52 research outputs found
Gaussian decomposition of HI surveys. IV. Galactic intermediate- and high-velocity clouds
Traditionally IVC and HVC were defined to be concentrations of HI gas, with
line-of-sight velocities that are inconsistent with data on the differential
rotation of the Galaxy. We demonstrate that IVCs and HVCs can be identified
from certain density enhancements in (V_C, FWHM) distribution of Gaussians,
representing the Galactic HI 21 cm radio lines. We study the Gaussians, which
parameters fall into the regions of the phase-space density enhancements about
$(V_C, FWHM) = (-131, 27), (164, 26) and (-49 km/s, 23 km/s). The sky
distribution of the Gaussians, corresponding to the first two concentrations,
very well represents the sky distribution of HVCs, as obtained on the basis of
the traditional definition of these objects. The Gaussians of the last
concentration correspond to IVCs. Based on this identification, the division
line between IVCs and HVCs can be drawn at about |V_C| = 74 km/s, and IVCs can
be identified down to velocities of about |V_C| = 24 km/s. Traces of both IVCs
and HVCs can also be seen in the sky distribution of Gaussians with FWHM = 7.3
km/s. In HVCs, these cold cores have small angular dimensions and low observed
brightness temperatures T_b. In IVCs, the cores are both larger and brighter.
This definition of IVCs and HVCs is less dependant than the traditional one, on
the differential rotation model of the Galaxy. The consideration of line-width
information may enable IVCs and HVCs to be better distinguished from each
other, and from the ordinary Galactic HIComment: 9 pages, 7 figures. Accepted for publication in A&A. High-resolution
version available at http://www.aai.ee/~urmas/ast/HVCc.pdf (12.4 MBaits
Turbulent power distribution in the local interstellar medium
Context: The interstellar medium (ISM) on all scales is full of structures
that can be used as tracers of processes that feed turbulence. Aims: We used HI
survey data to derive global properties of the angular power distribution of
the local ISM. Methods: HI4PI observations on an nside = 1024 HEALPix grid and
Gaussian components representing three phases, the cold, warm, and unstable
lukewarm neutral medium (CNM, WNM, and LNM), were used for velocities
\kms. For high latitudes we
generated apodized maps. After beam deconvolution we fitted angular power
spectra. Results: Power spectra for observed column densities are exceptionally
well defined and straight in log-log presentation with 3D power law indices
for the local gas. For intermediate velocity clouds (IVCs) we
derive and for high velocity clouds (HVCs) .
Single-phase power distributions for the CNM, LNM, and WNM are highly
correlated and shallow with for multipoles .
Excess power from cold filamentary structures is observed at larger multipoles.
The steepest single-channel power spectra for the CNM are found at velocities
with large CNM and low WNM phase fractions. Conclusions: The phase space
distribution in the local ISM is configured by phase transitions and needs to
be described with three distinct different phases, being highly correlated but
having distributions with different properties. Phase transitions cause locally
hierarchical structures in phase space. The CNM is structured on small scales
and is restricted in position-velocity space. The LNM as an interface to the
WNM envelops the CNM. It extends to larger scales than the CNM and covers a
wider range of velocities. Correlations between the phases are self-similar in
velocity.Comment: 23 pages, 33 figures, A&A in pres
HI anisotropies associated with radio-polarimetric filaments. Steep power spectra associated with cold gas
LOFAR detected toward 3C 196 linear polarization structures which were found
subsequently to be closely correlated with cold filamentary HI structures. The
derived direction-dependent HI power spectra revealed marked anisotropies for
narrow ranges in velocity, sharing the orientation of the magnetic field as
expected for magneto hydrodynamical turbulence. Using the Galactic portion of
the Effelsberg-Bonn HI Survey we continue our study of such anisotropies in the
HI distribution in direction of two WSRT fields, Horologium and Auriga; both
are well known for their prominent radio-polarimetric depolarization canals. At
349 MHz the observed pattern in total intensity is insignificant but polarized
intensity and polarization angle show prominent ubiquitous structures with so
far unknown origin. Apodizing the HI survey data by applying a rotational
symmetric 50 percent Tukey window, we derive average and position angle
dependent power spectra. We fit power laws and characterize anisotropies in the
power distribution. We use a Gaussian analysis to determine relative abundances
for the cold and warm neutral medium. For the analyzed radio-polarimetric
targets significant anisotropies are detected in the HI power spectra; their
position angles are aligned to the prominent depolarization canals, initially
detected by WSRT. HI anisotropies are associated with steep power spectra.
Steep power spectra, associated with cold gas, are detected also in other
fields. Radio-polarimetric depolarization canals are associated with
filamentary HI structures that belong to the cold neutral medium (CNM).
Anisotropies in the CNM are in this case linked to a steepening of the
power-spectrum spectral index, indicating that phase transitions in a turbulent
medium occur on all scales. Filamentary HI structures, driven by thermal
instabilities, and radio-polarimetric filaments are associated with each other.Comment: Accepted for publication by A&A, 28 pages, 41 figures, minor updates
in styl
Aspect ratios of far-infrared and H i filaments in the diffuse interstellar medium at high Galactic latitudes
Dusty magnetized structures observable in the far-infrared (FIR) at high
Galactic latitudes are ubiquitous and found to be closely related to HI
filaments with coherent velocity structures. Considering dimensionless
morphological characteristics based on Minkowski functionals, we determine the
distribution of filamentarities and aspect ratios for these structures.
Our data are based on Planck FIR and HI4PI HI observations. Filaments have
previously been extracted by applying the Hessian operator. We trace individual
filamentary structures along the plane of the sky and determine and .
Filaments in the diffuse interstellar medium (ISM) are seldom isolated
structures, but are rather part of a network of filaments with a well-defined,
continuous distribution in and . This distribution is self-replicating,
and the merger or disruption of individual filamentary structures leads only to
a repositioning of the filament in and without changing the course of
the distribution. FIR and HI filaments identified at high Galactic latitudes
are a close match to model expectations for narrow filaments with approximately
constant widths. This distribution is continuous without clear upper limits on
the observed aspect ratios. Filaments are associated with enhanced column
densities of CO-dark . Radial velocities along the filaments are coherent
and mostly linear with typical dispersions of
km/s. The magnetic field strength in the diffuse turbulent ISM scales with
hydrogen volume density as . At high Galactic
latitudes, we determine an average turbulent magnetic field strength of
G and an average mean strength of the
magnetic field in the plane of the sky of G.Comment: 22 pages, 30 figure
Local HI filaments driven by a small-scale dynamo. Unraveling velocities and tangling of dusty magnetized structures
Context. HI filaments are closely related to dusty magnetized structures that
are observable in the far infrared (FIR). Recently it was proposed that the
coherence of oriented HI structures in velocity traces the line of sight
magnetic field tangling. Aims. We study the velocity-dependent coherence
between FIR emission at 857 GHz and HI on angular scales of 18 arcmin. Methods.
We use HI4PI HI data and Planck FIR data and apply the Hessian operator to
extract filaments. For coherence, we require that local orientation angles
{\theta} in the FIR at 857 GHz along the filaments be correlated with the HI.
Results. We find some correlation for HI column densities at |v_LSR | < 50 km/,
but a tight agreement between FIR and HI orientation angles {\theta} exists
only in narrow velocity intervals of 1 km/s. Accordingly, we assign velocities
to FIR filaments. Along the line of sight these HI structures show a high
degree of the local alignment with {\theta}, as well as in velocity space.
Interpreting these aligned structures in analogy to the polarization of dust
emission defines an HI polarization. We observe polarization fractions of up to
80%, with averages of 30%. Orientation angles {\theta} along the filaments,
projected perpendicular to the line of sight, are fluctuating systematically
and allow a characteristic distribution of filament curvatures to be
determined. Conclusions. Local HI and FIR filaments identified by the Hessian
analysis are coherent structures with well-defined radial velocities. HI
structures are also organized along the line of sight with a high degree of
coherence. The observed bending of these structures in the plane of the sky is
consistent with models for magnetic field curvatures induced by a Galactic
small-scale turbulent dynamo.Comment: 25 pages, 25 figures, accepted to A&
Global properties of the HI high velocity sky, a statistical investigation based on the LAB survey
We study the properties of all major HVC complexes from a sample compiled
1991 by Wakker & van Woerden (WvW). We use the Leiden/Argentine/Bonn all sky
21-cm line survey and decompose the profiles into Gaussian components. We find
a well defined multi-component structure for most of the HVC complexes. The
cold HVC phase has lines with typical velocity dispersions of sigma = 3 km/s
and exists only within more extended broad line regions, typically with sigma =
12 km/s. The motions of the cores relative to the envelopes are characterized
by Mach numbers M = 1.5. The center velocities of the cores within a HVC
complex have typical dispersions of 20 km/s. Remarkable is the well defined
two-component structure for some prominent HVC complexes in the outskirts of
the Milky Way: Complex H, the Magellanic Stream and the Leading Arm. There
might be some indications for an interaction between HVCs and disk gas at
intermediate velocities. This is possible for complex H, M, C, WB, WD, WE, WC,
R, G, GCP, and OA, but not for complex A, MS, ACVHV, EN, WA, and P.
Conclusions: The line widths, determined by us, imply that estimates of HVC
masses, as far as derived from the WvW database, need to be scaled up by a
factor 1.4. Correspondingly, guesses for the external pressure of a confining
coronal gas need to be revised upward by a factor of 2. The HVC multi-phase
structure implies in general that currently the halo pressure is significantly
underestimated. In consequence, the HVC multi-phase structure may indicate that
most of the complexes are circum-galactic. HVCs have turbulent energy densities
which are an order of magnitude larger than that of comparable clumps in the
Galactic disk.Comment: Accepted for publication in A&
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