3,113 research outputs found
Change of Interest as a Function of Shift in Curricular Orientation
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67119/2/10.1177_001316445301300213.pd
The W51 Giant Molecular Cloud
We present 45"-47" angular resolution maps at 50" sampling of the 12CO and
13CO J=1-0 emission toward a 1.39 deg x 1.33 deg region in the W51 HII region
complex. These data permit the spatial and kinematic separation of several
spectral features observed along the line of sight to W51, and establish the
presence of a massive (1.2 x 10^6 Mo), large (83 pc x 114 pc) giant molecular
cloud (GMC), defined as the W51 GMC, centered at (l,b,V) = (49.5 deg, -0.2 deg,
61 km/s). A second massive (1.9 x 10^5 Mo), elongated (136 pc x 22 pc)
molecular cloud is found at velocities of about 68 km/s along the southern edge
of the W51 GMC. Of the five radio continuum sources that classically define the
W51 region, the brightest source at lambda 6cm (G49.5-0.4) is spatially and
kinematically coincident with the W51 GMC and three (G48.9-0.3, G49.1-0.4, and
G49.2-0.4) are associated with the 68 km/s cloud. Published absorption line
spectra indicate that the fifth prominent continuum source (G49.4-0.3) is
located behind the W51 molecular cloud. The W51 GMC is among the upper 1% of
clouds in the Galactic disk by size and the upper 5-10% by mass. While the W51
GMC is larger and more massive than any nearby molecular cloud, the average H2
column density is not unusual given its size and the mean H2 volume density is
comparable to that in nearby clouds. The W51 GMC is also similar to other
clouds in that most of the molecular mass is contained in a diffuse envelope
that is not currently forming massive stars. We speculate that much of the
massive star formation activity in this region has resulted from a collision
between the 68 km/s cloud and the W51 GMC.Comment: Accepted for publication by the Astronomical Journal. 21 pages, plus
7 figures and 1 tabl
High resolution imaging of molecular line emission from high redshift QSOs
We present moderate (1'') and high resolution (0.2'') observations of the
CO(2-1) emission at 43 GHz, and radio continuum emission at 1.47 GHz, from the
z=4.7 QSO BRI 1202-0725 and the z=4.4 QSO BRI 1335--0417 using the Very Large
Array. The moderate resolution observations show that in both cases the CO
emission is spatially resolved into two components separated by 1'' for
1335-0417 and 4'' for 1202-0725. The high resolution observations show that
each component has sub-structure on scales of 0.2'' to 0.5'', with intrinsic
brightness temperatures > 20K. The CO ladder from (2-1) up to (7-6) suggests a
high kinetic temperature for the gas (70 K), and a high column density (10^{24}
cm^{-2}). In both sources the continuum-to-line ratio: L_{FIR}/L'_{CO(1-0)} =
335. All these characteristics (brightness temperature, excitation temperature,
column density, and continuum-to-line ratio) are comparable to conditions found
in low redshift, ultra-luminous nuclear starburst galaxies. We find that the CO
emitting regions in 1202-0725 and 1335-0417 must be close to face-on in order
to avoid having the gas mass exceed the gravitational mass, implying perhaps
unreasonably large rotational velocities. While this problem is mitigated by
lowering the CO luminosity-to-H_2 mass conversion factor (X), the required X
values become comparable to, or lower than, the minimum values dictated by
optically thin CO emission. We considered the possibility of magnification by
gravitational lensing in order to reduce the molecular gas masses.Comment: aastex 12 postscript figures. to appear in the Astronomical Journa
Scaling and universality in the anisotropic Kondo model and the dissipative two-state system
Scaling and universality in the Ohmic two-state system is investigated by
exploiting the equivalence of this model to the anisotropic Kondo model. For
the Ohmic two-state system, we find universal scaling functions for the
specific heat, , static susceptibility, , and
spin relaxation function depending on the reduced
temperature (frequency ), with
the renormalized tunneling frequency, and uniquely specified by the dissipation
strength (). The scaling functions can be used to extract
and in experimental realizations.Comment: 5 pages (LaTeX), 4 EPS figures. Minor changes, typos corrected,
journal reference adde
Molecular gas in the galaxy M83. I - The molecular gas distribution
We present CO(1-0) and CO(2-1) Swedish-ESO Submillimetre Telescope (SEST)
observations of the barred spiral galaxy M83 (NGC5236). The maps cover the
entire optical disk. The CO emission is strongly peaked toward the nucleus,
which breaks up into two separate components in the CO(2-1) data due to the
higher spatial resolution. Emission from the bar is strong, in particular on
the leading edges of the bar. The molecular gas arms are clearly resolved and
can be traced for more than 360\degr . Emission in the inter-arm regions is
detected. The average CO CO(2-1)/CO(1-0) line ratio is 0.77. The ratio is lower
than this on the spiral arms and higher in the inter-arm regions. The arms show
regularly spaced concentrations of molecular gas, Giant Molecular Associations
(GMA's), whose masses are of the order 10^7 Msun. The total molecular gas mass
is estimated to be 3.9*10^9 Msun. This mass is comparable to the total HI mass,
but H_2 dominates in the optical disk. In the disk, H_2 and HI show very
similar distributions, including small scale clumping. We compare the molecular
gas distribution with those of other star formation tracers, such as B and
H_alpha images.Comment: 20 pages, 15 figures, A&A accepted. A higher resolution version
available at http://www.astro.su.se/~andreas/publications
Constraints on the Cosmic-Ray Density Gradient beyond the Solar Circle from Fermi gamma-ray Observations of the Third Galactic Quadrant
We report an analysis of the interstellar -ray emission in the third
Galactic quadrant measured by the {Fermi} Large Area Telescope. The window
encompassing the Galactic plane from longitude 210\arcdeg to 250\arcdeg has
kinematically well-defined segments of the Local and the Perseus arms, suitable
to study the cosmic-ray densities across the outer Galaxy. We measure no large
gradient with Galactocentric distance of the -ray emissivities per
interstellar H atom over the regions sampled in this study. The gradient
depends, however, on the optical depth correction applied to derive the \HI\
column densities. No significant variations are found in the interstellar
spectra in the outer Galaxy, indicating similar shapes of the cosmic-ray
spectrum up to the Perseus arm for particles with GeV to tens of GeV energies.
The emissivity as a function of Galactocentric radius does not show a large
enhancement in the spiral arms with respect to the interarm region. The
measured emissivity gradient is flatter than expectations based on a cosmic-ray
propagation model using the radial distribution of supernova remnants and
uniform diffusion properties. In this context, observations require a larger
halo size and/or a flatter CR source distribution than usually assumed. The
molecular mass calibrating ratio, , is
found to be
in the Local-arm clouds and is not significantly sensitive to the choice of
\HI\ spin temperature. No significant variations are found for clouds in the
interarm region.Comment: Corresponding authors: I. A. Grenier ([email protected]); T.
Mizuno ([email protected]); L. Tibaldo
([email protected]) accepted for publication in Ap
Molecular line intensities as measures of cloud masses - II. Conversion factors for specific galaxy types
We present theoretically-established values of the CO-to-H2 and C-to-H2
conversion factors that may be used to estimate the gas masses of external
galaxies. We consider four distinct galaxy types, represented by M51, NGC 6946,
M82 and SMC N27. The physical parameters that best represent the conditions
within the molecular clouds in each of the galaxy types are estimated using a
chi^2 analysis of several observed atomic fine structure and CO rotational
lines. This analysis is explored over a wide range of density, radiation field,
extinction, and other relevant parameters. Using these estimated physical
conditions in methods that we have previously established, CO-to-H2 conversion
factors are then computed for CO transitions up to J=9-8. For the conventional
CO(1-0) transition, the computed conversion factor varies significantly below
and above the canonical value for the Milky Way in the four galaxy types
considered. Since atomic carbon emission is now frequently used as a probe of
external galaxies, we also present, for the first time, the C-to-H2 conversion
factor for this emission in the four galaxy types considered.Comment: 14 pages, 11 figures, accepted for publication in MNRA
Gas and Star Formation in the Circinus Galaxy
We present a detailed study of the Circinus Galaxy, investigating its star
formation, dust and gas properties both in the inner and outer disk. To achieve
this, we obtained high-resolution Spitzer mid-infrared images with the IRAC
(3.6, 5.8, 4.5, 8.0 micron) and MIPS (24 and 70 micron) instruments and
sensitive HI data from the Australia Telescope Compact Array (ATCA) and the
64-m Parkes telescope. These were supplemented by CO maps from the Swedish-ESO
Submillimetre Telescope (SEST). Because Circinus is hidden behind the Galactic
Plane, we demonstrate the careful removal of foreground stars as well as large-
and small-scale Galactic emission from the Spitzer images. We derive a visual
extinction of Av = 2.1 mag from the Spectral Energy Distribution of the
Circinus Galaxy and total stellar and gas masses of 9.5 x 10^{10} Msun and 9 x
10^9 Msun, respectively. Using various wavelength calibrations, we find
obscured global star formation rates between 3 and 8 Msun yr^{-1}. Star forming
regions in the inner spiral arms of Circinus, which are rich in HI, are
beautifully unveiled in the Spitzer 8 micron image. The latter is dominated by
polycyclic aromatic hydrocarbon (PAH) emission from heated interstellar dust.
We find a good correlation between the 8 micron emission in the arms and
regions of dense HI gas. The (PAH 8 micron) / 24 micron surface brightness
ratio shows significant variations across the disk of Circinus.Comment: 18 pages, 14 figures. All figures have been compressed. Contact
authors for original figures. Accepted by MNRA
Thermodynamics of the dissipative two-state system: a Bethe Ansatz study
The thermodynamics of the dissipative two-state system is calculated exactly
for all temperatures and level asymmetries for the case of Ohmic dissipation.
We exploit the equivalence of the two-state system to the anisotropic Kondo
model and extract the thermodynamics of the former by solving the thermodynamic
Bethe Ansatz equations of the latter. The universal scaling functions for the
specific heat and static dielectric susceptibility
are extracted for all dissipation strengths for
both symmetric and asymmetric two-state systems. The logarithmic corrections to
these quantities at high temperatures are found in the Kondo limit , whereas for we find the expected power law temperature
dependences with the powers being functions of the dissipative coupling
. The low temperature behaviour is always that of a Fermi liquid.Comment: 24 pages, 32 PS figures. Typos corrected, final versio
The Herschel Virgo Cluster Survey. IX. Dust-to-gas mass ratio and metallicity gradients in four Virgo spiral galaxies
Using Herschel data from the Open Time Key Project the Herschel Virgo Cluster
Survey (HeViCS), we investigated the relationship between the metallicity
gradients expressed by metal abundances in the gas phase as traced by the
chemical composition of HII regions, and in the solid phase, as traced by the
dust-to-gas mass ratio. We derived the radial gradient of the dust-to-gas mass
ratio for all galaxies observed by HeViCS whose metallicity gradients are
available in the literature. They are all late type Sbc galaxies, namely
NGC4254, NGC4303, NGC4321, and NGC4501. We examined different dependencies on
metallicity of the CO-to-H conversion factor (\xco), used to transform the
CO observations into the amount of molecular hydrogen. We found that in
these galaxies the dust-to-gas mass ratio radial profile is extremely sensitive
to choice of the \xco\ value, since the molecular gas is the dominant component
in the inner parts. We found that for three galaxies of our sample, namely
NGC4254, NGC4321, and NGC4501, the slopes of the oxygen and of the dust-to-gas
radial gradients agree up to 0.6-0.7R using \xco\ values in the
range 1/3-1/2 Galactic \xco. For NGC4303 a lower value of \xco
10 is necessary. We suggest that such low \xco\ values might be due to a
metallicity dependence of \xco (from close to linear for NGC4254, NGC4321, and
NGC4501 to superlinear for NGC4303), especially in the radial regions
R0.6-0.7R where the molecular gas dominates. On the other hand, the
outer regions, where the atomic gas component is dominant, are less affected by
the choice of \xco, and thus we cannot put constraints on its value.Comment: 13 pages, 8 figures, A&A accepte
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