104 research outputs found
Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
We review the present observational knowledge on the spatial distribution and
the physical state of the different (molecular, atomic and ionized) components
of the interstellar gas in the innermost 3 kpc of our Galaxy -- a region which
we refer to as the interstellar Galactic bulge, to distinguish it from its
stellar counterpart. We try to interpret the observations in the framework of
recent dynamical models of interstellar gas flows in the gravitational
potential of a barred galaxy. Finally, relying on both the relevant
observations and their theoretical interpretation, we propose a model for the
space-averaged density of each component of the interstellar gas in the
interstellar Galactic bulge.Comment: 19 pages, 11 figure
A Dust-Penetrated Classification Scheme for Bars as Inferred from their Gravitational Force Fields
The division of galaxies into ``barred'' (SB) and ``normal'' (S) spirals is a
fundamental aspect of the Hubble galaxy classification system. This ``tuning
fork'' view was revised by de Vaucouleurs, whose classification volume
recognized apparent ``bar strength'' (SA, SAB, SB) as a continuous property of
galaxies called the ``family''. However, the SA, SAB, and SB families are
purely visual judgments that can have little bearing on the actual bar strength
in a given galaxy. Until very recently, published bar judgments were based
exclusively on blue light images, where internal extinction or star formation
can either mask a bar completely or give the false impression of a bar in a
nonbarred galaxy. Near-infrared camera arrays, which principally trace the old
stellar populations in both normal and barred galaxies, now facilitate a
quantification of bar strength in terms of their gravitational potentials and
force fields. In this paper, we show that the maximum value, Qb, of the ratio
of the tangential force to the mean radial force is a quantitative measure of
the strength of a bar. Qb does not measure bar ellipticity or bar shape, but
rather depends on the actual forcing due to the bar embedded in its disk. We
show that a wide range of true bar strengths characterizes the category ``SB'',
while de Vaucouleurs category ``SAB'' corresponds to a much narrower range of
bar strengths. We present Qb values for 36 galaxies, and we incorporate our bar
classes into a dust-penetrated classification system for spiral galaxies.Comment: Accepted for publication in the Astrophysical Journal (LaTex, 30
pages + 3 figures); Figs. 1 and 3 are in color and are also available at
http://bama.ua.edu/~rbuta/bars
Rotation Curves of Spiral Galaxies
Rotation curves of spiral galaxies are the major tool for determining the
distribution of mass in spiral galaxies. They provide fundamental information
for understanding the dynamics, evolution and formation of spiral galaxies. We
describe various methods to derive rotation curves, and review the results
obtained. We discuss the basic characteristics of observed rotation curves in
relation to various galaxy properties, such as Hubble type, structure,
activity, and environment.Comment: 40 pages, 6 gif figures; Ann. Rev. Astron. Astrophys. Vol. 39, p.137,
200
Gas flow models in the Milky Way embedded bars
The gas distribution and dynamics in the inner Galaxy present many unknowns
as the origin of the asymmetry of the longitude-velocity (lv) diagram of the
Central Molecular Zone (CMZ). On the other hand, there are recent evidences in
the stellar component of the presence of a nuclear bar that could be slightly
lopsided. Our goal is to characterize the nuclear bar observed in 2MASS maps
and to study the gas dynamics in the inner Milky Way taking into account this
secondary bar. We have derived a realistic mass distribution by fitting 2MASS
star counts maps with three components (disk, bulge and nuclear bar) and we
have simulated the gas dynamics, in the deduced gravitational potential, using
a sticky-particles code. Our simulations of the gas dynamics reproduce
successfully the main characteristics of the Milky Way for a bulge orientation
of 20-35 deg with respect to the Sun-Galactic Center (GC) line and a pattern
speed of 30-40 km/s/kpc. In our models the Galactic Molecular Ring (GMR) is not
an actual ring but the inner parts of the spiral arms, while the 3-kpc arm and
its far side counterpart are lateral arms that contour the bar. Our simulations
reproduce, for the first time, the parallelogram shape of the lv-diagram of the
CMZ as the gas response to the nuclear bar. This bar should be oriented by an
angle of ~60-75 deg with respect to the Sun-GC line and its mass amounts to
(2-5.5)10e9 Msun. We show that the observed asymmetry of the CMZ cannot be due
to lopsidedness of the nuclear bar as suggested by the 2MASS maps. We do not
find clear evidences of lopsidedness in the stellar potential. We propose that
the observed asymmetry of the central gas layer can be due to the infalling of
gas into the CMZ in the l=1.3-complexComment: 28 pages, 6 tables, 14 figures. To appear in Astronomy and
Astrophysic
Mapping the submillimeter spiral wave in NGC 6946
We have analysed SCUBA 850\mum images of the (near) face-on spiral galaxy NGC
6946, and found a tight correlation between dust thermal emission and molecular
gas. The map of visual optical depth relates well to the distribution of
neutral gas (HI+H2) and implies a global gas-to-dust ratio of 90. There is no
significant radial variation of this ratio: this can be understood, since the
gas content is dominated by far by the molecular gas. The latter is estimated
through the CO emission tracer, which is itself dependent on metallicity,
similarly to dust emission. By comparing the radial profile of our visual
optical depth map with that of the SCUBA image, we infer an emissivity (dust
absorption coefficient) at 850\mum that is 3 times lower than the value
measured by COBE in the Milky Way, and 9 times lower than in NGC 891. A
decomposition of the spiral structure half way out along the disk of NGC 6946
suggests an interarm optical depth of between 1 and 2. These surprisingly high
values represent 40-80% of the visual opacity that we measure for the arm
region (abridged).Comment: 12 pages, 9 figures, accepted in A&
The Distance of the Soft Gamma Repeater SGR 1806-20
We present CO(J=1-0) observations in the direction of the Soft Gamma Repeater
SGR 1806-20 with the SEST telescope. We detected several molecular clouds, and
we discuss in this paper the implications of these observations for the
distance to the X-ray counterpart AX 1805.7-2025, the supernova remnant
G10.0-0.3 and the very luminous O9-B2 star detected in the line of sight. The
distance of SGR 1806-20 is estimated to be 14.5 +/- 1.4 kpc and this Soft Gamma
Repeater is very likely associated with one of the brightest HII regions in the
Galaxy, W31. The large size of G10.0-0.3 (25 x 38 pc) for a young supernova
remnant possibly powered by a central pulsar (AX 1805.7-2025) indicates that
G10.0-0.3 could be expanding in the very low density region produced by the
wind of the blue star.Comment: 20 pages, four postscript figures, LaTeX. To appear in Astrophysical
Journal, 1997, in pres
A New Model for the Spiral Structure of the Galaxy. Superposition of 2+4-armed patterns
We investigate the possibility of describing the spiral pattern of the Milky
Way in terms of a model of superposition 2- and 4-armed wave harmonics (the
simplest description, besides pure modes). Two complementary methods are used:
a study of stellar kinematics, and direct tracing of positions of spiral arms.
In the first method, the parameters of the galactic rotation curve and the free
parameters of the spiral density waves were obtained from Cepheid kinematics,
under different assumptions. To turn visible the structure corresponding to
these models, we computed the evolution of an ensemble of N-particles,
simulating the ISM clouds, in the perturbed galactic gravitational field. In
the second method, we present a new analysis of the longitude-velocity (l-v)
diagram of the sample of galactic HII regions, converting positions of spiral
arms in the galactic plane into locii of these arms in the l-v diagram. Both
methods indicate that the ``self-sustained'' model, in which the 2-armed and
4-armed mode have different pitch angles (6 arcdeg and 12 arcdeg, respectively)
is a good description of the disk structure. An important conclusion is that
the Sun happens to be practically at the corotation circle. As an additional
result of our study, we propose an independent test for localization of the
corotation circle in a spiral galaxy: a gap in the radial distribution of
interstellar gas has to be observed in the corotation region.Comment: 17 pages, 9 figures, Latex, uses aas2pp4.st
Dark matter in the Milky Way, II. the HI gas distribution as a tracer of the gravitational potential
Context. Gas within a galaxy is forced to establish pressure balance against
gravitational forces. The shape of an unperturbed gaseous disk can be used to
constrain dark matter models. Aims. We derive the 3-D HI volume density
distribution for the Milky Way out to a galactocentric radius of 40 kpc and a
height of 20 kpc to constrain the Galactic mass distribution. Methods. We used
the Leiden/Argentine/Bonn all sky 21-cm line survey. The transformation from
brightness temperatures to densities depends on the rotation curve. We explored
several models, reflecting different dark matter distributions. Each of these
models was set up to solve the combined Poisson-Boltzmann equation in a
self-consistent way and optimized to reproduce the observed flaring. Results.
Besides a massive extended halo of M ~ 1.8 10^{12} Msun, we find a
self-gravitating dark matter disk with M=2 to 3 10^{11} Msun, including a dark
matter ring at 13 < R < 18.5 kpc with M = 2.2 to 2.8 10^{10} Msun. The
existence of the ring was previously postulated from EGRET data and coincides
with a giant stellar structure that surrounds the Galaxy. The resulting Milky
Way rotation curve is flat up to R~27 kpc and slowly decreases outwards. The
\hi gas layer is strongly flaring. The HWHM scale height is 60 pc at R = 4 kpc
and increases to ~2700R \la 40$ kpc can be successfully modeled by a self-gravitating
isothermal disk raises the question of whether this massive disk may have been
caused by similar merger events in the past.Comment: 19 pages, 21 figures, accepted for publication by A&
The surface brightness of the Galaxy at the Solar Neighbourhood
We present a new determination of the surface brightness of our Galaxy at the
Solar Neighbourhood as observed from outside the Galaxy. We rely on various
existing optical and infra-red surveys to obtain a multiwavelength estimate. On
the one hand, scattered light does not contribute significantly to the surface
brightness. On the other hand, optical and infrared integrated all-sky surveys
(Pioneer 10/11 and COBE/DIRBE) show a systematically larger value than our
synthetic local estimate based on Hipparcos data. This local estimate is also
compatible with our Galactic simulations normalised at the Solar Neighbourhood
and assuming an homogeneous stellar distribution. We interpret this
disagreement as a signature of the presence of a local minimum of the stellar
density compatible with Gould's belt. According to this result, the global
luminosity of the Milky Way should follow the Tully-Fisher relation established
for external galaxies.Comment: 27 pages (including 13 pages of appendices), 45 figures; accepted for
publication in Astronomy and Astrophysic
Who Is at Risk for Diagnostic Discrepancies? Comparison of Pre- and Postmortal Diagnoses in 1800 Patients of 3 Medical Decades in East and West Berlin
<div><h3>Background</h3><p>Autopsy rates in Western countries consistently decline to an average of <5%, although clinical autopsies represent a reasonable tool for quality control in hospitals, medically and economically. Comparing pre- and postmortal diagnoses, diagnostic discrepancies as uncovered by clinical autopsies supply crucial information on how to improve clinical treatment. The study aimed at analyzing current diagnostic discrepancy rates, investigating their influencing factors and identifying risk profiles of patients that could be affected by a diagnostic discrepancy.</p> <h3>Methods and Findings</h3><p>Of all adult autopsy cases of the Charité Institute of Pathology from the years 1988, 1993, 1998, 2003 and 2008, the pre- and postmortal diagnoses and all demographic data were analyzed retrospectively. Based on power analysis, 1,800 cases were randomly selected to perform discrepancy classification (class I-VI) according to modified Goldman criteria. The rate of discrepancies in major diagnoses (class I) was 10.7% (95% CI: 7.7%–14.7%) in 2008 representing a reduction by 15.1%. Subgroup analysis revealed several influencing factors to significantly correlate with the discrepancy rate. Cardiovascular diseases had the highest frequency among class-I-discrepancies. Comparing the 1988-data of East- and West-Berlin, no significant differences were found in diagnostic discrepancies despite an autopsy rate differing by nearly 50%. A risk profile analysis visualized by intuitive heatmaps revealed a significantly high discrepancy rate in patients treated in low or intermediate care units at community hospitals. In this collective, patients with genitourinary/renal or infectious diseases were at particularly high risk.</p> <h3>Conclusions</h3><p>This is the current largest and most comprehensive study on diagnostic discrepancies worldwide. Our well-powered analysis revealed a significant rate of class-I-discrepancies indicating that autopsies are still of value. The identified risk profiles may aid both pathologists and clinicians to identify patients at increased risk for a discrepant diagnosis and possibly suboptimal treatment intra vitam.</p> </div
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