of 150 objects at low redshift
(z 0.5), and a group of 21 objects with redshift 1.7z2.6. For the low
redshift objects, the "blue" FUV-NUV color may be caused by enhanced Ly
emission, since Ly transits the GALEX FUV band from z=0.1 to z=0.47.
Synthetic QSO templates constructed with Ly up to 3 times stronger than
in standard templates match the observed UV colors of our low redshift sample.
The H emission increases, and the optical spectra become bluer, with
increasing absolute UV luminosity. The UV-blue QSOs at redshift about 2, where
the GALEX bands sample restframe about 450-590A (FUV) and about 590-940A(NUV),
are fainter than the average of UV-normal QSOs at similar redshift in NUV,
while they have comparable luminosities in other bands. Therefore we speculate
that their observed FUV-NUV color may be explained by a combination of steep
flux rise towards short wavelengths and dust absorption below the Lyman limit,
such as from small grains or crystalline carbon. The ratio of Ly to CIV
could be measured in 10 objects; it is higher (30% on average) than for
UV-normal QSOs, and close to the value expected for shock or collisional
ionization. FULL VERSION AVAILABLE FROM AUTHOR'S WEB SITE:
http://dolomiti.pha.jhu.edu/papers/2009_AJ_Extreme_UV_QSOs.pdfComment: Astronomical Journal, in pres
QSOs in the combined SDSS/GALEX database
We discuss selection of QSO candidates from the combined SDSS and GALEX
catalogues. We discuss properties of QSOs within the combined sample, and note
uncertainties in number counts and completeness, compared with other SDSS-based
samples. We discuss colour and other properties with redshift within the sample
and the SEDs for subsets. We estimate the numbers of faint QSOs that are
classified as extended objects in the SDSS, and consequent uncertainties that
follow.Comment: To appear in PAS
Identifying short motifs by means of extreme value analysis
The problem of detecting a binding site -- a substring of DNA where
transcription factors attach -- on a long DNA sequence requires the recognition
of a small pattern in a large background. For short binding sites, the matching
probability can display large fluctuations from one putative binding site to
another. Here we use a self-consistent statistical procedure that accounts
correctly for the large deviations of the matching probability to predict the
location of short binding sites. We apply it in two distinct situations: (a)
the detection of the binding sites for three specific transcription factors on
a set of 134 estrogen-regulated genes; (b) the identification, in a set of 138
possible transcription factors, of the ones binding a specific set of nine
genes. In both instances, experimental findings are reproduced (when available)
and the number of false positives is significantly reduced with respect to the
other methods commonly employed.Comment: 6 pages, 5 figure
Dust Emissivity in the Far-Infrared
We have derived the dust emissivity in the Far-Infrared (FIR) using data
available in the literature. We use two wavelength dependences derived from
spectra of Galactic FIR emission (Reach et al. 1995). A value for the
emissivity, normalised to the extinction efficiency in the V band, has been
retrieved from maps of Galactic FIR emission, dust temperature and extinction
(Schlegel et al. 1998).
Our results are similar to other measurements in the Galaxy but only
marginally consistent with the widely quoted values of Hildebrand (1983)
derived on one reflection nebula. The discrepancy with measurements on other
reflection nebulae (Casey 1991) is higher and suggests a different grain
composition in these environments with respect to the diffuse interstellar
medium.
We measure dust masses for a sample of six spiral galaxies with FIR
observations and obtain gas-to-dust ratios close to the Galactic value.Comment: 5 pages, 1 ps file, A&A letter accepte
Monte Carlo Predictions of Far-Infrared Emission from Spiral Galaxies
We present simulations of Far Infrared (FIR) emission by dust in spiral
galaxies, based on the Monte Carlo radiative transfer code of Bianchi, Ferrara
& Giovanardi (1996). The radiative transfer is carried out at several
wavelength in the Ultraviolet, optical and Near Infrared, to cover the range of
the stellar Spectral Energy Distribution (SED). Together with the images of the
galactic model, a map of the energy absorbed by dust is produced. Using
Galactic dust properties, the spatial distribution of dust temperature is
derived under the assumption of thermal equilibrium. A correction is applied
for non-equilibrium emission in the Mid Infrared. Images of dust emission can
then be produced at any wavelength in the FIR.
We show the application of the model to the spiral galaxy NGC 6946. The
observed stellar SED is used as input and models are produced for different
star-dust geometries. It is found that only optically thick dust disks can
reproduce the observed amount of FIR radiation. However, it is not possible to
reproduce the large FIR scalelength suggested by recent observation of spirals
at 200 um, even when the scalelength of the dust disk is larger than that for
stars. Optically thin models have ratios of optical/FIR scalelengths closer to
the 200um observations, but with smaller absolute scalelengths than optically
thick cases. The modelled temperature distributions are compatible with
observations of the Galaxy and other spirals. We finally discuss the
approximations of the model and the impact of a clumpy stellar and dust
structure on the FIR simulations.Comment: 19 pages, 6 figures, accepted by A&
A Curious Truncation of N=4 Yang-Mills
The coupling constant dependence of correlation functions of BPS operators in
N=4 Yang-Mills can be expressed in terms of integrated correlation functions.
We approximate these integrated correlators by using a truncated OPE expansion.
This leads to differential equations for the coupling dependence. When applied
to a particular sixteen point correlator, the coupling dependence we find
agrees with the corresponding amplitude computed via the AdS/CFT
correspondence. We conjecture that this truncation becomes exact in the large N
and large 't Hooft coupling limit.Comment: 10 pages, LaTeX; additional comments, added reference
ISO observations of spirals: modelling the FIR emission
ISO observations at 200 micron have modified our view of the dust component
in spiral galaxies. For a sample of seven resolved spirals we have retrieved a
mean temperature of 20K, about 10K lower than previous estimates based on IRAS
data at shorter wavelengths. Because of the steep dependence of far-infrared
emission on the dust temperature, the dust masses inferred from ISO fluxes are
a factor of 10 higher than those derived from IRAS data only, leading to
gas-to-dust ratios close to the value observed in the Galaxy. The scale-length
of the 200 micron emission is larger than for the IRAS 100 micron emission,
with colder dust at larger distances from the galactic centre, as expected if
the interstellar radiation field is the main source of dust heating. The 200
micron scale-length is also larger than the optical, for all the galaxies in
the sample. This suggests that the dust distribution is more extended than that
of the stars.A model of the dust heating is needed to derive the parameters of
the dust distribution from the FIR emission. Therefore, we have adapted an
existing radiative transfer code to deal with dust emission. Simulated maps of
the temperature distribution within the dust disk and of the dust emission at
any wavelength can be produced. The stellar spectral energy distribution is
derived from observations in the ultraviolet, optical and near infrared. The
parameters of the dust distribution (scale-lengths and optical depth) are
chosen to reproduce the observed characteristics of the FIR emission, i.e. the
shape of the spectrum, the flux and the spatial distribution. We describe the
application of the model to one of the galaxies in the sample, NGC 6946.Comment: 6 pages, 5 figures. Contribution to the proceedings of the workshop
"ISO Beyond Point Sources" held at VILSPA 14-17 September 199
The Resolved Outer Population of NGC6822 with WFPC2
We present F336W (U), F439W (B), F555W (V), and F675W (R) Wide Field
Planetary Camera 2 (WFPC2) photometry of two outer regions of the Local Group
dwarf irregular galaxy NGC6822. The NE region is ~13 arcmin from the galaxy
centre, while the W region lies 10 arcmin out, and within the wispy low surface
brightness outer regions of the galaxy. The fields are not crowded and contain
few NGC 6822 stars. We discuss errors and uncertainties and find that the W
region contains a main sequence that extends to stars of about 2 solar masses,
with an age of about 200 Myr. The NE region has no main sequence or stars
younger than 1 Gyr, but does contain some luminous red stars that are not
matched in the W field. These stars are not clumped in the field. The results
suggest that the W region may be a trace of a tidal event that triggered the
current star-formation in this isolated galaxy.Comment: 12 pages including 2 tables, plus 4 figures (#1 omitted) To appear in
PAS
Instantons in supersymmetric Yang-Mills and D-instantons in IIB superstring theory
The one-instanton contributions to various correlation functions of
supercurrents in four-dimensional N=4 supersymmetric SU(2) Yang-Mills theory
are evaluated to the lowest order in perturbation theory.Expressions of the
same form are obtained from the leading effects of a single D-instanton
extracted from the IIB superstring effective action around the AdS5*S5
background. This is in line with the suggested AdS/Yang-Mills correspondence.
The relation between Yang--Mills instantons and D-instantons is further
confirmed by the explicit form of the classical D-instanton solution in the
AdS5*S5 background and its associated supermultiplet of zero modes.
Speculations are made concerning instanton effects in the large-N_c limit of
the SU(N_c) Yang-Mills theory.Comment: 41 pages, LaTeX. Typos corrected and minor clarifications adde
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