365 research outputs found
Effects of dust absorption on spectroscopic studies of turbulence
We study the effect of dust absorption on the recovery velocity and density
spectra as well as on the anisotropies of magnetohydrodynamic turbulence using
the Velocity Channel Analysis (VCA), Velocity Coordinate Spectrum (VCS) and
Velocity Centroids. The dust limits volume up to an optical depth of unity. We
show that in the case of the emissivity proportional to the density of
emitters, the effects of random density get suppressed for strong dust
absorption intensity variations arise from the velocity fluctuations only.
However, for the emissivity proportional to squared density, both density and
velocity fluctuations affect the observed intensities. We predict a new
asymptotic regime for the spectrum of fluctuations for large scales exceeding
the physical depths to unit optical depth. The spectrum gets shallower by unity
in this regime. In addition, the dust absorption removes the degeneracy
resulted in the universal spectrum of intensity fluctuations of
self-absorbing medium reported by Lazarian \& Pogosyan. We show that the
predicted result is consistent with the available HII region emission data. We
find that for sub-Alfv\'enic and trans-Alfv\'enic turbulence one can get the
information about both the magnetic field direction and the fundamental
Alfv\'en, fast and slow modes that constitute MHD turbulence.Comment: Published in MNRAS, minor changes to match the published versio
The three dimensional skeleton: tracing the filamentary structure of the universe
The skeleton formalism aims at extracting and quantifying the filamentary
structure of the universe is generalized to 3D density fields; a numerical
method for computating a local approximation of the skeleton is presented and
validated here on Gaussian random fields. This method manages to trace well the
filamentary structure in 3D fields such as given by numerical simulations of
the dark matter distribution on large scales and is insensitive to monotonic
biasing. Two of its characteristics, namely its length and differential length,
are analyzed for Gaussian random fields. Its differential length per unit
normalized density contrast scales like the PDF of the underlying density
contrast times the total length times a quadratic Edgeworth correction
involving the square of the spectral parameter. The total length scales like
the inverse square smoothing length, with a scaling factor given by 0.21 (5.28+
n) where n is the power index of the underlying field. This dependency implies
that the total length can be used to constrain the shape of the underlying
power spectrum, hence the cosmology. Possible applications of the skeleton to
galaxy formation and cosmology are discussed. As an illustration, the
orientation of the spin of dark halos and the orientation of the flow near the
skeleton is computed for dark matter simulations. The flow is laminar along the
filaments, while spins of dark halos within 500 kpc of the skeleton are
preferentially orthogonal to the direction of the flow at a level of 25%.Comment: 17 pages, 11 figures, submitted to MNRA
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