arXiv subjects

B. Winkel

Publications and source records attributed to B. Winkel.

45 records · Page 3Linked to original sources

arXiv2010

Absorption and emission line studies of gas in the Milky Way halo

We perform a systematic study of physical properties and distribution of neutral and ionised gas in the halo of the Milky Way (MW). Beside the large neutral intermediate- and high-velocity cloud (IVC, HVC) complexes there exists a population of partly ionised gaseous structures with low-column densities that have a substantial area filling factor. The origin and nature of these structures are still under debate. We analyse the physical parameters of the MW halo gas and the relation to quasar (QSO) metal-absorption line systems at low and high redshifts. For this purpose we combine new HI 21-cm data from the EBHIS and GASS surveys with optical quasar absorption line data to study the filling factor and distribution of these gaseous clouds in the halo at HI densities below 10^19 1/cm^2. This study is important to understand the evolution of the MW in particular and the gas accretion mechanisms of galaxies in general.

astro-ph.GA↗

arXiv2010

RFI mitigation for the Effelsberg-Bonn HI Survey (EBHIS)

A new L-band 7 feed array at the 100-m telescope is used to perform an unbiased, fully sampled HI survey of the whole northern hemisphere - the Effelsberg-Bonn HI Survey (EBHIS). The use of state-of-the-art digital Fast Fourier Transform spectrometers based on FPGAs - superior in dynamic range and allowing fast dumping of spectra - makes it possible to apply sophisticated RFI mitigation schemes. Based on the current status of the survey we discuss the RFI situation at the 100-m telescope and present a fast algorithm to automatically identify RFI in the raw data output from the spectrometer. Using simulations we show that it is feasible to detect more than 95% of all RFI in excess of 1 sigma amplitude with less than 1% false positives.

astro-ph.IM↗

arXiv2010

GASS: The Parkes Galactic All-Sky Survey. II. Stray-Radiation Correction and Second Data Release

The Parkes Galactic All-Sky Survey (GASS) is a survey of Galactic atomic hydrogen (HI) emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release was published by McClure-Griffiths et al. (2009). We remove instrumental effects that affect the GASS and present the second data release. We calculate the stray-radiation by convolving the all-sky response of the Parkes antenna with the brightness temperature distribution from the Leiden/Argentine/Bonn (LAB) all sky 21-cm line survey, with major contributions from the 30-m dish of the Instituto Argentino de Radioastronomia (IAR) in the southern sky. Remaining instrumental baselines are corrected using the LAB data for a first guess of emission-free baseline regions. Radio frequency interference is removed by median filtering. After applying these corrections to the GASS we find an excellent agreement with the Leiden/Argentine/Bonn (LAB) survey. The GASS is the highest spatial resolution, most sensitive, and is currently the most accurate HI survey of the Galactic HI emission in the southern sky. We provide a web interface for generation and download of FITS cubes.

astro-ph.GA↗

arXiv2010

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.

astro-ph.IM↗

arXiv2009

The Effelsberg Bonn HI Survey EBHIS

The Effelsberg-Bonn HI Survey (EBHIS) covers the whole sky north of Dec(2000) = -5 deg. on a fully sampled angular grid. Using state-of-the-art FPGA-spectrometers we perform a Milky Way and an extragalactic HI survey in parallel. Moreover, the high dynamic range and short dump time of the HI spectra allow to overcome the vast majority of all radio-frequency-interference (RFI) events. The Milky Way data will be corrected for the stray-radiation bias which warrants a main-beam efficiency of 99%. Towards the whole survey area we exceed the sensitivity limit of HIPASS, while towards the Sloan-Digital-Sky-Survey (SDSS) area EBHIS offers an order of magnitude higher mass sensitivity. The Milky Way data will be a cornerstone for multi-frequency astrophysics, while the extragalactic part will disclose detailed information on the structure formation of the local universe.

astro-ph.GA↗

arXiv2009

Data reduction strategy of the Effelsberg-Bonn HI Survey (EBHIS)

Since autumn 2008 a new L-band 7-Feed-Array receiver is used for an HI 21-cm line survey performed with the 100-m Effelsberg telescope. The survey will cover the whole northern hemisphere comprising both, the galactic and extragalactic sky in parallel. Using state-of-the-art FPGA based digital Fast Fourier Transform spectrometers, superior in dynamic range and temporal resolution, allows to apply sophisticated radio frequency interferences (RFI) mitigation schemes to the survey data. The EBHIS data reduction software includes the RFI mitigation, gain-curve correction, intensity calibration, stray-radiation correction, gridding, and source detection. We discuss the severe degradation of radio astronomical HI data by RFI signals and the gain in scientific yield when applying modern RFI mitigation schemes. For this aim simulations of the galaxy distribution within the local volume (z<0.07) with and without RFI degradation were performed. These simulations, allow us to investigate potential biases and selection effects introduced by the data reduction software and the applied source parametrization methods.

astro-ph.IM↗

arXiv2009

High-resolution 21-cm observations of low-column density gas clumps in the Milky Way halo

We study the properties of low-column density gas clumps in the halo of the Milky Way based on high-resolution 21-cm observations. Using interferometric data from the WSRT and the VLA we study HI emission at low-, intermediate- and high radial velocities along four lines of sight towards quasars. Along these sightlines we previously detected weak CaII and NaI absorbers in their optical spectra. The analysis of the high-resolution HI data reveals the presence of several compact and cold clumps of neutral gas at velocities similar to the optical absorption. The clumps have narrow HI line widths in the range of 1.8 to 13 km/s, yielding upper limits for the kinetic temperature of the gas of 70 to 3700 K. The neutral gas has low HI column densities in the range of 5E18 to 3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes. Our high-resolution 21-cm observations indicate that many of the CaII and NaI absorbers seen in our optical quasar spectra are associated with low-column density HI clumps at small angular scales. This suggests that next to the massive, high-column density neutral gas clouds in the halo (the common 21-cm LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas structures in the halo that remain mostly unseen in the existing 21-cm all-sky surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected HI clumps are consistent with what is expected from theoretical models of gas in the inner and outer Milky Way halo.

astro-ph.GA↗

arXiv2007

The Effelsberg-Bonn HI Survey (EBHIS)

The new L-band 7-feed-array at the 100-m telescope in Effelsberg will be used to perform an unbiased fully sampled HI survey of the entire northern hemisphere observing the galactic and extragalactic sky using simultaneously two different backends. The survey will be extremely valuable for a broad range of research topics: study of the low-mass end of the HI mass function (HIMF) in the local volume, environmental and evolutionary effects (as seen in the HIMF), the search for galaxies near low-redshift Lyman-alpha absorbers, and analysis of multiphase and extraplanar gas, HI shells, and ultra-compact high-velocity-clouds.

astro-ph↗

arXiv2007

The Robustness of Least-Squares Frequency Switching (LSFS)

Least-squares frequency switching (LSFS) is a new method to reconstruct signal and gain function (known as bandpass or baseline) from spectral line observations using the frequency switching method. LSFS utilizes not only two but a set of three or more local oscillator (LO) frequencies. The reconstruction is based on a least squares fitting scheme. Here we present a detailed investigation on the stability of the LSFS method in a statistical sense and test the robustness against radio frequency interference (RFI), receiver gain instabilities and continuum sources. It turns out, that the LSFS method is indeed a very powerful method and is robust against most of these problems. Nevertheless, LSFS fails in presence of RFI signals or strong line emission. We present solutions to overcome these limitations using a flagging mechanism or remapping of measured signals, respectively.

astro-ph↗