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J. Kerp

Publications and source records attributed to J. Kerp.

At least 55 records · Page 3Linked to original sources
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.

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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.

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arXiv2007

Dark matter in the Milky Way, II. the HI gas distribution as a tracer of the gravitational potential

Context. Gas within a galaxy is forced to establish pressure balance against gravitational forces. The shape of an unperturbed gaseous disk can be used to constrain dark matter models. Aims. We derive the 3-D HI volume density distribution for the Milky Way out to a galactocentric radius of 40 kpc and a height of 20 kpc to constrain the Galactic mass distribution. Methods. We used the Leiden/Argentine/Bonn all sky 21-cm line survey. The transformation from brightness temperatures to densities depends on the rotation curve. We explored several models, reflecting different dark matter distributions. Each of these models was set up to solve the combined Poisson-Boltzmann equation in a self-consistent way and optimized to reproduce the observed flaring. Results. Besides a massive extended halo of M ~ 1.8 10^{12} Msun, we find a self-gravitating dark matter disk with M=2 to 3 10^{11} Msun, including a dark matter ring at 13 < R < 18.5 kpc with M = 2.2 to 2.8 10^{10} Msun. The existence of the ring was previously postulated from EGRET data and coincides with a giant stellar structure that surrounds the Galaxy. The resulting Milky Way rotation curve is flat up to R~27 kpc and slowly decreases outwards. The \hi gas layer is strongly flaring. The HWHM scale height is 60 pc at R = 4 kpc and increases to ~2700$ pc at R=40 kpc. Spiral arms cause a noticeable imprint on the gravitational field, at least out to R = 30 kpc. Conclusions. Our mass model supports previous proposals that the giant stellar ring structure is due to a merging dwarf galaxy. The fact that the majority of the dark matter in the Milky Way for $R \la 40$ kpc can be successfully modeled by a self-gravitating isothermal disk raises the question of whether this massive disk may have been caused by similar merger events in the past.

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arXiv2006

Searching for Stars in Compact High-Velocity Clouds. II

We address the hypothesis that High Velocity Clouds correspond to the "missing" dwarf galaxies of the Local Group predicted by cosmological simulations. To this end, we present optical and near-infrared photometry of five additional High Velocity Clouds, one of which produces Lyman series absorption on the sight line towards the Quasar Ton S210, with sufficient resolution and sensitivity to enable the detection of an associated stellar content. We do not detect significant stellar populations intrinsic to any of the five clouds. In combination with the results from our paper I, which had yielded non detections of stellar content in another five cases, we find that there is a 50% chance of getting a null result in ten trials if fewer than 7% of all High Velocity Clouds contain stars. We conclude that the population of High Velocity Clouds is an unlikely repository for the "missing" dwarfs of the Local Group.

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arXiv2005

XMM-Newton Data Processing for Faint Diffuse Emission: Proton Flares, Exposure Maps and Report on EPIC MOS1 Bright CCDs Contamination

We present a study of the in-flight performance of the XMM-Newton EPIC MOS and pn detectors, with focus on the influence of proton flares and vignetting on the data. The very wide range in the conditions of our sample of observations, in terms of exposure length and background intensities, allows the detection of a wide range in the spectra of the proton flares, in contrast to the hard-spectrum flares proposed by Lumb et al.(2002) or Read et al.(2003) We also find an up to now unreported contamination in the low energy regime (E<0.5 keV) of the MOS1 observations, consisting of a significant increase in the measured intensities in two CCDs at the edges of the detector. This contamination yields in "bright CCDs" in the observations. Its effect must be taken into account for the study of sources detected in the affected CCDs. With respect to vignetting, we present in-flight exposure maps and we propose a method to repeat this calculation for user-definable energy bands. All the results presented here, have the goal to enable the study of very faint extended sources with XMM-Newton, like nearby galactic X-ray halos or the soft X-ray background.

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arXiv2005

Weak lensing study of dark matter filaments and application to the binary cluster A 222 and A 223

We present a weak lensing analysis of the double cluster system Abell 222 and Abell 223. The lensing reconstruction shows evidence for a possible dark matter filament connecting both clusters. The case for a filamentary connection between A 222/223 is supported by an analysis of the galaxy density and X-ray emission between the clusters. Using the results of N-body simulations, we try to develop a criterion that separates this system into cluster and filament regions. The aim is to find a technique that allows the quantification of the significance of (weak lensing) filament candidates in close pairs of clusters. While this mostly fails, the aperture quadrupole statistics (Schneider & Bartelmann 1997) shows some promise in this area. The cluster masses determined from weak lensing in this system are considerably lower than those previously determined from spectroscopic and X-ray observations (Dietrich et al. 2002; Proust et al. 2000; Wang et al. 1999). Additionally, we report the serendipitous weak lensing detection of a previously unknown cluster in the field of this double cluster system.

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arXiv2005

Compact high-velocity clouds around the Galaxy and M31

We have observed 11 compact high-velocity clouds in the 21-cm line of neutral hydrogen with the 100-m telescope in Effelsberg. Our observations show that most of the clouds have a rather complex morphology. Head-tail structures and bow-shock shapes suggest that many of the CHVCs are interacting with an ambient medium, providing additional evidence for a circumgalactic CHVC population. In order to verify the concept of a circumgalactic population directly, we have started an HI blind survey to search for CHVCs in the neighbourhood of M31, using the Effelsberg telescope. We can confirm some of the extended HI structures discovered by Thilker et al. (2004) with the Green Bank Telescope (GBT), whereas we have not yet found any cloud similar to the CHVCs observed near the Milky Way. Our non-detection suggests an upper limit for the distance of CHVCs from their host galaxies of about 60 kpc.

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arXiv2005

A Field Programmable Gate Array Spectrometer for Radio Astronomy

We describe the technological concept and the first-light results of a 1024-channel spectrometer based on field programmable gate array (FPGA) hardware. This spectrometer is the prototype for the seven beam L-band receiver to be installed at the Effelsberg 100-m telescope in autumn 2005. Using "of-the-shelf" hardware and software products, we designed and constructed an extremely flexible Fast-Fourier-Transform (FFT) spectrometer with unprecedented sensitivity and dynamic range, which can be considered prototypical for spectrometer development in future radio astronomy.

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arXiv2005

Effelsberg HI observations of compact high-velocity clouds

We have mapped 11 compact high-velocity clouds (CHVCs) in the 21-cm line emission of neutral, atomic hydrogen, using the Effelsberg 100-m radio telescope. The aim of our observations was to study the overall distribution of the warm neutral medium of CHVCs with high sensitivity. The achieved baseline rms of about 50 mK at the original 2.6 km/s velocity resolution allows us to search for evidence of ram-pressure interaction with the ambient medium. In addition, we have obtained spectra along an appropriate axis across each CHVC with longer integration times and denser angular sampling. These deep slices with a spectral baseline rms of about 25...35 mK allow us to determine the column density profile in greater detail as well as the velocity and line width gradient across each cloud. The most outstanding result of our observations is the complexity of the HI column density distribution and the line profiles of the investigated CHVCs. We have found only one cloud with a spherically-symmetric appearance. Among the remaining clouds we observe head-tail structures, bow-shock shapes, and objects with irregular shapes. These complex morphologies in combination with the obtained physical parameters suggest that ram-pressure interactions with an ambient medium may play a significant role in shaping some of the CHVCs from our sample. These results are consistent with a circumgalactic distribution of CHVCs with typical distances of the order of 100 kpc. The pressure of the ambient medium might also stabilise CHVCs in addition to their own gravitational potential.

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arXiv2004

A Multiband Approach to AGN: Radioscopy & Radio Astronomy

Only in the radio-loud population of active galactic nuclei (AGN) does the production, collimation, and acceleration of powerful relativistic jets take place. We introduce here a concept of combined VLBI- and X-ray spectroscopic observations of sources with relativistic, broad iron lines. This approach has enormous potential to yield deep insights into the accretion/jet-production process in AGN. Better knowledge of the milliarcsecond-resolution radio structure of the nuclear radio cores in so-called ``radio-quiet'' broad-iron-line Seyfert galaxies is essential for future combined radio/X-ray studies of the different modes of radio-jet production in accreting black hole systems.

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arXiv2004

The Parkes HI Survey of the Magellanic System

We present the first fully and uniformly sampled, spatially complete HI survey of the entire Magellanic System with high velocity resolution, performed with the Parkes Telescope. The final data-cubes have an rms noise of sigma ~ 0.05 K and an effective angular resolution of 16 arcmin. The Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) are associated with huge gaseous features with a total HI mass of M(HI) = 4.87 10^8 M_sun [d/55 kpc]^2, if all HI gas is at the same distance of 55 kpc. Approximately two thirds of this HI gas is located close to the Magellanic Clouds (Magellanic Bridge and Interface Region), and 25% of the HI gas is associated with the Magellanic Stream. The Leading Arm has a four times lower HI mass than the Magellanic Stream, corresponding to 6% of the total HI mass of the gaseous features. We have analyzed the velocity field of the Magellanic Clouds and their neighborhood introducing a LMC-standard-of-rest frame. The HI in the Magellanic Bridge shows low velocities relative to the Magellanic Clouds suggesting an almost parallel motion, while the gas in the Interface Region has significantly higher relative velocities indicating that this gas is leaving the Magellanic Bridge building up a new section of the Magellanic Stream. The clouds in the Magellanic Stream and the Leading Arm show significant differences, both in the column density distribution and in the shapes of the line profiles. The HI gas in the Magellanic Stream is more smoothly distributed than the gas in the Leading Arm. These morphological differences can be explained if the Leading Arm is at considerably lower z-heights and embedded in a higher pressure ambient medium.

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arXiv2004

Combined VLBI- and X-ray Observations of Active Galactic Nuclei

Compact radio cores in radio-loud active galactic nuclei (AGNs) are classical targets of Very-Long-Baseline interferometric (VLBI) research. Until today little has been known about their X-ray properties in comparison to radio-quiet AGNs. Here, we present results from a systematic X-ray spectral survey of radio-loud AGNs. This study is based on a statistically complete sample of regularly monitored compact, extragalactic radio jets from the VLBA 2 cm Survey and makes use of high-quality X-ray spectroscopic data from the archives of the X-ray observatories ASCA, BeppoSax, CHANDRA, and XMM-Newton. Combined VLBI and X-ray observations of AGNs can yield direct observational links between the central parsecs of extragalactic jets and dynamical processes within the accretion flows around supermassive black holes. It is essential to establish such links, which might have the power to attack the unsolved problem: "What makes an AGN radio-loud?" We present some early results from our survey project and report briefly on detailed studies of 0716+714, NGC 1052, and 3C 390.3.

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arXiv2004

Probes of Jet-Disk-Coupling in AGN from Combined VLBI and X-Ray Observations

The formation of powerful extragalactic jets is not well understood at present as well as the associated key question:``What makes an AGN radio loud?''. Here we discuss how the combination of VLBI and X-ray spectroscopic observations allows the inter-relation between the accretion flow and the formation of relativistic jets in AGN to be explored.

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arXiv2004

Jet emission in NGC1052 at radio, optical, and X-ray frequencies

We present a combined radio, optical, and X-ray study of the nearby LINER galaxy NGC 1052. Data from a short (2.3 ksec) {\it CHANDRA} observation of NGC 1052 reveal the presence of various jet-related X-ray emitting regions, a bright compact core and unresolved knots in the jet structure as well as an extended emitting region inside the galaxy well aligned with the radio synchrotron jet-emission. The spectrum of the extended X-ray emission can best be fitted with a thermal model with $kT = (0.4-0.5)$ keV, while the compact core exhibits a very flat spectrum, best approximated by an absorbed power-law with $N_{\rm H} = (0.6-0.8) \times 10^{22} {\rm cm^{-2}}$. We compare the radio structure to an optical ``structure map'' from a {\it Hubble Space Telescope} ({\it HST}) observation and find a good positional correlation between the radio jet and the optical emission cone. Bright, compact knots in the jet structure are visible in all three frequency bands whose spectrum is inconsistent with synchrotron emission.

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arXiv2003

ESO422-G028: The Host Galaxy of a GRG

ESO422-G028 denotes the very center cD-galaxy of the giant radio source B0503-286 (Saripalli et al. 1986, Subrahmanya and Hunstead, 1986). The angular extent of the associated radio structure is about 42.4 arcmin, which corresponds to a linear size of 1.89 Mpc (with Omega_M=0.27, Omega_Lambda=0.73, and H_0=71 km/s/Mpc). Here we present new high-frequency total-power and polarization radio maps obtained with the Effelsberg 100-m dish. In addition, we correlate the radio data with optical and X-ray observations to investigate the physical conditions of both, the host galaxy and the extended structure.

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arXiv2003

Effelsberg HI Survey of Compact High-Velocity Clouds

We studied 11 compact high-velocity clouds (CHVCs) in the 21-cm line emission of neutral hydrogen with the 100-m telescope in Effelsberg. We find that most of our CHVCs are not spherically-symmetric as we would expect in case of a non-interacting, intergalactic population. Instead, many CHVCs reveal a complex morphology suggesting that they are disturbed by ram-pressure interaction with an ambient medium. Thus, CHVCs are presumably located in the neighborhood of the Milky Way instead of being spread across the entire Local Group.

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arXiv2003

The Twin-Jet of NGC1052 at Radio, Optical, and X-ray Frequencies

We present results from a combined radio, optical, and X-ray study of the jet-associated emission features in NGC1052. We analyse the radio-optical morphology and find a good positional correlation between the radio jet and the optical emission cone. Two optical emission knots are directly associated with radio counterparts exhibiting a radio to X-ray broadband spectrum not compatible with synchrotron emission. We discuss the possibility that the thermal soft spectrum of the extended X-ray emission originates from jet driven shocks produced in the interaction between the jet-plasma and its surrounding medium.

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