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 $-2.9 < \gamma < -2.6$. 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 $|v_{\mathrm{LSR}}| \leq 25$ \kms. For high latitudes $|b| > 20\deg$ 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 $\gamma \geq -3$ for the local gas. For intermediate velocity clouds (IVCs) we derive $\gamma = -2.6$ and for high velocity clouds (HVCs) $\gamma = -2.0$. Single-phase power distributions for the CNM, LNM, and WNM are highly correlated and shallow with $\gamma \sim -2.5$ for multipoles $l \leq 100$. 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

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

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