2,130 research outputs found
A Search for Extraplanar Dust in Nearby Edge-On Spirals
We present high resolution BV images of 12 edge-on spiral galaxies observed
with the WIYN 3.5-m telescope. These images were obtained to search for
extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those
previously found in NGC 891 (Howk & Savage 1997). Our imaged galaxies include a
sample of seven massive L_*-like spiral galaxies within D<25 Mpc that have
inclinations i > 87 deg from the plane of the sky. We find that five of these
seven systems show extraplanar dust, visible as highly-structured absorbing
clouds against the background stellar light of the galaxies. The more prominent
structures are estimated to have associated gas masses >10^5 M_sun; the implied
potential energies are > 10^(52) ergs. All of the galaxies in our sample that
show detectable halpha emission at large z also show extraplanar dust
structures. None of those galaxies for which extraplanar halpha searches were
negative show evidence for extensive high-z dust. The existence of extraplanar
dust is a common property of massive spiral galaxies. We discuss several
mechanisms for shaping the observed dust features, emphasizing the possibility
that these dusty clouds represent the dense phase of a multiphase medium at
high-z in spiral galaxies. The correlation between high-z dust and extraplanar
Halpha emission may simply suggest that both trace the high-z interstellar
medium in its various forms (or phases), the existence of which may ultimately
be driven by vigorous star formation in the underlying disk. (Abstract
abridged)Comment: 26 pages; 15 jpeg figures. To appear in The Astronomical Journal, May
1999. Gzipped tar files of high-resolution figures in postscript and jpeg
formats are available at
http://www.astro.wisc.edu/~howk/Papers/papers.html#surve
First optical detection from the supernova remnant G 15.1-1.6
Deep optical CCD images of the supernova remnant G 15.1-1.6 were obtained and
filamentary and diffuse emission has been discovered. The images, taken in the
emission lines of Halpha+[N II], [S II] and [O III], reveal filamentary and
diffuse structures all around the remnant. The radio emission at 4850 MHz in
the same area is found to be well correlated with the brightest optical
filaments. The IRAS 60 micron emission may also be correlated with the optical
emission but to a lesser extent. The flux calibrated images suggest that the
optical emission originates from shock-heated gas ([S II]/Halpha > 0.4), while
there is a possible HII region ([S II]/Halpha ~0.3) contaminating the supernova
remnant's emission to the east. Furthermore, deep long-slit spectra were taken
at two bright filaments and also show that the emission originates from shock
heated gas. An [O III] filamentary structure has also been detected further to
the west but it lies outside the remnant's boundaries and possibly is not
associated to it. The [O III] flux suggests shock velocities into the
interstellar "clouds" ~100 km/s, while the [S II] 6716/6731 ratio indicates
electron densities up to ~250 cm^{-3}. Finally, the Halpha emission has been
measured to be between 2 to 7 x 10^{-16} erg/s/cm^2/arcsec^2, while the lower
limit to the distance is estimated at 2.2 kpc.Comment: 12 pages, 6 figures, 3 tables. Accepted for pubication in A&
Relating dust, gas and the rate of star formation in M31
We derive distributions of dust temperature and dust opacity across M31 at
45" resolution using the Spitzer data. With the opacity map and a standard dust
model we de-redden the Ha emission yielding the first de-reddened Ha map of
M31. We compare the emissions from dust, Ha, HI and H2 by means of radial
distributions, pixel-to-pixel correlations and wavelet cross-correlations. The
dust temperature steeply decreases from 30K near the center to 15K at large
radii. The mean dust optical depth at the Ha wavelength along the line of sight
is about 0.7. The radial decrease of the dust-to-gas ratio is similar to that
of the oxygen abundance. On scales<2kpc, cold dust emission is best correlated
with that of neutral gas and warm dust emission with that of ionized gas. Ha
emission is slightly better correlated with emission at 70um than at 24um. In
the area 6kpc<R< 17kpc, the total SFR is ~0.3Msun/yr. The Kennicutt-Schmidt law
between SFR and total gas has a power-law index of 1.30+-0.05 in the radial
range of R=7-11kpc increasing by about 0.3 for R=11-13kpc. The lack of H2 in
the central region could be related to the lack of HI and the low opacity/high
temperature of the dust. Since neither SFR nor SFE is well correlated with the
surface density of H2 or total gas, other factors than gas density must play an
important role in the formation of massive stars in M31. The molecular
depletion time scale of 1.1 Gyr indicates that M31 is about three times less
efficient in forming young massive stars than M33.Comment: 22 pages accepted for publication in A&
Serologic testing for symptomatic coccidioidomycosis in immunocompetent and immunosuppressed hosts
Serologic studies are an important diagnostic tool in the clinical evaluation and follow-up of persons with coccidioidomycosis. Numerous types of serologic tests are available, including immunodiffusion, enzyme immunoassay, and complement fixation. We conducted a retrospective review of the results of 1,797 serologic tests spanning 12 months from the onset of coccidioidomycosis in 298 immunocompetent and 62 immunosuppressed persons with symptomatic infection. Using the onset of symptoms as a reference point, we plotted the positive or negative serologic results over time for both groups. Compared with the immunocompetent group, immunosuppressed persons had lower rates of seropositivity for every type of test during the first year after onset of symptoms for coccidioidomycosis, although many results did not achieve statistical significance. Combining the results of these tests increased the sensitivity of the serologic evaluation in immunocompromised patients. Immunosuppressed persons have the ability to mount a serologic response to coccidioidomycosis, but in some circumstances, multiple methods may be required to improve detection
The Relation Between the Surface Brightness and the Diameter for Galactic Supernova Remnants
In this work, we have constructed a relation between the surface brightness
() and diameter (D) of Galactic C- and S-type supernova remnants
(SNRs). In order to calibrate the -D dependence, we have carefully
examined some intrinsic (e.g. explosion energy) and extrinsic (e.g. density of
the ambient medium) properties of the remnants and, taking into account also
the distance values given in the literature, we have adopted distances for some
of the SNRs which have relatively more reliable distance values. These
calibrator SNRs are all C- and S-type SNRs, i.e. F-type SNRs (and S-type SNR
Cas A which has an exceptionally high surface brightness) are excluded. The
Sigma-D relation has 2 slopes with a turning point at D=36.5 pc: (at 1
GHz)=8.4 D
WmHzster (for
WmHzster and D36.5 pc) and (at 1
GHz)=2.7 10 D
WmHzster (for
WmHzster and D36.5 pc). We discussed the theoretical
basis for the -D dependence and particularly the reasons for the change
in slope of the relation were stated. Added to this, we have shown the
dependence between the radio luminosity and the diameter which seems to have a
slope close to zero up to about D=36.5 pc. We have also adopted distance and
diameter values for all of the observed Galactic SNRs by examining all the
available distance values presented in the literature together with the
distances found from our -D relation.Comment: 45 pages, 2 figures, accepted for publication in Astronomical and
Astrophysical Transaction
Estimating Fixed Effects: Perfect Prediction and Bias in Binary Response Panel Models, with an Application to the Hospital Readmissions Reduction Program
The maximum likelihood estimator for the regression coefficients, β, in a panel binary response model with fixed effects can be severely biased if N is large and T is small, a consequence of the incidental parameters problem. This has led to the development of conditional maximum likelihood estimators and, more recently, to estimators that remove the O(T–1) bias in β^. We add to this literature in two important ways. First, we focus on estimation of the fixed effects proper, as these have become increasingly important in applied work. Second, we build on a bias-reduction approach originally developed by Kosmidis and Firth (2009) for cross-section data, and show that in contrast to other proposals, the new estimator ensures finiteness of the fixed effects even in the absence of within-unit variation in the outcome. Results from a simulation study document favourable small sample properties. In an application to hospital data on patient readmission rates under the 2010 Affo
Search for a Higgs Boson Produced in Association with a W Boson in pbar-p Collisions at sqrt{s} = 1.96 TeV
We present a search for a standard model Higgs boson produced in association
with a W boson using 2.7 1/fb of integrated luminosity of pbar-p collision data
taken at sqrt{s} = 1.96 TeV. Limits on the Higgs boson production rate are
obtained for masses between 100 GeV and 150 GeV. Through the use of
multivariate techniques, the analysis achieves an observed (expected) 95%
confidence level upper limit of 5.6 (4.8) times the theoretically expected
production cross section for a standard model Higgs boson with a mass of 115
GeV.Comment: submitted to Phys. Rev. Let
Search for the standard model Higgs boson decaying to a bb pair in events with one charged lepton and large missing transverse energy using the full CDF data set
We present a search for the standard model Higgs boson produced in
association with a W boson in sqrt(s) = 1.96 TeV p-pbar collision data
collected with the CDF II detector at the Tevatron corresponding to an
integrated luminosity of 9.45 fb-1. In events consistent with the decay of the
Higgs boson to a bottom-quark pair and the W boson to an electron or muon and a
neutrino, we set 95% credibility level upper limits on the WH production cross
section times the H->bb branching ratio as a function of Higgs boson mass. At a
Higgs boson mass of 125 GeV/c2 we observe (expect) a limit of 4.9 (2.8) times
the standard model value.Comment: Submitted to Phys. Rev. Lett (v2 contains clarifications suggested by
PRL
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