367 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
Dust-temperature of an isolated star-forming cloud: Herschel observations of the Bok globule CB244
We present Herschel observations of the isolated, low-mass star-forming Bok
globule CB244. It contains two cold sources, a low-mass Class 0 protostar and a
starless core, which is likely to be prestellar in nature, separated by 90
arcsec (~ 18000 AU). The Herschel data sample the peak of the Planck spectrum
for these sources, and are therefore ideal for dust-temperature and column
density modeling. With these data and a near-IR extinction map, the MIPS 70
micron mosaic, the SCUBA 850 micron map, and the IRAM 1.3 mm map, we model the
dust-temperature and column density of CB244 and present the first measured
dust-temperature map of an entire star-forming molecular cloud. We find that
the column-averaged dust-temperature near the protostar is ~ 17.7 K, while for
the starless core it is ~ 10.6K, and that the effect of external heating causes
the cloud dust-temperature to rise to ~ 17 K where the hydrogen column density
drops below 10^21 cm^-2. The total hydrogen mass of CB244 (assuming a distance
of 200 pc) is 15 +/- 5 M_sun. The mass of the protostellar core is 1.6 +/- 0.1
M_sun and the mass of the starless core is 5 +/- 2 M_sun, indicating that ~ 45%
of the mass in the globule is participating in the star-formation process.Comment: Accepted for A&A Herschel Special Issue; 5 pages, 2 figure
A Broadband Study of Galactic Dust Emission
We have combined infrared data with HI, H2 and HII surveys in order to
spatially decompose the observed dust emission into components associated with
different phases of the gas. An inversion technique is applied. For the
decomposition, we use the IRAS 60 and 100 micron bands, the DIRBE 140 and 240
micron bands, as well as Archeops 850 and 2096 micron wavelengths. In addition,
we apply the decomposition to all five WMAP bands. We obtain longitude and
latitude profiles for each wavelength and for each gas component in carefully
selected Galactic radius bins.We also derive emissivity coefficients for dust
in atomic, molecular and ionized gas in each of the bins.The HI emissivity
appears to decrease with increasing Galactic radius indicating that dust
associated with atomic gas is heated by the ambient interstellar radiation
field (ISRF). By contrast, we find evidence that dust mixed with molecular
clouds is significantly heated by O/B stars still embedded in their progenitor
clouds. By assuming a modified black-body with emissivity law lambda^(-1.5), we
also derive the radial distribution of temperature for each phase of the gas.
All of the WMAP bands except W appear to be dominated by emission from
something other than normal dust, most likely a mixture of thermal
bremstrahlung from diffuse ionized gas, synchrotron emission and spinning dust.
Furthermore, we find indications of an emissivity excess at long wavelengths
(lambda > 850 micron) in the outer Galaxy (R > 8.9 kpc). This suggests either
the existence of a very cold dust component in the outer Galaxy or a
temperature dependence of the spectral emissivity index. Finally, it is shown
that ~ 80% of the total FIR luminosity is produced by dust associated with
atomic hydrogen, in agreement with earlier findings by Sodroski et al. (1997).Comment: accepted for publication by A&
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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