1,477 research outputs found
Fragmented molecular complexes: The role of the magnetic field in feeding internal supersonic motions
A hierarchical structure for molecular complexes in their cold phase i.e., preceeding the formation of massive stars, was derived from extensive large scale CO(13)(J=1=0) observations: the mass is found to be distributed into virialized clouds which fill only a very low fraction approx. 01 of the volume of the complex and are supported against gravity by internal supersonic motions. An efficient mechanism was found to transfer kinetic energy from the orbital motions of the clouds to their internal random motions. The large perturbations of the magnetic field induced at the cloud boundaries by their interactions with their neighbors generate systems of hydromagnetic waves trapped inside the clouds. The magnetic field lines being closely coupled to the gas at the densities which prevail in the bulk of the clouds volume, internal velocity dispersion is thus generated. Some conclusions derived from this data are given
Correlations in the Far Infrared Background
We compute the expected angular power spectrum of the cosmic Far Infrared
Background (FIRB). We find that the signal due to source correlations dominates
the shot--noise for \ell \la 1000 and results in anisotropies with rms
amplitudes between 5% and 10% of the mean
for l \ga 150. The angular power spectrum depends on several unknown
quantities, such as the UV flux density evolution, optical properties of the
dust, biasing of the sources of the FIRB, and cosmological parameters. However,
when we require our models to reproduce the observed DC level of the FIRB, we
find that the anisotropy is at least a few percent in all cases. This
anisotropy is detectable with proposed instruments, and its measurement will
provide strong constraints on models of galaxy evolution and large-scale
structure at redshifts up to at least .Comment: 7 pages, 4 figures included, uses emulateapj.sty. More models
explored than in original version. Accepted for publication in Ap
Diffuse infrared emission of the galaxy: Large scale properties
The Infrared Astronomy Satellite (IRAS) survey is used to study large scale properties and the origin of the diffuse emission of the Galaxy. A careful subtraction of the zodiacal light enables longitude profiles of the galactic emission at 12, 25, 60, and 100 microns to be presented
Jeans criterion in a turbulent medium
According to the classical Jeans analysis, all the molecular clouds of mass larger than a few 100 M(solar), size larger than about 1pc and kinetic temperature Tk less than 30K are gravitationally unstable. We have shown that in clouds supported by internal supersonic motions, local gravitational instabilities may appear within molecular clouds which are globally stable. The argument is threefold: (1) when the turbulent kinetic energy is included into the internal energy term, the virial equilibrium condition shows that molecular clouds such as those observed, which are gravitationally unstable according to the Jeans criterion, are indeed globally stable if supported by a turbulent velocity field of power spectrum steeper than 3; (2) 2D compressible hydrodynamical simulations show that a supersonic turbulent velocity field generates a turbulent pressure within clouds, the gradients of which stabilize the unstable scales (i.e., the largest scales and the cloud itself) against gravitational collapse; (3) an analysis similar to the Jeans approach but including the turbulent pressure gradient term, gives basically the same results as those given in (1). Clouds of mean density lower than a critical value are found to be stable even though more massive than their Jeans mass. In clouds of mean density larger than that critical value, the gravitational instability appears only over a range of scales smaller than the cloud size, the largest scales being stable. In practice, the observed mean densities are lower than this critical value: the observation of a small number of cores and stars of a few solar masses embedded in clouds of several hundred solar masses can only be understood in terms of small scale density fluctuations of large amplitude generated by the supersonic turbulence which would occasionally overtake the limit of gravitational stability
Comment on "On the Origin of the Highest Energy Cosmic Rays"
We show that the photodisintegration of heavy cosmic ray nuclei with energies
above 10^20 eV is dominated by interactions with photons from the cosmic
microwave background radiation, rather than from infrared ones. This implies
that the observed air shower events with energies 2-3 10^20 eV cannot originate
from Fe nuclei coming from distances beyond 10 MpcComment: 1 page, 2 figure
The Subillimeter Properties of Extremely Red Objects in the CUDSS Fields
We discuss the submillimeter properties of Extremely Red Objects (EROs) in
the two Canada-UK Deep Submillimeter Survey (CUDSS) Fields. We measure the mean
submillimeter flux of the ERO population (to K < 20.7) and find 0.4 +/- 0.07
mJy for EROs selected by (I-K) > 4.0 and 0.56 +/- 0.09 mJy for EROs selected by
(R-K) > 5.3 but, these measurements are dominated by discrete, bright
submillimeter sources. We estimate that EROs produce 7-11% of the far-infrared
background at 850um. This is substantially less than a previous measurement by
Wehner, Barger & Kneib (2002) and we discuss possible reasons for this
discrepancy. We show that ERO counterparts to bright submillimeter sources lie
within the starburst region of the near-infrared color-color plot of Pozzetti &
Mannucci (2000). Finally, we claim that pairs or small groups of EROs with
separations of < 10 arcseconds often mark regions of strong submillimeter flux.Comment: 9 pages, 8 figures, accepted for publication in Ap
Statistical properties of dust far-infrared emission
The description of the statistical properties of dust emission gives
important constraints on the physics of the interstellar medium but it is also
a useful way to estimate the contamination of diffuse interstellar emission in
the cases where it is considered a nuisance. The main goals of this analysis of
the power spectrum and non-Gaussian properties of 100 micron dust emission are
1) to estimate the power spectrum of interstellar matter density in three
dimensions, 2) to review and extend previous estimates of the cirrus noise due
to dust emission and 3) to produce simulated dust emission maps that reproduce
the observed statistical properties. The main results are the following. 1) The
cirrus noise level as a function of brightness has been previously
overestimated. It is found to be proportional to instead of ^1.5, where
is the local average brightness at 100 micron. This scaling is in
accordance with the fact that the brightness fluctuation level observed at a
given angular scale on the sky is the sum of fluctuations of increasing
amplitude with distance on the line of sight. 2) The spectral index of dust
emission at scales between 5 arcmin and 12.5 degrees is =-2.9 on average
but shows significant variations over the sky. Bright regions have
systematically steeper power spectra than diffuse regions. 3) The skewness and
kurtosis of brightness fluctuations is high, indicative of strong
non-Gaussianity. 4) Based on our characterization of the 100 micron power
spectrum we provide a prescription of the cirrus confusion noise as a function
of wavelength and scale. 5) Finally we present a method based on a modification
of Gaussian random fields to produce simulations of dust maps which reproduce
the power spectrum and non-Gaussian properties of interstellar dust emission.Comment: 13 pages, 13 figures. Accepted for publication in A&
Cosmic Background dipole measurements with Planck-High Frequency Instrument
This paper discusses the Cosmic Background (CB) dipoles observations in the
framework of the Planck mission. Dipoles observations can be used in three
ways: (i) It gives a measurement of the peculiar velocity of our Galaxy which
is an important observation in large scale structures formation model. (ii)
Measuring the dipole can give unprecedent information on the monopole (that can
be in some cases hard to obtain due to large foreground contaminations). (iii)
The dipole can be an ideal absolute calibrator, easily detectable in
cosmological experiments. Following the last two objectives, the main goal of
the work presented here is twofold. First, we study the accuracy of the
Planck-HFI calibration using the Cosmic Microwave Background (CMB) dipole
measured by COBE as well as the Earth orbital motion dipole. We show that we
can reach for HFI, a relative calibration between rings of about 1% and an
absolute calibration better than 0.4% for the CMB channels (in the end, the
absolute calibration will be limited by the uncertainties on the CMB
temperature). We also show that Planck will be able to measure the CMB dipole
direction at better than 1.7 arcmin and improve on the amplitude. Second, we
investigate the detection of the Cosmic Far-Infrared Background (FIRB) dipole.
Measuring this dipole could give a new and independent determination of the
FIRB for which a direct determination is quite difficult due to Galactic dust
emission contamination. We show that such a detection would require a Galactic
dust emission removal at better than 1%, which will be very hard to achieve.Comment: 10 pages, 13 figures, submitted to A&A, uses aa.sty V5.
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