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Martin Krause

Publications and source records attributed to Martin Krause.

52 records · Page 3Linked to original sources
arXiv2010

Stability of Cloud Orbits in the Broad Line Region of Active Galactic Nuclei

We investigate the global dynamic stability of spherical clouds in the Broad Line Region (BLR) of Active Galactic Nuclei (AGN), exposed to radial radiation pressure, gravity of the central black hole (BH), and centrifugal forces assuming the clouds adapt their size according to the local pressure. We consider both, isotropic and anisotropic light sources. In both cases, stable orbits exist also for very sub-Keplerian rotation for which the radiation pressure contributes substantially to the force budget. We demonstrate that highly eccentric, very sub-Keplerian stable orbits may be found. This gives further support for the model of Marconi et al. 2008, who pointed out that black hole masses might be significantly underestimated if radiation pressure is neglected. That model improved the agreement between black hole masses derived in certain active galaxies based on BLR dynamics, and black hole masses derived by other means in other galaxies by inclusion of a luminosity dependent term. For anisotropic illumination, energy is conserved for averages over long time intervals, only, but not for individual orbits. This leads to Rosetta orbits that are systematically less extended in the direction of maximum radiation force. Initially isotropic relatively low column density systems would therefore turn into a disk when an anisotropic AGN is switched on.

astro-ph.CO↗
arXiv2010

Asymmetries in Extragalactic Double Radio Sources: Clues from 3D Simulations of Jet - Disc Interaction

Observational and theoretical studies of extragalactic radio sources have suggested that an inhomogeneous environment may be responsible for observed arm length asymmetries of jets and the properties of extended emission line regions in high redshift radio galaxies. We perform 3D hydrodynamic simulations of the interaction of a powerful extragalactic bipolar jet with a disc-shaped clumpy interstellar medium of log-normal density distribution and analyze the asymmetry. Furthermore, we compute the relation between jet asymmetry and the ISM properties by means of Monte Carlo simulations based on a 1D propagation model for the jet through the dense medium. We find that the properties of the ISM can be related to a probability distribution of jet arm length asymmetries: Disc density and height are found to have the largest effect on the asymmetry for realistic parameter ranges, while the Fourier energy spectrum of the ISM and turbulent Mach number only have a smaller effect. The hydrodynamic simulations show that asymmetries generally may be even larger than expected from the 1D model due to the complex interaction of the jet and its bow shock with gaseous clumps, which goes much beyond simple energy disposal. From our results, observed asymmetries of medium-sized local radio galaxies may be explained by gas masses of 10^9 to 10^10 solar masses in massive elliptical galaxies. Furthermore, the simulations provide a theoretical basis for the observed correlation that emission line nebulae are generally found to be brighter on the side of the shorter lobe in high redshift radio galaxies (McCarthy et al. 1991). This interaction of jets with the cold gas phase suggests that star formation in evolving high redshift galaxies may be affected considerably by jet activity.

astro-ph.HE↗
arXiv2009

Measurements of the Cosmological Evolution of Magnetic Fields with the Square Kilometre Array

We investigate the potential of the Square Kilometre Array (SKA) for measuring the magnetic fields in clusters of galaxies via Faraday rotation of background polarised sources. [...] We find that about 10 per cent of the sky is covered by a significant extragalactic Faraday screen. Most of it has rotation measures between 10 and 100 rad/m/m. We argue that the cluster centres should have up to about 5000 rad/m/m. We show that the proposed mid frequency aperture array of the SKA as well as the lowest band of the SKA dish array are well suited to make measurements for most of these rotation measure values, typically requiring a signal-to-noise of ten. We calculate the spacing of sources forming a grid for the purpose of measuring foreground rotation measures: it reaches a spacing of 36 arcsec for a 100 hour SKA observation per field. We also calculate the statistics for background RM measurements in clusters of galaxies. We find that a first phase of the SKA would allow us to take stacking experiments out to high redshifts (>1), and provide improved magnetic field structure measurements for individual nearby clusters. The full SKA aperture array would be able to make very detailed magnetic field structure measurements of clusters with more than 100 background sources per cluster up to a redshift of 0.5 and more than 1000 background sources per cluster for nearby clusters, and could for reasonable assumptions about future measurements of electron densities in high redshift clusters constrain the power law index for the magnetic field evolution to better than dm=0.4, if the magnetic field in clusters should follow B ~ (1+z)^m.

astro-ph.CO↗
arXiv2009

Physics and fate of jet related emission line regions

At high redshift, extragalactic jets are associated with extended emission line regions. We present global and local hydrodynamic simulations of the interaction of jets with their environment relevant to this phenomenon. We determine the fraction of kinetic energy and momentum of the jet that should appear in the emission line gas. From momentum considerations, we argue that the gas seen in outward motion in many examples must to a large part originate from within the galaxy. The host galaxies should have been massive, gas rich and heavily star forming galaxies. The expelled gas is not expected to return to the galaxy quickly. The observed kinetic energy in the emission line gas constrains the jet powers to be greater than 10^47 erg/s.

astro-ph.CO↗
arXiv2008

Dynamical and chemical effects of FR II radio sources on the intra-cluster medium

We investigate the effects of intermittent strong jets from an Active Galactic Nuclei (AGN) of a massive galaxy in the core of a cool core galaxy cluster, on the dynamics and metal distribution of the intra-cluster medium (ICM). We use a simple model for the metal distribution within the host galaxy which includes metal injection via star formation. We carry out 2D axisymmetric hydrodynamic simulations of these systems. After having established a cooling flow, two light jets are injected in opposite directions with a range of (intermittent) active phases. We follow the time evolution of the system from the jets' active phases up to 3 Gyr. The general metallicity evolution for all our simulations is very similar on large-scales. The convective flows driven by the jets advect gas and metals from the central galaxy to distances beyond 1.5 Mpc within the cluster. Intermittent jets are able to distribute metals to grater radii. The metal injection has effects on the general metal abundances present in the ICM, the faster the metal replenishment time the higher the metal abundance. Although metallicity gradients in the very central regions of cool core clusters are likely to be shaped by less-energetic phenomena, we find evidence in our simulations for metallicity gradients similar to those observed out to ~400 kpc radius. The metal distribution details of the central galaxy have no effects on the dynamical evolution of the ICM metals.

astro-ph↗
arXiv2006

Chemical enrichment of the intracluster medium by FR II radio sources

We present 2D axisymmetric hydrodynamic simulations investigating the long term effect of FR II radio galaxies on the metal distribution of the surrounding intra-cluster medium (ICM). A light jet is injected into a cooling flow atmosphere for 10-30 Myr. We then follow the subsequent evolution for 3 Gyr on a spherical grid spanning 3 Mpc in radius. A series of passive tracer particles were placed in an annulus about the cluster core to simulate metal carrying clouds in order to calculate the metallicity (Z) as a function of time and radial distance from the cluster centre. The jet has a significant effect on the ICM over the entire 3 Gyr period. By the end of the simulations, the jets produced metallicities of ~10% of the initial metallicity of the cluster core throughout much of the cluster. The jets transport the metals not only in mixing regions, but also through upwelling ICM behind the jet, enriching the cluster over both long and short distances.

astro-ph↗
arXiv2006

Simulations of multi-phase turbulence in jet cocoons

The interaction of optically emitting clouds with warm X-ray gas and hot, tenuous radio plasma in radio jet cocoons is modelled by 2D compressible hydrodynamic simulations. The initial setup is the Kelvin-Helmholtz instability at a contact surface of density contrast 10,000. The denser medium contains clouds of higher density. Optically thin radiation is realised via a cooling source term. The cool phase effectively extracts energy from the other gas which is both, radiated away and used for acceleration of the cold phase. This increases the system's cooling rate substantially and leads to a massively amplified cold mass dropout. We show that it is feasible, given small seed clouds of order 100 solar masses, that all of the optically emitting gas in a radio jet cocoon may be produced by this mechanism on the propagation timescale of the jet. The mass is generally distributed as T^-1/2 with temperature, with a prominent peak at 14,000 K. This peak is likely to be related to the counteracting effects of shock heating and a strong rise in the cooling function. The volume filling factor of cold gas in this peak is of the order 10^-5 to 10^-3 and generally increases during the simulation time. The simulations tend towards an isotropic scale free Kolmogorov-type energy spectrum over the simulation timescale. We find the same Mach-number density relation as Kritsuk & Norman (2004) and show that this relation may explain the velocity widths of emission lines associated with high redshift radio galaxies, if the environmental temperature is lower, or the jet-ambient density ratio is less extreme than in their low redshift counterparts.

astro-ph↗
arXiv2006

Simulations of the interaction of cold gas with radio jets

A new scenario for the interaction of a jet with a background medium with cold clouds is investigated by means of hydrodynamic turbulence simulations with cooling. The idea is that the cold clouds are overtaken by a radio cocoon and stirred up by turbulence in this cocoon. The 2D multiphase turbulence simulations contain all the three gas phases and have a number of interesting properties. The produced power spectrum is proportional to the inverse square of the wavevector. The Mach number - density relation may explain the observed velocities in emission line gas associated with radio galaxies. The model also explains the increased optical visibility within the radio structures, the correlation between emission line and radio luminosity, and the evolution of the alignment effect with source size. I also propose this to be a useful model for the recently discovered neutral outflows in nearby radio sources.

astro-ph↗
arXiv2005

Galactic Wind Shells and High Redshift Radio Galaxies On the Nature of Associated Absorbers

A jet is simulated on the background of a galactic wind headed by a radiative bow shock. The wind shell, which is due to the radiative bow shock, is effectively destroyed by the impact of the jet cocoon, thanks to Rayleigh-Taylor instabilities. Associated strong HI absorption, and possibly also molecular emission, in high redshift radio galaxies which is observed preferentially in the smaller ones may be explained by that model, which is an improvement of an earlier radiative bow shock model. The model requires temperatures of $\approx 10^6$ K in the proto-clusters hosting these objects, and may be tested by high resolution spectroscopy of the Ly$α$ line. The simulations show that -- before destruction -- the jet cocoon fills the wind shell entirely for a considerable time with intact absorption system. Therefore, radio imaging of sources smaller than the critical size should reveal the round central bubbles, if the model is correct.

astro-ph↗
arXiv2005

Evolution of the ISM and Galactic Activity

We study the effects of time-dependent mass injection and heating on the evolution of the interstellar medium (ISM) in elliptical galaxies. As the large and luminous ellipticals have supermassive black holes at their cores, which were probably much less massive in the young universe, feeding these black holes is essential. We examine steady state solutions and describe the impact of the initial starburst on the evolution of the ISM and consequences for galactic activity, based on results from Starburst99.

astro-ph↗
arXiv2004

Very Light Jets II: Bipolar large scale simulations in King atmospheres

Hydrodynamic jets, underdense with respect to their environment by a factor of up to 10,000, were computed in axisymmetry as well as in 3D. They finally reached a size of up to 220 jet radii, corresponding to a 100 kpc sized radio galaxy. The simulations are "bipolar", involving both jets. These are injected into a King type density profile with small stochastic density variations. The back-reaction of the cocoons on the beams in the center produces armlength asymmetries of a few percent, with the longer jets on the side with the higher average density. Two distinguishable bow shock phases were observed: an inner elliptical part, and a later cylindrical, cigar-like phase, which is known from previous simulations. The sideways motion of the inner elliptical bow shock part is shown to follow the law of motion for spherical blast waves also in the late phase, where the aspect ratio is high, with good accuracy. X-ray emission maps are calculated and the two bow shock phases are shown to appear as rings and elongated or elliptical regions, depending on the viewing angle. Such structures are observed in the X-ray data of several radio galaxies (e.g. in Abell 2052 and Hercules A), the best example being Cygnus A. In this case, an elliptical bow shock is infered from the observations, a jet power of 10^47 erg/s is derived, and the Lorentz factor can be limited to Gamma>10. Based on the simulation results and the comparison to the observations, the emission line gas producing the alignment effect in radio galaxies at high redshift is suggested to be cooled gas entrained over the cocoon boundary.

astro-ph↗
arXiv2004

Interaction of Jets with Galactic Winds

We have used the vectorised and parallelised magnetohydrodynamics code NIRVANA on the NEC SX-5 and the new SX-6 installation in parallel mode to simulate the interaction of jets with a galactic wind that might be typical for the star-bursting radio-galaxies of the early universe. The two simulations, one axisymmetric and one in 3D show that the jet pierces and destroys a thin and dense shell produced by the pre-installed superwind. We suggest that small radio galaxies at high redshift might be absorbed on the blue wing due to the galactic wind shell, and possibly on the red wing due to a cooling flow. In larger sources the jet cocoon will fill the wind cavity and accelerate the shell. The Rayleigh-Taylor instability will then disrupt the shell and disperse dense, possibly star-forming fragments throughout the region.

astro-ph↗
arXiv2003

Parameters for Very Light Jets of cD Galaxies

Recent Chandra X-ray observations of jets in central cluster galaxies show interesting features around the symmetry plane. We have carried out simulations involving bipolar jets, removing the artificial boundary condition at the symmetry plane. We use a very low jet density (IGM/jet roughly 10000) and take into account a decreasing density profile. We find that the jet bow shock undergoes two phases: First a nearly spherical one and second the well-known cigar-shaped one. We propose Cygnus A to be in a transition phase, clear signs from both phases. Due to inward growing of Kelvin Helmholtz instabilities (KHI) between cocoon and shocked IGM, mass entrainment is observed predominantly in the symmetry plane. We propose this mechanism to produce some of the so far enigmatic X-ray features in the symmetry plane in Cygnus A.

astro-ph↗
arXiv2002

Very Light Jets I. Axisymmetric Parameter Study and Analytic Approximation

The propagation of extragalactic jets is studied by a series of twelve axisymmetric hydrodynamic simulations. Motivated by observational constraints, but unlike most previous simulations, the regime of jet to external medium density (eta) from 10^-5 to 10^-2 is explored, for Mach numbers (M) between 2.6 and 26. The computational domain contained the bow shocks for the whole simulation time. The bow shocks are found to be spherical at source sizes below a critical value r1 (blastwave phase), which can reach up to 10 jet radii. [...] The numerical work is complemented by an analytic approach for the spherical phase. Extending previous work, the radial force balance could be integrated for arbitrary background density and energy input, which results in a global solution. The analytic results are shown to be consistent with the numerical work, and a lower limit to r1 can be calculated, which falls below the numerical results by a few jet radii. It is shown explicitely how a King density distribution changes the discussed aspects of the bow shock propagation. Because the jet head propagates very fast in the blastwave phase, it turns out that it is not possible to ``frustrate'' a jet by a high density environment. This is very important for the class of small radio galaxies (compact symmetric objects / GHz peaked sources): They have to be young. During its blastwave phase, a powerful jet can transfer typically 10^60 erg to the environmental gas. This is enough to balance the radiative losses in a cooling flow, if one of the cluster galaxies harbours a powerful jet every Gigayear.

astro-ph↗
arXiv2002

Absorbers and Globular Cluster Formation in Powerful High Redshift Radio Galaxies

A radiative hydrodynamic simulation for a typical, powerful high redshift radio galaxy is presented. The jet is injected at one third the speed of light into a 10000 times denser, homogeneous medium. In the beginning of the simulation, the bow shock consists of a spherical shell that is similar to a spherical blast wave. This shell cools radiatively down to approximately 10,000 K, providing after 6 Mio yrs a neutral column of 3.8 x 10^21 cm^-2 around the whole system. The shell starts to fragment and forms condensations. This absorbing screen will cover a smaller and smaller fraction of the radio source, and therefore the emission line region, and eventually form stars in typically 10,000 globular clusters of one million solar masses. Approximately 10^9 solar masses are entrained into the radio cocoon. This gas, cooling and illuminated from the radio source, could be the emission line gas observed in high redshifted radio galaxies and radio loud quasars. The neutral column behind the bow shock can account for the absorption found in almost all of the small sources. The globular cluster excess of approximately 10^4 systems found in present day brightest cluster galaxies (BCGs), which are believed to be the vestiges of these objects, is consistent with the presented scenario.

astro-ph↗