Context. There are significant amounts of H2 in the Milky Way. Due to its symmetry H2 does not radiate at radio frequencies. CO is thought to be a tracer for H2; however, CO is formed at significantly higher opacities than H2. Thus, toward high Galactic latitudes significant amounts of H2 are hidden and are called CO–dark. Aims. We demonstrate that the dust-to-gas ratio is a tool for identifying locations and column densities of CO–dark H2. Methods. We adopt the hypothesis of a constant E(B−V)∕NH ratio, independent of phase transitions from H I to H2. We investigate the Doppler temperatures TD, from a Gaussian decomposition of HI4PI data, to study temperature dependences of E(B−V)∕NHI. Results. The E(B−V)∕NHI ratio in the cold H I gas phase is high in comparison to the warmer phase. We consider this as evidence that cold H I gas toward high Galactic latitudes is associated with H2. Beyond CO–bright regions, for TD ≤ 1165 K we find a correlation (NHI + 2NH2)∕NHI ∝−logTD. In combination with a factor XCO = 4.0 × 1020 cm−2 (K km s−1)−1 this yields NH∕E(B−V) ~ 5.1 to 6.7 × 1021 cm−2 mag−1 for the full sky, which is compatible with X-ray scattering and UV absorption line observations. Conclusions. Cold H I with TD ≤ 1165 K contains on average 46% CO–dark H2. Prominent filaments have TD ≤ 220 K and typical excitation temperatures Tex ~ 50 K. With a molecular gas fraction of ≥61% they are dominated dynamically by H2.

Abstract. -This article describes the stray-radiation correction that was applied to the HI observations of the Leiden/Dwingeloo survey of Hartmann & Burton. This correction involved convolving the empirically-determined antenna pattern with the measured all-sky HI distribution. The importance of the correction is demonstrated and practice regarding its application described. The general algorithm used here is presented. The results obtained with this algorithm are compared to those following from other methods. The 0.07 K sensitivity level of the survey depends critically on the success of the stray-radiation correction.

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 H i 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 LSR | ≤ 25 km s −1 . For high latitudes |b| > 20 • 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 γ ≥ −3 for the local gas. For intermediate velocity clouds (IVCs) we derive γ = −2.6 and for high velocity clouds (HVCs) γ = −2.0. Single-phase power distributions for the CNM, LNM, and WNM are highly correlated and shallow with γ ∼ −2.5 for multipoles l ≤ 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.

Abstract. Diffuse excess 1/4 keV soft X-ray emission was found to be positionally correlated with the column density distribution of the high velocity cloud (HVC) complex C (Kerp et al. 1996). Here we point out that the detected diffuse X-ray emission is indeed associated with the HVC phenomenon. For this purpose we study the 1/4 keV radiation transfer as well as the Hi column density distribution of HVCs and intermediate velocity clouds (IVCs) towards HVC complex C in detail. We present evidence that on arcmin scales the 3/4 keV soft X-ray emission is positionally anticorrelated with the HVC column density distribution of an individual HVC filament of complex C. IntroductionUp to now HVCs have been detected, in emission, only in the line radiation of the neutral atomic hydrogen (for a comprehensive review see Wakker & van Woerden 1997). Recent analyses of ROSAT data, suggest that HVCs also emit soft X-rays (Herbstmeier et al. 1995and Kerp et al. 1994, 1995, 1996. Here we focus on HVC complex C towards high galactic latitudes. We present a comparative analysis of X-ray and HI 21 cm line data available in this region (73° < I < 118°, 38° < b < 63°). In the l/4keV energy regime one has to disentangle the X-ray emission of the individual soft X-ray sources which contribute to the observed X-ray background in order to reveal the excess emission associated with the HVCs. Especially the intermediate-and low-velocity gas determines the intensity distribution of the soft X-ray sky, by photoelectric absorption. Because of the much smaller absorption cross section in the 3/4 keV band such a decomposition is not necessary. We will demonstrate that it is feasible to separate the emission of the "normal" soft X-ray background (SXRB) from that of the excess emission associated with HVCs. In consequence we have to solve the radiation transport of soft X-ray photons through the galactic interstellar medium (ISM) to identify sky areas which reveal deviations from this "normal" SXRB intensity distribution. Here we focus on two aspects which are important to understanding the recent results on X-ray emission of HVCs. First, we will show that the ROSAT 1/4 keV all-sky survey data reveal soft X-ray emission which is positionally correlated with the HVCs of complex C. We show new H121 cm line maps of the total and IVC column density distribution and discuss the relation of the individual cloud populations to the appearance of the soft X-ray intensity distribution on the sky. Second, we will discuss in Sect. 3 the small scale positional association of an individual HVC filament with excess soft X-ray emission in the ROSAT 3/4keV energy band. This is of importance because the 3/4 keV radiation can penetrate the galactic ISM without significant attenuation towards high galactic latitudes. T h e Soft X-ray BackgroundIt is a matter of discussion whether the SXRB has a patchy source intensity distribution or not (see also Pietz et al., Snowden, and Wang; this volume). For our purpose knowledge of the soft X-ray intensity distribution on ...

Context. The interstellar medium is affected by turbulence and observed H i structures in channel maps are shaped by turbulent motions. It is taken for granted by a few theoreticians that observed H i structures do not represent real density enhancement but velocity caustics, caused by velocity crowding. This interpretation was questioned and objections by Clark et al. led to violent debates. Aims. To settle the discussion we verify theoretical key parameters by using Effelsberg Bonn H i Survey (EBHIS) observations. Methods. We apply unsharp masking to determine filamentary H i structures at high spatial frequencies. In addition we use Gaussian parameters to distinguish the cold neutral medium (CNM) from observed H i column densities. We compare power spectra and spatial distributions of dust and H i column densities, distinguishing CNM and multiphase column densities at various velocity widths. Results. Observations contradict the Velocity Channel Analysis (VCA) postulate that the spectral index should steepen with the width of the velocity window. We rather find that the thin slice spectral index depends strongly on the H i phase composition. Multiphase power spectra are steeper for regions with cold gas. VCA denies such H i phase dependencies on the power distribution. Separating the CNM we find that the power spectra are significantly flatter than those for the multiphase H i composite. We observe excess CNM power for small scale structures originating from cold dust bearing filaments that are embedded in the CNM. Spectral indices for narrow channel widths depend on the Doppler temperature of the H i gas. In presence of enhanced small scale H i structure the far infrared emission from dust is also enhanced. Conclusions. Small scale cold filamentary H i structures are predominantly caused by density enhancements due to phase transitions rather than by velocity caustics.