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 $|v_{\mathrm{LSR}}| \leq 25$ \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

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 $F$ and aspect ratios $A$ 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 $A$ and $F$. 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 $A$ and $F$. This distribution is self-replicating, and the merger or disruption of individual filamentary structures leads only to a repositioning of the filament in $A$ and $F$ 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 $H_2$. Radial velocities along the filaments are coherent and mostly linear with typical dispersions of $\Delta v_{\mathrm{LSR}} = 5.24$ km/s. The magnetic field strength in the diffuse turbulent ISM scales with hydrogen volume density as $B \propto n_{\mathrm{H}}^{0.58}$. At high Galactic latitudes, we determine an average turbulent magnetic field strength of $\langle \delta B \rangle = 5.3 ~\mu$G and an average mean strength of the magnetic field in the plane of the sky of $\langle B_{\mathrm{POS}} \rangle = 4.4 ~\mu$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&