The Effelsberg-Bonn HI Survey (EBHIS)

The Effelsberg-Bonn HI survey (EBHIS) comprises an all-sky survey north of Dec = -5 degrees of the Milky Way and the local volume out to a red-shift of z ~ 0.07. Using state of the art Field Programmable Gate Array (FPGA) spectrometers it is feasible to cover the 100 MHz bandwidth with 16.384 spectral channels. High speed storage of HI spectra allows us to minimize the degradation by Radio Frequency Interference (RFI) signals. Regular EBHIS survey observations started during the winter season 2008/2009 after extensive system evaluation and verification tests. Until today, we surveyed about 8000 square degrees, focusing during the first all-sky coverage of the Sloan-Digital Sky Survey (SDSS) area and the northern extension of the Magellanic stream. The first whole sky coverage will be finished in 2011. Already this first coverage will reach the same sensitivity level as the Parkes Milky Way (GASS) and extragalactic surveys (HIPASS). EBHIS data will be calibrated, stray-radiation corrected and freely accessible for the scientific community via a web-interface. In this paper we demonstrate the scientific data quality and explore the expected harvest of this new all-sky survey.Comment: 19 pages, 11 figures, accepted for publication by Astronomical Note

RFI Mitigation for the Parkes Galactic All-Sky Survey (GASS)

The GASS is a survey of Galactic atomic hydrogen (HI) emission in the southern sky observed with the Parkes 64-m Radio Telescope. With a sensitivity of 60 mK for a channel width of 1 km/s the GASS is the most sensitive and most accurate survey of the Galactic HI emission in the southern sky. We discuss RFI mitigation strategies that have been applied during the data reduction. Most of the RFI could be cleaned by using prior information on the HI distribution as well as statistical methods based on median filtering. Narrow line RFI spikes have been flagged during the first steps of the data processing. Most of these lines were found to be constant over long periods of time, such data were replaced by interpolating profiles from the Leiden/Argentine/Bonn (LAB) survey. Remaining RFI was searched for at any position by a statistical comparison of all observations within a distance of 0.1 deg. The median and mean of the line emission was calculated. In cases of significant deviations between both it was checked in addition whether the associated rms fluctuations exceeded the typical scatter by a factor of 3. Such data were replaced by the median, which is found to be least biased by RFI and other artifacts. The median estimator was found to be equivalent to the mean, which was obtained after rejecting outliers.Comment: accepted for publication in the Proceedings of the RFI mitigation workshop 29-31 March 2010 Groningen, the Netherland

The Effelsberg-Bonn HI Survey: Data reduction

Starting in winter 2008/2009 an L-band 7-Feed-Array receiver is used for a 21-cm line survey performed with the 100-m telescope, the Effelsberg-Bonn HI survey (EBHIS). The EBHIS will cover the whole northern hemisphere for decl.>-5 deg comprising both the galactic and extragalactic sky out to a distance of about 230 Mpc. Using state-of-the-art FPGA-based digital fast Fourier transform spectrometers, superior in dynamic range and temporal resolution to conventional correlators, allows us to apply sophisticated radio frequency interference (RFI) mitigation schemes. In this paper, the EBHIS data reduction package and first results are presented. The reduction software consists of RFI detection schemes, flux and gain-curve calibration, stray-radiation removal, baseline fitting, and finally the gridding to produce data cubes. The whole software chain is successfully tested using multi-feed data toward many smaller test fields (1--100 square degrees) and recently applied for the first time to data of two large sky areas, each covering about 2000 square degrees. The first large area is toward the northern galactic pole and the second one toward the northern tip of the Magellanic Leading Arm. Here, we demonstrate the data quality of EBHIS Milky Way data and give a first impression on the first data release in 2011.Comment: 17 pages, 14 figures; to be published in ApJ

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&

The Leiden/Argentine/Bonn (LAB) Survey of Galactic HI: Final data release of the combined LDS and IAR surveys with improved stray-radiation corrections

We present the final data release of observations of lambda 21-cm emission from Galactic neutral hydrogen over the entire sky, merging the Leiden/Dwingeloo Survey (LDS: Hartmann & Burton, 1997) of the sky north of delta = -30 deg with the Instituto Argentino de Radioastronomia Survey (IAR: Arnal et al., 2000, and Bajaja et al., 2005) of the sky south of delta = -25 deg. The angular resolution of the combined material is HPBW ~ 0.6 deg. The LSR velocity coverage spans the interval -450 km/s to +400 km/s, at a resolution of 1.3 km/s. The data were corrected for stray radiation at the Institute for Radioastronomy of the University of Bonn, refining the original correction applied to the LDS. The rms brightness-temperature noise of the merged database is 0.07 - 0.09 K. Residual errors in the profile wings due to defects in the correction for stray radiation are for most of the data below a level of 20 - 40 mK. It would be necessary to construct a telescope with a main beam efficiency of eta_{MB} > 99% to achieve the same accuracy. The merged and refined material entering the LAB Survey of Galactic HI is intended to be a general resource useful to a wide range of studies of the physical and structural characteristices of the Galactic interstellar environment. The LAB Survey is the most sensitive Milky Way HI survey to date, with the most extensive coverage both spatially and kinematically.Comment: 9 pages, 5 figures, accepted for publication by Astronomy & Astrophysic

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&

Global properties of the HI distribution in the outer Milky Way

Aims: We derive the 3-D HI volume density distribution for the Galactic disk out to R = 60 kpc. Methods: Our analysis is based on parameters for the warp and rotation curve derived previously. The data are taken from the Leiden/Argentine/Bonn all sky 21-cm line survey. Results: The Milky Way HI disk is significantly warped but shows a coherent structure out to R = 35 kpc. The radial surface density distribution, the densities in the middle of the warped plane, and the HI scale heights all follow exponential relations. The radial scale length for the surface density distribution of the HI disk is 3.75 kpc. Gas at the outskirts for 40 < R < 60 kpc is described best by a distribution with an exponential radial scale length of 7.5 kpc and a velocity dispersion of 74 km/s. Such a highly turbulent medium fits also well with the average shape of the high velocity profile wings observed at high latitudes. The turbulent pressure gradient of such extra-planar gas is on average in balance with the gravitational forces. About 10% of the Milky Way HI gas is in this state. The large scale HI distribution is lopsided; for R < 15 kpc there is more gas in the south. The HI flaring indicates that this asymmetry is caused by a dark matter wake, located at R = 25 kpc in direction of the Magellanic System. Conclusions: The HI disk is made up of two major components. Most prominent is the normal HI disk which can be traced to R = 35 kpc. This is surrounded by a patchy distribution of highly turbulent gas reaching large scale heights but also large radial distances. At the position of the Sun the exponential scale height in the z direction is 3.9 kpc. This component resembles the anomalous gas discovered previously in some galaxies.Comment: to be published in A&