arXiv subjects

J. Kerp

Publications and source records attributed to J. Kerp.

70 records · Page 4Linked to original sources

arXiv2002

The soft X-ray background towards the northern sky. A detailed analysis of the Milky Way halo

We present a correlation analysis of the diffuse X-ray background emission of the ROSAT all-sky survey with the Leiden/Dwingeloo 21-cm HI line survey. We derive a consistent model for the diffuse X-ray background emission over about 50% of the sky. Only three diffuse X-ray components are necessary to fit the ROSAT data from 0.1 keV to 2.4 keV: a) the Local Hot Bubble, b) the Milky Way Halo, and c) the extragalactic X-ray background. Only one temperature of the hot coronal gas in the Milky Way Halo is needed. Our model predicts, that a major fraction of the 1/4 keV and about 50% of the 3/4 keV diffuse X-ray emission originates from the Milky Way Halo. We detect a difference between the intensities towards the Galactic center and its anti-center, which is consistent with the electron density distribution independently derived from pulsar dispersion measurements.

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arXiv2002

The 3-D composition of the Galactic interstellar medium. The hot phases and X-ray absorbing material

We present maps of the Galactic X-ray Halo and constrain the location of X-ray absorbing gas using the X-ray radiation transfer as distance a indicator. Based on a consistent model of the soft X-ray background (SXRB) and the ROSAT all-sky survey, we derive the morphology of some components involved in the SXRB. In particular we focus on their spatial and temperature distributions across the sky. Starting from our SXRB model, we extract information on the 3-dimensional composition of the X-ray emitting and absorbing material. Using this approach, it is possible to differentiate between absorbing gas within or behind the Galactic X-ray Halo.

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arXiv2002

A window to the Galactic X-ray halo: The ISM towards the Lockman hole

We present a combined X-ray/HI-analysis of the ISM towards the Lockman hole. This sky region is considered as the "window to the distant universe" because of its absolute lowest HI column density on the whole sky. The Lockman hole appears to be not as transparent as the HI data suggest. We propose that about half of the ISM towards the Lockman hole is in form of ionized hydrogen rather than HI.

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arXiv2002

Searching for Stars in Compact High-Velocity Clouds. I First Results from VLT and 2MASS

We investigate the hypothesis that compact high-velocity clouds (CHVC) are the "missing" dwarf galaxies of the Local Group, by searching them for populations of resolved stars. To this end we conducted two distinct tests based on optical and near-infrared single-star photometry. The optical and the near-infrared experiments complement one another; the optical data help us to rule out distant populations but they are restricted to the central regions of the gas distributions, whereas the near-infrared photometry allows us to set limits on nearby populations spread over the typical cloud size. First, we discuss deep optical single-star photometry of five CHVCs in the V and I filters, obtained with the FORS instrument at the Very Large Telecope (VLT). We find that their optical colour-magnitude diagrams are indistinguishable from that of a population of Galactic stars, and attribute all of the resolved stars to Galactic foreground. We present simulations which address the question of how much of a "normal" dwarf-galaxy type population we might hide in the data. A Kolmogorov-Smirnov test allows us to set very stringent limits on the absence of a resolved stellar population in CHVCs. Second, we also culled near infrared single-star photometry in the J, H, and K_S bands for four of the CHVCs from the Two Micron All Sky Survey (2MASS). The infrared data do not reveal any stellar contents in the CHVCs which resembles that of nearby dwarf galaxies either, and are explained with Galactic foreground as well. We interpret our null detections to indicate that the five CHVCs investigated by us do not host an associated stellar content which is similar to that of the known dwarf galaxies of the Local Group. These CHVCs are very likely pure hydrogen clouds in which no star formation has taken place over cosmic time.

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arXiv2002

Searching for an Intrinsic Stellar Population in Compact High-Velocity Clouds

We are investigating the hypothesis that Compact High--Velocity Clouds (CHVCs) are the left-over building blocks of Local Group galaxies. To this end, we are searching for their embedded stellar populations using FORS at the VLT. The search is done with single-star photometry in V and I bands, which is sensitive to both, young and old, stellar populations. Five CHVCs of our sample have been observed so far down to I=24. We pointed the VLT towards the highest HI column density regions, as determined in Effelsberg radio data. In an alternate approach, we searched 2MASS public data towards those 5 CHVCs down to K=16. While the VLT data probe the central regions out to distance moduli of about 27, the 2MASS data are sensitive to a population of red giant stars to distance moduli of about 20. The 2MASS data, on the other hand, cover a much wider field of view than the VLT data (radius of 1 degree versus FORS field of 6.8 arcmin). We did not find a stellar population intrinsic to the CHVCs in either data. In this paper, we illustrate our search methods.

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arXiv2002

Hard X-ray emission from a young massive star-forming cluster

We report the detection of hard X-ray emission (>2 keV) from a number of point sources associated with the very young massive star-forming region IRAS 19410+2336. The X-ray emission is detected from several sources located around the central and most deeply embedded mm continuum source, which remains undetected in the X-ray regime. All X-ray sources have K-band counterparts, and those likely belonging to the evolving massive cluster show near-infrared colors in the 2MASS data indicative of pre-main-sequence stages. The X-ray luminosities around 10^{31} erg/s are at the upper end of luminosities known for low-mass pre-main-sequence sources, and mass estimates based on the infrared data indicate that at least some of the X-ray detected sources are intermediate-mass objects. Therefore, we conclude that the X-ray emission is due to intermediate-mass pre-main-sequence Herbig Ae/Be stars or their precursors. The emission process is possibly due to magnetic star-disk interaction as proposed for their low-mass counterparts.

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arXiv2002

On the Temperature and Intensity Distribution of the Galactic X-ray Plasma

We present the results of our investigation of the composition of the diffuse soft X-ray background emission (SXRB). Combining data of the Leiden/Dwingeloo HI Survey and the ROSAT All-Sky Survey (RASS), we set up a radiation transport equation in order to model the SXRB. Two different techniques lead to the model parameters: An image oriented approach which compares observed and modeled maps of the 1/4 and 3/4 keV X-ray energy regime and a more analytic approach using scatter diagrams. The analysis shows that only three independent components of the emitting plasma (local, halo and extragalactic) are needed to explain the SXRB. The results for the temperatures and X-ray intensities, which characterize the three components, are given and compared to an alternative model.

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arXiv2001

Deep H I observations of the compact high-velocity cloud HVC125+41-207

We present deep HI observations of the compact high-velocity cloud HVC125+41-207 using the 100-m Effelsberg telescope. Our goal was in particular to study the warm neutral medium (WNM) in detail. The Effelsberg data reveal a two phase core/halo structure - one component with a velocity width of FWHM approx 5 km/s (Westerbork data show FWHM approx 2 km/s, Braun & Burton 2000) and one with FWHM approx 18 km/s. The column density distribution of the warmer component is highly asymmetric and shows a head-tail structure. We performed a Gaussian decomposition of the cloud and found that 52% of the HI mass of the cloud is in the WNM. 24% of the WNM is located in the tail. The overall structure and the systematic variation of the observational parameters radial velocity, velocity dispersion and column density indicate that this cloud is currently interacting with the ambient medium. The Westerbork HI data of this HVC (Braun & Burton 2000) reveal an interesting dense condensation. Assuming that this condensation is virialized and in pressure equilibrium with the ambient medium, we derive a distance of 130 kpc for HVC125+41-207. Following these considerations, it is possible to constrain the parameters n_IGM < 7.8 10^-6 cm^-3 and T_IGM > 1.1 10^5 K of the intergalactic medium of the Local Group.

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arXiv2000

In search for the oldest star forming regions in Holmberg II

We present an X-ray study of the irregular dwarf galaxy Holmberg II based on deep ROSAT PSPC observations. Holmberg II is one of the most famous examples of an irregular dwarf galaxy with a disrupted interstellar medium (ISM): expanding HI holes are present across the entire face of the galaxy. Huge HI cavities of kpc extent are found even outside current starforming regions. We search for faint X-ray sources (stellar end points as well as hot gas), touching the limits of the ROSAT PSPC detector and link the newly detected X-ray sources to features detected in other wavelengths. Using an X-ray hardness ratio color-color diagram we show that it is possible to differentiate between thermal plasma and power-law X-ray spectra, which helps to track down the physical nature of the individual sources.

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arXiv2000

The First Complete and Fully Sampled HI Survey of the Tidal Arms of the Magellanic System

The LMC, SMC, and the Milky Way are a spectacular set of interacting galaxies. The prominent tidal arms which protrude from the Magellanic Clouds extend for tens of kiloparsecs and cover a large part of the southern sky. These arms, and the Magellanic Clouds themselves, are therefore a highly suitable subject for the detailed investigation of an interacting galaxy system. We present the first results of the Parkes narrow-band HI survey of the tidal arms of the Magellanic System. It is the first fully sampled, spatially complete survey of this system. The data provide information on the column density distribution and the variation of the shape of the line profiles over the extent of the tidal features. We present a cloud catalogue of the tidal features of the Magellanic System and discuss the distribution relative to the observational parameters longitude, latitude and radial velocity (v_LSR and v_GSR). We find a new stream-like feature that is most likely related to the Leading Arm. Detailed investigation reveals a number of clouds showing a cometary appearance in the position-velocity representation (so-called head-tail structures). These head-tail structures are interpreted as clouds that are currently interacting with an ambient medium.

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arXiv2000

The head-tail structure of high-velocity clouds - A survey of the northern sky

We present new observational results on high-velocity clouds (HVCs) based on an analysis of the Leiden/Dwingeloo HI survey. We cataloged all HVCs with NHI > 1 10^19 cm^-2 and found 252 clouds that form a representative flux limited sample. The detailed analysis of each individual HVC in this sample revealed a significant number of HVCs (nearly 20%) having simultaneously a velocity and a column density gradient. These HVCs have a cometary appearance in the position-velocity representation and are called henceforward head-tail HVCs (HT HVCs). The head is the region with the highest column density of the HVC, while the column density of the tail is in general much lower (by a factor of 2 - 4). The absolute majority of the cataloged HVCs belongs to the well known HVC complexes. With exception of the very faint HVC complex L, all HVC complexes contain HT HVCs. The HT HVCs were analyzed statistically with respect to their physical parameters like position, velocity (v_LSR,v_GSR), and column density. We found a linear correlation between the fraction of HVCs having a head-tail structure and the peak column density of the HVCs. While there is no correlation between the fraction of HT HVCs and v_LSR, we found a dependence of the fraction of HT HVCs and v_GSR. There is no significant correlation between the fraction of HT HVCs and the parameters galactic longitude and latitude. The HT HVCs may be interpreted as HVCs that are currently interacting with their ambient medium. In the context of this model the tails represent material that is stripped off from the HVC core. We discuss the implications of this model for galactic and extragalactic HVCs.

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arXiv1998

Soft X-rays from High-Velocity Clouds

The ROSAT mission gave us the unique possibility to study the galactic interstellar medium by ``shadowing'' observations on arcmin angular resolution level. The high sensitivity of the ROSAT PSPC in combination with the large field of view allows the detailed study of individual clouds, while the ROSAT all-sky survey is the main source for studies of the large-scale intensity distribution of the diffuse soft X-ray emission. We show that neutral hydrogen clouds are associated with soft X-ray emission, E(X-ray) < 3/4 keV. These neutral hydrogen clouds, known as high-velocity clouds (HVCs), are located partly within the galactic halo, and partly in intergalactic space. The ROSAT detection of soft X-ray emission from some of these HVCs is the first detection of HVCs in emission other than HI 21-cm line radiation.

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arXiv1998

HVCs probing a gaseous Galactic halo

We study the hypothesis that high-velocity clouds (HVCs) may originate from instabilities within the gaseous phase of the Galactic halo. According to the hydrostatic equilibrium model of Kalberla & Kerp (1998), we determine the probability distribution of occurrence of instabilities within the Galactic halo. Instabilities may produce condensations within the Galactic halo beyond a z-distances of 4 kpc, which are accelerated by gravity and decelerated by drag-forces. We present evidence that the HI high-velocity dispersion component, discovered by Kalberla et al. (1998) is associated with high-velocity-halo gas. The physical properties of this high-velocity-halo gas are similar to the recently detected highly-ionized HVCs by Sembach et al. (1995, 1998). Accordingly, the HI high-velocity dispersion component may be the neutral tracer of the turbulent gas motions within the Galactic halo. Our calculations demonstrate, that the sky-averaged signal of in-falling condensations does not differ significantly from the HI emission of the turbulent Galactic halo gas.

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arXiv1998

The gaseous Milky Way Halo

In 1998 several papers claim the detection of an ubiquitous gaseous phase within the Galactic halo. Here we like to focus on the detections of X-ray emitting gas within the Galactic halo as well as the discovery of a pervasive neutral Galactic halo gas. We discuss critically the major differences between the recent publications as well as the limitations of the analyses.

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arXiv1998

Hydrostatics of the Galactic Halo

We investigated a hydrostatic equilibrium model of the Milky Way following Parker (1966), to constrain the large scale properties of the interstellar medium. In our approach we found an excellent agreement between our simple hydrostatic equilibrium model of the Milky Way and the recent all-sky survey data rangeing from the gamma-ray to the radio regime. On large scales the galactic disk-halo system is found to be stable against Parker-instabilities. Pressure support from the Galactic disk is essential to stabilise the halo. In particular the diffuse ionised gas layer acts as a disk-halo interface. Assuming that the distribution of the soft X-ray emitting plasma traces the gravitational potential, we derived the dark matter content of the Milky Way to be about M ~ 2.8 10^11 M_o. Our findings are consistent with the rotation curve of the Galaxy.

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arXiv1998

A search for soft X-ray emission associated with prominent high-velocity-cloud complexes

We correlate the ROSAT 1/4 keV all-sky survey with the Leiden/Dwingeloo HI survey, looking for soft X-ray signatures of prominent high-velocity-cloud (HVC) complexes. We study the transfer of 1/4 keV photons through the interstellar medium in order to distinguish variations in the soft X-ray background (SXRB) intensity caused by photoelectric absorption effects from those due to excess X-ray emission. The X-ray data are modelled as a combination of emission from the Local Hot Bubble (LHB) and emission from a distant plasma in the galactic halo and extragalactic sources. The X-ray radiation intensity of the galactic halo and extragalactic X-ray background is modulated by the photoelectric absorption of the intervening galactic interstellar matter. We show that large- and small-scale intensity variations of the 1/4 keV SXRB are caused by photoelectric absorption which is predominantly traced by the total N(HI) distribution. The extensive coverage of the two surveys supports evidence for a hot, X-ray emitting corona. We show that this leads to a good representation of the SXRB observations. For four large areas on the sky, we search for regions where the modelled and observed X-ray emission differ. We find that there is excess X-ray emission towards regions near HVC complexes C, D, and GCN. We suggest that the excess X-ray emission is positionally correlated with the high-velocity clouds. Some lines of sight towards HVCs also pass through significant amounts of intermediate-velocity gas, so we cannot constrain the possible role played by IVC gas in these directions of HVC and IVC overlap, in determining the X-ray excesses.

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