arXiv subjects

Dieter Breitschwerdt

Publications and source records attributed to Dieter Breitschwerdt.

50 records · Page 3Linked to original sources

arXiv2005

Ovi Column Density Distribution in the Local Bubble - Results from 3D Adaptive Mesh Refinement Simulations

The Local Bubble (LB) is an X-ray emitting region extending 100 pc in radius in the Galactic plane and 300 pc perpendicular to it, and it is embedded in a somewhat larger H{\sc i} deficient cavity. Its origin and spectral properties in UV, EUV and X-rays are still poorly understood. We have performed 3D high resolution (down to 1.25 pc) hydrodynamic superbubble simulations of the LB and Loop I superbubble in a realistic \emph{inhomogeneous} background ISM, disturbed by supernova (SN) explosions at the Galactic rate. We can reproduce (i) the size of the bubbles (in contrast to similarity solutions), (ii) the interaction shell with Loop I, discovered with ROSAT, (iii) predict the merging of the two bubbles in about 3 Myr, when the interaction shell starts to fragment, and, (iv) the generation of blobs like the Local Cloud as a consequence of a dynamical instability. The \ion{O}{6} column densities are monitored and found to be in excellent agreement with \textsc{Copernicus} and FUSE absorption line data, showing LB column densities $<1.7 \times 10^{13} {\rm cm}^{-2}$, in contrast to other existing models.

astro-ph↗

arXiv2004

Volume Filling Factors of the ISM Phases in Star Forming Galaxies - I. The Role of the Disk-Halo Interaction

The role of matter circulation between the disk and halo in establishing the volume filling factors of the different ISM phases in the Galactic disk is investigated, using a modified version of the three-dimensional supernova-driven ISM model of Avillez (2000). We carried out adaptive mesh refinement simulations of the ISM with five supernova rates (in units of the Galactic value), $σ/σ_{Gal}=1$, 2, 4, 8 and 16 (corresponding to starburst conditions) using three finer level resolutions of 2.5, 1.25 and 0.625 pc, allowing us to understand how resolution would affect the volumes of gas phases in pressure equilibrium. We find that the volume filling factors of the different ISM phases depend sensitively on the existence of a duty cycle between the disk and halo acting as a pressure release valve for the hot (T> 10^5.5 K) phase in the disk.

astro-ph↗

arXiv2004

3D Simulations of the Local Bubble

We have performed high resolution 3D simulations of the Local Bubble (with 1.25 pc finest resolution) in a realistic background ISM, jointly with the dynamical evolution of the neighbouring Loop I superbubble. We can reproduce (i) the size of the bubbles (in contrast to similarity solutions), (ii) the interaction shell with Loop I, discovered with ROSAT, (iii) predict the merging of the two bubbles in about 3 Myr, when the interaction shell starts to fragment, and, (iv) the generation of blobs like the Local Cloud as a consequence of a dynamical instability.

astro-ph↗

arXiv2004

3D HD and MHD Adaptive Mesh Refinement Simulations of the Global and Local ISM

We have performed high resolution 3D simulations with adaptive mesh refinement, following the ISM evolution in a star forming galaxy both on small (<1 pc) and large (>10 kpc) scales, enabling us to track structures in cooling shock compressed regions as well as the entire Galactic fountain flow. It is shown in an MHD run that the latter one is not inhibited by a large scale disk parallel magnetic field. The fountain plays a vital role in limiting the volume filling factor of the hot gas. Contrary to classical models most of the gas between 100K and 8000 K is found to be thermally unstable. On scales of superbubbles we find that the internal temperature structure is rather inhomogeneous for an old object like our Local Bubble, leading to low OVI column densities, consistent with observations.

astro-ph↗

arXiv2004

Is the Local Bubble dead?

We give a summary of the current state of Local Bubble research, resulting from the discussions of a dedicated panel meeting. After more than 25 years of intense observational and theoretical work, we are still far from a coherent picture, although a probable one emerges at the horizon. A multi-supernova origin seems to be the best guess, with non-equilibrium cooling and soft X-ray emission accompanying its expansion. In addition our vantage point may force us to accept a substantial but quantitatively unknown contribution from heliospheric emission.

astro-ph↗

arXiv2003

High Resolution Simulations of the Global and Local ISM

We present the first to date high resolution calculations of the ISM down to scales of 0.625 pc of the global and local ISM. The simulations show the morphology and structure of the different ISM phases and reproduce many of the features that have been observed in the Milky Way and other galaxies. In particular, they show that the hot gas has a moderately low volume filling factor (~20%) even in the absence of magnetic fields. Also, cold gas is mainly concentrated in filamentary structures running perpendicular to the midplane forming and dissipating within 10-12 Myr. Compression is the dominant process for their formation, but thermal instability also plays a role. Also the evolution of the Local Bubble is simulated by multi-supernova explosions; calculated extensions after ~13 Myr match observations.

astro-ph↗

arXiv2003

Observation and Modelling of Starburst Driven Galactic Winds: A Review in honour of John Dyson

Starburst galaxies are generally associated with extended X-ray and radio halos, giving a clear hint of an outflow of gas and relativistic particles from the disk into the halo. The driving agents are, not surprisingly, active star forming regions, injecting hot gas and cosmic rays generated by supernova remnant and superbubble shock waves. The dynamics of the outflow and the thermal evolution of the plasma are strongly coupled, and therefore a self-consistent model is necessary for a satisfactory description. Since the plasma temperature is between one and a few million Kelvin, the halo is most conspicuous in soft X-rays. It will be shown that X-ray data obtained recently with XMM-Newton EPIC pn bear clear spectral signatures that are in strong disagreement with an isothermal halo in collisional ionization equilibrium (CIE). Instead a temperature structure in X-ray halos is observed. It will be demonstrated that a galactic wind outflow model in which the non-equilibrium ionization structure is calculated self-consistently can give a satisfactory and physical explanation for the X-ray spectral characteristics, e.g. in case of the local starburst galaxy NGC 3079. In particular, high subsolar abundances, that have been reported from X-ray observations in some cases, are shown to be artefacts of CIE spectral fit models. It is found that spectral models are strongly constrained by the presence of diagnostic lines such as OVII and OVIII as well as Fe-L line complexes. Thus a detailed spectral X-ray model will help to provide important galactic wind parameters, such as mass loss rate and velocity profile, which may also serve as valuable input for galactic outflows in the early universe.

astro-ph↗

arXiv2003

Self-consistent Modelling of the interstellar medium

The dynamical evolution of hot optically thin plasmas in the ISM crucially depends on the heating and cooling processes. It is essential to realize that all physical processes that contribute operate on different time scales. In particular detailed balancing is often violated since the statistically inverse process of e.g. collisional ionization is recombination of an ion with two electrons, which as a three-body collision is usually dominated by radiative recombination, causing a departure from collisional ionization equilibrium. On top of these differences in atomic time scales, hot plasmas are often in a dynamical state, thereby introducing another time scale, which can be the shortest one. The non-equilibrium effects will be illustrated and discussed in the case of galactic outflows. It will be shown, that spectral analyses of X-ray data of edge-on galaxies show a clear signature in the form of "multi-temperature" halos, which can most naturally be explained by the "freezing-in" of highly ionized species in the outflow, which contribute to the overall spectrum by DELAYED RECOMBINATION. This naturally leads to a non-equilibrium cooling function, which modifies the dynamics, which in turn changes the plasma densities and thermal energy budget, thus feeding back on the ionization structure. Therefore SELF-CONSISTENT modelling is needed.

astro-ph↗

arXiv2001

Chandra Detection of a Hot Gaseous Corona Around the Edge-on Galaxy NGC 4631

We present a Chandra X-ray observation that shows, unambiguously for the first time, the presence of a giant diffuse X-ray-emitting corona around the edge-on disk galaxy NGC 4631. This corona, with a characteristic thermal temperature of 2-7\times10^6 K, extends as far as 8 kpc away from the galactic plane. The X-ray morphology resembles the radio halo of the galaxy, indicating a close connection between outflows of hot gas, cosmic rays, and magnetic field from the galactic disk. Enhanced diffuse X-ray emission is apparently enclosed by numerous Hα-emitting loops blistered out from the central disk of the galaxy, as is evident in a comparison with our deep Hubble Space Telescope imaging.

astro-ph↗

arXiv1999

Galactic Fountains and Galactic Winds

The development of galactic fountain theories is reviewed with special emphasis on the different approaches and concepts that have been used in the past. In particular the outstanding contribution of Franz Kahn to our physical understanding is appreciated. It is argued that galactic outflows represent an essential phase during galaxy evolution. The dynamics of the outflow imprints its signature on the emission spectra of soft X-rays, which may well be observable with AXAF and XMM. Finally, some remarks about winds in starburst galaxies and AGN are made.

astro-ph↗

arXiv1999

X-ray emission mechanisms of LINERs

We present here an analysis of the X-ray properties of a sample of LINER galaxies observed with the ROSAT PSPC and HRI instruments. A spatial analysis shows that the bulk of the X-ray emission is consistent with arising from a point source; some extended emission appears at weak emission levels. The X-ray spectra are formally best described by a powerlaw with photon index Gamma_x ~ -2 or thermal emission from a Raymond-Smith plasma with highly subsolar abundances (Z < 0.1). Several emission mechanisms that might contribute to the observed X-ray spectra are discussed. In particular, we take the very subsolar abundances derived from Raymond-Smith fits as an indication of a more complex emission mechanism, like the presence of a second hard component or plasma out of collisional-ionization equilibrium.

astro-ph↗

arXiv1999

Dusty warm absorbers: The case of IRAS 13349+2438

Warm absorbers are an important new probe of the central region of active galaxies (AGN). Observing and modeling this component provides a wealth of information on the nature of the warm absorber itself, its relation to other components of the active nucleus, and the intrinsic AGN X-ray spectral shape. We briefly review the general properties of dusty warm gas. For the first time, we then apply such a model to the IR loud quasar IRAS 13349+2438. It was the first to be suggested to host a dusty warm absorber (Brandt et al. 1996), but has not yet been modeled as such.

astro-ph↗

arXiv1999

The exceptional X-ray properties of the NLSy1 galaxy RXJ 0134.3-4258

The Narrow-line Seyfert 1 galaxy RXJ0134.3-4258 was detected during the ROSAT all-sky survey with an ultrasoft spectrum (Greiner 1996). Our later pointed observation led to the discovery of drastic spectral variability. Here, we present the first detailed analysis of the soft X-ray properties of this peculiar source.

astro-ph↗

arXiv1999

The dynamical signature of the ISM in soft X-rays -- I. Diffuse soft X-rays from galaxies

We present the first dynamically and thermally self-consistent calculations of fast adiabatically expanding gas flows from the Galactic disk into the halo. It is shown that in a hot plasma (T >= 10^6 K) with a high overpressure with respect to the ambient medium, the dynamical time scale is much shorter than the intrinsic time scales (e.g. for recombination, collisional excitation and ionization etc.). Therefore dynamical models that use collisional ionization equilibrium (CIE) cooling functions for the evolution of the plasma are in general not correct. In particular, the emission spectra obtained from non-equilibrium calculations are radically different. We describe a method to obtain self-consistent solutions using an iterative procedure. It is demonstrated that soft X-ray background emission between 0.3 and 1.5 keV can be well explained by a superposition of line emission and delayed recombination of an initially hot plasma streaming away from the Galactic disk (outflow and/or winds). In addition to these local winds we also present calculations on global winds from spiral galaxies, which originate from a hot and quiescent galactic corona. We also emphasize that it is dangerous to derive plasma temperatures merely from line ratios of ionized species, such as NV/OVI, unless the dynamical and thermal history of the plasma is known.

astro-ph↗