448 research outputs found
The large scale gas and dust distribution in the galaxy: Implications for star formation
Infrared Astronomy Observations are presented for the diffuse infrared (IR) emissions from the galactic plane at wavelengths of 60 and 100 microns and the total far infrared intensity and its longitudinal variations in the disk were derived. Using available CO, 5 GHz radio-continuum, and HI data, the IR luminosity per hydrogen mass and the ingrared excess (IRE) ratio in the Galaxy were derived. The longitudinal profiles of the 60 and 100 micron emission were linearly decomposed into three components that are associated with molecular (H2), neutral (HI), and ionized (HII) phases in the interstellar medium (ISM), and the relevant dust properties were derived in each phase. Implications of the findings for various models of the diffuse IR emisison and for star formation in the galactic disk are discussed
Phenotypic knockout of the high-affinity human interleukin 2 receptor by intracellular single-chain antibodies against the alpha subunit of the receptor.
Molecular hydrogen beyond the optical edge of an isolated spiral galaxy
We know little about the outermost portions of galaxies because there is
little light coming from them. We do know that in many cases atomic hydrogen
(HI) extends well beyond the optical radius \cite{Casertano91}. In the centers
of galaxies, however, molecular hydrogen (H2) usually dominates by a large
factor, raising the question of whether H2 is abundant also in the outer
regions but hitherto unseen.Here we report the detection of emission from
carbon monoxide (CO), the most abundant tracer of H2, beyond the optical radius
of the nearby galaxy NGC 4414. The molecular clouds probably formed in the
regions of relatively high HI column density and in the absence of spiral
density waves. The relative strength of the lines from the two lowest
rotational levels indicates that both the temperature and density of the H2 are
quite low compared to conditions closer to the center. The inferred surface
density of the molecular material continues the monotonic decrease from the
inner regions. We conclude that while molecular clouds can form in the outer
region of this galaxy, there is little mass associated with them.Comment: 3 Nature page
Clumpy outer Galaxy molecular clouds and the steepening of the IMF
We report the results of high-resolution (~0.2 pc) CO(1-0) and CS(2-1)
observations of the central regions of three star-forming molecular clouds in
the far-outer Galaxy (~16 kpc from the Galactic Center): WB89 85 (Sh 2-127),
WB89 380, and WB89 437. We used the BIMA array in combination with IRAM 30-m
and NRAO 12-m observations. The GMC's in which the regions are embedded were
studied by means of KOSMA 3-m CO(2-1) observations. The properties the CO and
CS clumps are analyzed and compared with newly derived results of previously
published single-dish measurements of local clouds (OrionB South and Rosette).
We find that the slopes of the clump mass distributions (-1.28 and -1.49, for
WB89 85 and WB89 380, respectively) are somewhat less steep than found for most
local clouds, but similar to those of clouds which have been analyzed with the
same clumpfind program. We investigate the clump stability by using the virial
theorem, including all possible contributions (gravity, turbulence, magnetic
fields, and pressure due to the interclump gas). It appears that under
reasonable assumptions a combination of these forces would render most clumps
stable. Comparing only gravity and turbulence, we find that in the far-outer
Galaxy clouds, these forces are in equilibium (virial parameter alpha~1) for
clumps down to the lowest masses found (a few Msol). For clumps in the local
clouds alpha~1 only for clumps with masses larger than a few tens of Msol. Thus
it appears that in these outer Galaxy clumps gravity is the dominant force down
to a much lower mass than in local clouds, implying that gravitational collapse
and star formation may occur more readily even in the smallest clumps. Although
there are some caveats, due to the inhomogeneity of the data used, this might
explain the apparently steeper IMF found in the outer Galaxy.Comment: 29 pages, including 9 tables, 21 figures. Accepted for Astron.
Astrop
The spiral structure of our Milky Way Galaxy
The spiral structure of our Milky Way Galaxy is not yet known. HII regions
and giant molecular clouds are the most prominent spiral tracers. We collected
the spiral tracer data of our Milky Way from the literature, namely, HII
regions and giant molecular clouds (GMCs). With weighting factors based on the
excitation parameters of HII regions or the masses of GMCs, we fitted the
distribution of these tracers with models of two, three, four spiral-arms or
polynomial spiral arms. The distances of tracers, if not available from stellar
or direct measurements, were estimated kinetically from the standard rotation
curve of Brand & Blitz (1993) with =8.5 kpc, and =220 km
s or the newly fitted rotation curves with =8.0 kpc and
=220 km s or =8.4 kpc and =254 km s. We
found that the two-arm logarithmic model cannot fit the data in many regions.
The three- and the four-arm logarithmic models are able to connect most
tracers. However, at least two observed tangential directions cannot be matched
by the three- or four-arm model. We composed a polynomial spiral arm model,
which can not only fit the tracer distribution but also match observed
tangential directions. Using new rotation curves with =8.0 kpc and
=220 km s and =8.4 kpc and =254 km s for
the estimation of kinematic distances, we found that the distribution of HII
regions and GMCs can fit the models well, although the results do not change
significantly compared to the parameters with the standard and
.Comment: 34 Pages, 10 Figures, 5 Tables. Accepted for publication in A&A.
Edited
DART-RAY: a 3D ray-tracing radiative transfer code for calculating the propagation of light in dusty galaxies
We present DART-Ray, a new ray-tracing 3D dust radiative transfer (RT) code designed specifically to calculate radiation field energy density (RFED) distributions within dusty galaxy models with arbitrary geometries. In this paper, we introduce the basic algorithm implemented in . DART-Ray which is based on a pre-calculation of a lower limit for the RFED distribution. This pre-calculation allows us to estimate the extent of regions around the radiation sources within which these sources contribute significantly to the RFED. In this way, ray-tracing calculations can be restricted to take place only within these regions, thus substantially reducing the computational time compared to a complete ray-tracing RT calculation. Anisotropic scattering is included in the code and handled in a similar fashion. Furthermore, the code utilizes a Cartesian adaptive spatial grid and an iterative method has been implemented to optimize the angular densities of the rays originated from each emitting cell. In order to verify the accuracy of the RT calculations performed by DART-Ray, we present results of comparisons with solutions obtained using the dusty 1D RT code for a dust shell illuminated by a central point source and existing 2D RT calculations of disc galaxies with diffusely distributed stellar emission and dust opacity. Finally, we show the application of the code on a spiral galaxy model with logarithmic spiral arms in order to measure the effect of the spiral pattern on the attenuation and RFED. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society
Molecular gas in late-type galaxies
We present CO(J=1--0) line observations of 22 low-luminosity spiral
galaxies in the Virgo cluster. These data, together with 244 others available
in the literature, allow us to build a large sample that we use to study the
molecular gas properties of galaxies spanning a large range of morphological
types and luminosities and belonging to different environments (clusters -
field). The molecular gas content of the target galaxies is estimated using a
luminosity-dependent X = conversion factor that has been
calibrated on a sample of nearby galaxies. spans from 10 mol
cm (K km s in giant spirals to 10 mol
cm (K km s in dwarf irregulars. The value of the
conversion factor is found consistent with a value derived independently from
dust masses estimated from FIR fluxes, with a metallicity-dependent dust to gas
ratio. The relationships between X and the UV radiation field (as traced by the
), the metallicity and the H band luminosity are analysed.
We show that the molecular gas contained in molecular clouds or complexes is of
the order of 15% of the total gas on average whatever the luminosity or the
Hubble type of the galaxies. We discuss the relation between the star formation
rate and the molecular gas content and estimate the average star formation
efficiency of late-type galaxies.Comment: accepted for publication on Astronomy and Astrophysic
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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