286 research outputs found
Anisotropies in the HI gas distribution toward 3C196
The local Galactic HI gas was found to contain cold neutral medium (CNM)
filaments that are aligned with polarized dust emission. These filaments appear
to be dominated by the magnetic field and in this case turbulence is expected
to show distinct anisotropies. We use the Galactic Effelsberg--Bonn HI Survey
(EBHIS) to derive 2D turbulence spectra for the HI distribution in direction to
3C196 and two more comparison fields. Prior to Fourier transform we apply a
rotational symmetric 50% Tukey window to apodize the data. We derive average as
well as position angle dependent power spectra. Anisotropies in the power
distribution are defined as the ratio of the spectral power in orthogonal
directions. We find strong anisotropies. For a narrow range in position angle,
in direction perpendicular to the filaments and the magnetic field, the
spectral power is on average more than an order of magnitude larger than
parallel. In the most extreme case the anisotropy reaches locally a factor of
130. Anisotropies increase on average with spatial frequency as predicted by
Goldreich and Sridhar, at the same time the Kolmogorov spectral index remains
almost unchanged. The strongest anisotropies are observable for a narrow range
in velocity and decay with a power law index close to --8/3, almost identical
to the average isotropic spectral index of . HI
filaments, associated with linear polarization structures in LOFAR observations
in direction to 3C196, show turbulence spectra with marked anisotropies.
Decaying anisotropies appear to indicate that we witness an ongoing shock
passing the HI and affecting the observed Faraday depth.Comment: minor errors corrected, 15 pages, 29 figures, accepted for
publication by A&
Properties of extra-planar HI clouds in the outer part of the Milky Way
There is mounting evidence for an extra-planar gas layer around the Milky Way
disk, similar to the anomalous HI gas detected in a few other galaxies. As much
as 10% of the gas may be in this phase. We analyze HI clouds located in the
disk-halo interface outside the solar circle to probe the properties of the
extra-planar HI gas, which is following Galactic rotation. We use the
Leiden/Argentine/Bonn (LAB) 21-cm line survey to search for HI clouds which
take part in the rotation of the Galactic plane, but are located above the disk
layer. Selected regions are mapped with the Effelsberg 100-m telescope. Two of
the HI halo clouds are studied in detail for their small scale structure using
the Westerbork Synthesis Radio Telescope (WSRT). Data from the 100m telescope
allow for the parameterization of 25 distinct HI halo clouds at Galactocentric
radii 10 kpc <R<15 kpc and heights 1 kpc <z<5 kpc. The clouds have a median
temperature of 620 K, column densities of NH~10E19 cm^-2, and most of them are
surrounded by an extended envelope of warmer HI gas. Interferometer
observations for two selected regions resolve the HI clouds into several
arc-minute sized cores. These cores show narrow line widths (FWHM ~3 km/s),
they have volume densities of n > 1.3 cm^-3, masses up to 24 M_{sol}, and are
on average in pressure equilibrium with the surrounding envelopes. Pressures
and densities fall within the expectations from theoretical phase diagrams (P
vs ). The HI cores tend to be unstable if one assumes a thermally
bistable medium, but are in better agreement with models that predict thermal
fragmentation driven by a turbulent flow.Comment: 9 pages, 5 figures, 3 tables, Accepted for publication in A&
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
Caustics and velocity caustics in the diffuse interstellar medium at high Galactic latitudes
The FIR distribution at high Galactic latitudes, observed with Planck, is
filamentary with coherent structures in polarization. These structures are also
closely related to HI filaments with coherent velocity structures. There is a
long-standing debate about the physical nature of these structures. They are
considered either as velocity caustics, fluctuations engraved by the turbulent
velocity field or as cold three-dimensional density structures in the
interstellar medium (ISM). We discuss different approaches to data analysis and
interpretation in order to work out the differences. We considered mathematical
preliminaries for the derivation of caustics that characterize filamentary
structures in the ISM. Using the Hessian operator, we traced individual FIR
filamentary structures in HI from channel maps as observed and alternatively
from data that are provided by the velocity decomposition algorithm (VDA). VDA
is claimed to separate velocity caustics from density effects. Based on the
strict mathematical definition, the so-called velocity caustics are not
actually caustics. These VDA data products may contain caustics in the same way
as the original HI observations. Caustics derived by a Hessian analysis of both
databases are nearly identical with a correlation coefficient of 98%. However,
the VDA algorithm leads to a 30% increase in the alignment uncertainties when
fitting FIR/HI orientation angles. We used HI absorption data to constrain the
physical nature of FIR/HI filaments and determine spin temperatures and volume
densities of FIR/HI filaments. HI filaments exist as CNM structures; outside
the filaments no CNM absorption is detectable. The CNM in the diffuse ISM is
exclusively located in filaments with FIR counterparts. These filaments at high
Galactic latitudes exist as cold density structures; velocity crowding effects
are negligible.Comment: 16 pages, 11 figure
The soft X-ray background towards the northern sky. A detailed analysis of the Milky Way halo
We present a correlation analysis of the diffuse X-ray background emission of
the ROSAT all-sky survey with the Leiden/Dwingeloo 21-cm HI line survey. We
derive a consistent model for the diffuse X-ray background emission over about
50% of the sky. Only three diffuse X-ray components are necessary to fit the
ROSAT data from 0.1 keV to 2.4 keV: a) the Local Hot Bubble, b) the Milky Way
Halo, and c) the extragalactic X-ray background. Only one temperature of the
hot coronal gas in the Milky Way Halo is needed. Our model predicts, that a
major fraction of the 1/4 keV and about 50% of the 3/4 keV diffuse X-ray
emission originates from the Milky Way Halo. We detect a difference between the
intensities towards the Galactic center and its anti-center, which is
consistent with the electron density distribution independently derived from
pulsar dispersion measurements.Comment: Astron. Nachr. in press, issue dedicated to the proceedings of the
workshop "X-ray Surveys in the light of New Observatories", Sep. 2002,
Santander, Spai
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