arXiv subjects

M. Krause

Publications and source records attributed to M. Krause.

At least 91 records · Page 5Linked to original sources

arXiv2009

[The Impact of Nuclear Star Formation on Gas Inflow to AGN

Our adaptive optics observations of nearby AGN at spatial resolutions as small as 0.085arcsec show strong evidence for recent, but no longer active, nuclear star formation. We begin by describing observations that highlight two contrasting methods by which gas can flow into the central tens of parsecs. Gas accumulation in this region will inevitably lead to a starburst, and we discuss the evidence for such events. We then turn to the impact of stellar evolution on the further inflow of gas by combining a phenomenological approach with analytical modelling and hydrodynamic simulations. These complementary perspectives paint a picture in which all the processes are ultimately regulated by the mass accretion rate into the central hundred parsecs, and the ensuing starburst that occurs there. The resulting supernovae delay accretion by generating a starburst wind, which leaves behind a clumpy interstellar medium. This provides an ideal environment for slower stellar outflows to accrete inwards and form a dense turbulent disk on scales of a few parsecs. Such a scenario may resolve the discrepancy between the larger scale structure seen with adaptive optics and the small scale structure seen with VLTI.

astro-ph.CO↗

arXiv2009

Very Light Magnetized Jets on Large Scales - I. Evolution and Magnetic Fields

Magnetic fields, which are undoubtedly present in extragalactic jets and responsible for the observed synchrotron radiation, can affect the morphology and dynamics of the jets and their interaction with the ambient cluster medium. We examine the jet propagation, morphology and magnetic field structure for a wide range of density contrasts, using a globally consistent setup for both the jet interaction and the magnetic field. The MHD code NIRVANA is used to evolve the simulation, using the constrained-transport method. The density contrasts are varied between η= 10^{-1} and 10^{-4} with constant sonic Mach number 6. The jets are supermagnetosonic and simulated bipolarly due to the low jet densities and their strong backflows. The helical magnetic field is largely confined to the jet, leaving the ambient medium nonmagnetic. We find magnetic fields with plasma β\sim 10 already stabilize and widen the jet head. Furthermore they are efficiently amplified by a shearing mechanism in the jet head and are strong enough to damp Kelvin-Helmholtz instabilities of the contact discontinuity. The cocoon magnetic fields are found to be stronger than expected from simple flux conservation and capable to produce smoother lobes, as found observationally. The bow shocks and jet lengths evolve self-similarly. The radio cocoon aspect ratios are generally higher for heavier jets and grow only slowly (roughly self-similar) while overpressured, but much faster when they approach pressure balance with the ambient medium. In this regime, self-similar models can no longer be applied. Bow shocks are found to be of low excentricity for very light jets and have low Mach numbers. Cocoon turbulence and a dissolving bow shock create and excite waves and ripples in the ambient gas. Thermalization is found to be very efficient for low jet densities.

astro-ph.HE↗

arXiv2009

Origin of magnetic moments in defective TiO2 single crystals

In this paper we show that ferromagnetism can be induced in pure TiO2 single crystals by oxygen ion irradiation. By combining x-ray diffraction, Raman-scattering, and electron spin resonance spectroscopy, a defect complex, \emph{i.e.} Ti$^{3+}$ ions on the substitutional sites accompanied by oxygen vacancies, has been identified in irradiated TiO2. This kind of defect complex results in a local (TiO$_{6-x}$) stretching Raman mode. We elucidate that Ti$^{3+}$ ions with one unpaired 3d electron provide the local magnetic moments.

cond-mat.mtrl-sci↗

arXiv2009

Magnetism in the nearby galaxy M33

Using high-resolution data of the linearly polarized intensity and polarization angle at 3.6, 6.2, and 20 cm together with a 3-D model of the regular magnetic field, we study variations of the structure, strength, and energy density of the magnetic field in the Scd galaxy M33. The regular magnetic field consists of a horizontal component (represented by an axisymmetric mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric modes from 3 to 5 kpc radius) and a vertical component. However, the inferred `vertical field' may be partly due to a galactic warp. We estimate the average total and regular magnetic field strengths as ~ 6.4 and 2.5 $μ$G, respectively. Generation of interstellar magnetic fields by turbulent gas motion in M33 is indicated as the turbulent and magnetic energy densities are about equal.

astro-ph.GA↗

arXiv2008

The magnetic field structure in NGC 253 in presence of a galactic wind

We present radio continuum polarimetry observations of the nearby edge-on galaxy NGC 253 which possesses a very bright radio halo. Using the vertical synchrotron emission profiles and the lifetimes of cosmic-ray electrons, we determined the cosmic-ray bulk speed as (300+/-30) km/s, indicating the presence of a galactic wind in this galaxy. The large-scale magnetic field was decomposed into a toroidal axisymmetric component in the disk and a poloidal component in the halo. The poloidal component shows a prominent X-shaped magnetic field structure centered on the nucleus, similar to the magnetic field observed in other edge-on galaxies. Faraday rotation measures indicate that the poloidal field has an odd parity (antisymmetric). NGC 253 offers the possibility to compare the magnetic field structure with models of galactic dynamos and/or galactic wind flows.

astro-ph↗

arXiv2008

High-resolution radio continuum survey of M33: III. Magnetic fields

Using the linearly polarized intensity and polarization angle data at 3.6, 6.2 and 20 cm, we determine variations of Faraday rotation and depolarization across the nearby galaxy M33. A 3-D model of the regular magnetic field is fitted to the observed azimuthal distribution of polarization angles. Faraday rotation, measured between 3.6 and 6.2 cm at a linear resolution of 0.7 kpc, shows more variation in the south than in the north of the galaxy. About 10% of the nonthermal emission from M33 at 3.6 cm is polarized. We estimate the average total and regular magnetic field strengths in M33 as ~ 6.4 and 2.5 $μ$G, respectively. Under the assumption that the disk of M33 is flat, the regular magnetic field consists of horizontal and vertical components: however the inferred vertical field may be partly due to a galactic warp. The horizontal field is represented by an axisymmetric (m=0) mode from 1 to 3 kpc radius and a superposition of axisymmetric and bisymmetric (m=0+1) modes from 3 to 5 kpc radius. An excess of differential Faraday rotation in the southern half together with strong Faraday dispersion in the southern spiral arms seem to be responsible for the north-south asymmetry in the observed wavelength dependent depolarization. The presence of an axisymmetric m=0 mode of the regular magnetic field in each ring suggests that a galactic dynamo is operating in M33. The pitch angles of the spiral regular magnetic field are generally smaller than the pitch angles of the optical spiral arms but are twice as big as simple estimates based on the mean-field dynamo theory and M33's rotation curve. Generation of interstellar magnetic fields from turbulent gas motions in M33 is indicated by the equipartition of turbulent and magnetic energy densities.

astro-ph↗

arXiv2008

Polarisation of mJy radio sources

Predictions of the number of faint polarised radio sources that can be detected by SKA pathfinder telescopes and the SKA depend on the polarisation properties of radio sources with a total flux density around 1 mJy. Total intensity source counts suggest a transition in the dominant population from AGN to galaxies around this flux density, and the properties of brighter radio sources may not be representative for this fainter population. We show that unresolved spiral galaxies can be highly polarised radio sources, up to ~ 20% polarised at 4.8 GHz. This result is partly based on observations of nearby galaxies, including galaxies with significant deviations from axial symmetry and other peculiarities. A first analysis of polarised source counts divided into steep-spectrum AGN, flat-spectrum AGN and star forming galaxies is presented, including a prediction of polarised source counts to microjansky levels.

astro-ph↗

arXiv2008

A Multi-Scale Study of IR and Radio Emission from M33

The origin of the tight radio--IR correlation in galaxies has not been fully understood. One reason is the uncertainty about which heating sources (stars or diffuse interstellar radiation field)provide the energy that is absorbed by dust and re-radiated in IR. Another problem is caused by comparing the IR emission with the thermal and nonthermal components of the radio continuum emission separated by simplistically assuming a constant nonthermal spectral index. We use the data at the $Spitzer$ MIPS wavelengths of 24, 70, and 160 $μ$m, as well as recent radio continuum map at 3.6 cm observed with the 100--m Effelsberg telescope. Using the wavelet transformation, we separate diffuse emission components from compact sources and study the radio-IR correlation at various scales. We also investigate the IR correlations with the thermal and nonthermal radio emissions separated by our developed method. A H$α$ map serves as a tracer of star forming regions.

astro-ph↗

arXiv2008

Variations of the Radio Synchrotron Spectral Index in M33

We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M33. We separate the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 $μ$m and a standard dust model, we de-redden the H$α$ emission. The extinction-corrected H$α$ emission serves as a template for the thermal free-free radio emission. Subtracting from the observed 3.6 and 20cm emission (Effelsberg and the VLA) this free-free emission, we obtain maps of the nonthermal intensity and spectral index.

astro-ph↗

arXiv2008

Cosmic rays and the magnetic field of the nearby starburst galaxy NGC 253

Using radio polarimetry we study the connection between the transport of cosmic rays (CR's), the three-dimensional magnetic field structure, and features of other ISM phases in the halo of NGC 253. We present a new sensitive radio continuum map of NGC 253 obtained from combined VLA and Effelsberg observations at lambda 6.2 cm. We find a prominent radio halo with a scaleheight of the thick radio disk of 1.7 kpc. The linear dependence between the local scaleheight of the vertical continuum emission and the cosmic ray electron (CRE) lifetime requires a vertical CR bulk speed of 270 km s^-1. The magnetic field structure of NGC 253 resembles an ``X''-shaped configuration where the orientation of the large-scale magnetic field is plane-parallel only in the inner regions of the disk and at small distances from the galactic midplane. At larger galactocentric radii and further away from the midplane the vertical component becomes important. This is most clearly visible at the location of the ``radio spur'' southeast of the nucleus, where the magnetic field orientation is almost vertical. We made a simple model for the dominant toroidal (r,phi) magnetic field component using a spiral magnetic field with prescribed inclination and pitch angle. The residual poloidal (r,phi,z) magnetic field component which was revealed by subtracting the model from the observations shows a distinct ``X''-shaped magnetic field orientation centered on the nucleus. The orientation angle of the poloidal magnetic field is consistent with a magnetic field transport described by the superposition of the vertical CR bulk speed and the rotation velocity. Hence, we propose a disk wind which transports cosmic rays, magnetic field, and (partially) ionized gas from the disk into the halo.

astro-ph↗

arXiv2007

Testing the magnetic field models of galaxies with the SKA

The future new-generation radio telescope SKA (Square Kilometre Array) and its precursors will provide a rapidly growing number of polarized radio sources. Hundred and thousands polarized background sources can be measured towards nearby galaxies thus allowing their detailed magnetic field mapping by means of Faraday rotation measures (RM). We aim to estimate the required density of the background polarized sources detected with the SKA for reliable recognition and reconstruction of the magnetic field structure in nearby spiral galaxies. We construct a galaxy model which includes the ionized gas and magnetic field patterns of different azimuthal symmetry (axisymmetric (ASS), bisymmetric (BSS) and quadrisymmetric spiral (QSS), and superpositions) plus a halo magnetic field. RM fluctuations with a Kolmogorov spectrum due to turbulent fields and/or fluctuations in ionized gas density are superimposed. Recognition of magnetic structures is possible from RM towards background sources behind galaxies or a continuous RM map obtained from the diffuse polarized emission from the galaxy itself. Under favourite conditions, about a few dozens of polarized sources are sufficient for a reliable recognition. Reconstruction of the field structure without precognition becomes possible for a large number of background sources. A reliable reconstruction of the field structure needs at least 20 RM values on a cut along the projected minor axis which translates to approximately 1200 sources towards the galaxy. Radio telescopes operating at low frequencies (LOFAR, ASKAP and the low-frequency SKA array) may also be useful instruments for field recognition or reconstruction with the help of RM, if background sources are still significantly polarized at low frequencies (abriged).

astro-ph↗

arXiv2007

Large-scale Propagation of Very Light Jets in Galaxy Clusters

We performed MHD simulations of very light bipolar jets with density contrasts down to 10^-4 in axisymmetry, which were injected into a medium of constant density and evolved up to 200 kpc (200 r_j) full length. These jets show weak and roundish bow shocks as well as broad cocoons and thermalize their kinetic energy very efficiently. We argue that very light jets are necessary to match low-frequency radio observations of radio lobes as well as the bow shocks seen in X-rays. Due to the slow propagation, the backflows and their turbulent interaction in the midplane are important for a realistic global appearance.

astro-ph↗

arXiv2007

High-resolution radio continuum survey of M33 II. Thermal and nonthermal emission

We determine the variation in the nonthermal radio spectral index in the nearby spiral galaxy M33 at a linear resolution of 360 pc. We separate the thermal and nonthermal components of the radio continuum emission without the assumption of a constant nonthermal spectral index. Using the Spitzer FIR data at 70 and 160 $μ$m and a standard dust model, we deredden the H$α$ emission. The extinction corrected H$α$ emission serves as a template for the thermal free-free radio emission. Subtracting from the observed 3.6 cm and 20 cm emission (Effelsberg and the VLA) this free-free emission, we obtain the nonthermal maps. A constant electron temperature used to obtain the thermal radio intensity seems appropriate for M~33 which, unlike the Milky Way, has a shallow metallicity gradient. For the first time, we derive the distribution of the nonthermal spectral index across a galaxy, M33. We detect strong nonthermal emission from the spiral arms and star-forming regions. Wavelet analysis shows that at 3.6 cm the nonthermal emission is dominated by contributions from star-forming regions, while it is smoothly distributed at 20 cm. For the whole galaxy, we obtain thermal fractions of 51% and 18% at 3.6 cm and 20 cm, respectively. The thermal emission is slightly stronger in the southern than in the northern half of the galaxy. We find a clear radial gradient of mean extinction in the galactic plane. The nonthermal spectral index map indicates that the relativistic electrons suffer energy-loss when diffusing from their origin in star-forming regions towards interarm regions and the outer parts of the galaxy. We also conclude that the radio emission is mostly nonthermal at R $>$ 5 kpc in M33.

astro-ph↗

arXiv2007

High resolution radio continuum survey of M33: I. The radio maps

We study the exponential scale length of total radio emission, the spectral index distribution, and the linear radio polarization in the Scd galaxy M33. Observations were carried out using the 3.6 cm dual channel and the 6.2 cm four channel receivers of the 100--m Effelsberg telescope along with the L-band VLA D--array at 20 cm. High spatial resolution and sensitivity in both total and linearly polarized radio continuum emission from M33 were achieved. We found considerable extended emission, not only from the main arms I S and I N, but also from the weaker arms. The large--scale magnetic field exhibits well--ordered spiral structure with almost the same orientation as that of the optical spiral arms, however, it does not show a clear structural correlation or anti--correlation with the optical arms. There is a north-south asymmetry in polarization that is frequency-dependent. We found that the ring mean spectral index versus radius increases faster beyond $R$ = 4 kpc. At each wavelength, the exponential scale length is larger inside than outside $R$ = 4 kpc. From the larger scales lengths at $R$ $<$ 4 kpc, we conclude that star forming regions are mainly spread over the region $R$ $<$ 4 kpc without a dominant nuclear concentration. Furthermore, at $R$ $<$ 4 kpc, a spatial correlation between cosmic rays and star forming regions may exist. From the behaviour of the mean spectral indices obtained from different pairs of the radio continuum data at 3.6, 6.2, and 20 cm, we confirm that a decrease in the thermal fraction causes an increase in the spectral index. The frequency-dependent asymmetry in the polarization hints to an asymmetry in Faraday depolarization.

astro-ph↗

arXiv2007

Superposition of Quantum and Classical Rotational Motions in Sc2C2@C84 Fullerite

The superposition of the quantum rotational motion (tunneling) of the encapsulated Sc2C2 complex with the classical rotational motion of the surrounding C84 molecule in a powder crystal of Sc2C2@C84 fullerite is investigated by theory. Since the quantum rotor is dragged along by the C84 molecule, any detection method which couples to the quantum rotor (in casu the C2 bond of the Sc2C2 complex also probes the thermally excited classical motion (uniaxial rotational diffusion and stochastic meroaxial jumps) of the surrounding fullerene. The dynamic rotation-rotation response functions in frequency space are obtained as convolutions of quantum and classical dynamic correlation functions. The corresponding Raman scattering laws are derived, the overall shape of the spectra and the width of the resonance lines are studied as functions of temperature. The results of the theory are confronted with experimental low-frequency Raman spectra on powder crystals of Sc2C2@C84 [M. Krause et al., Phys. Rev. Lett. 93, 137403 (2004)]. The agreement of theory with experiment is very satisfactory in a broad temperature range.

cond-mat.mtrl-sci↗

arXiv2007

A multi-scale study of infrared and radio emission from Scd galaxy M33

We investigate the energy sources of the infrared (IR) emission and their relation to the radio continuum emission at various spatial scales within the Scd galaxy M33. We use the wavelet transform to analyze IR data at the Spitzer wavelengths of 24, 70, and 160$μ$m, as well as recent radio continuum data at 3.6cm and 20cm. An H$α$ map serves as a tracer of the star forming regions and as an indicator of the thermal radio emission. We find that the dominant scale of the 70$μ$m emission is larger than that of the 24$μ$m emission, while the 160$μ$m emission shows a smooth wavelet spectrum. The radio and H$α$ maps are well correlated with all 3 MIPS maps, although their correlations with the 160$μ$m map are weaker. After subtracting the bright HII regions, the 24 and 70$μ$m maps show weaker correlations with the 20cm map than with the 3.6cm map at most scales. We also find a strong correlation between the 3.6cm and H$α$ emission at all scales. Comparing the results with and without the bright HII regions, we conclude that the IR emission is influenced by young, massive stars increasingly with decreasing wavelength from 160 to 24$μ$m. The radio-IR correlations indicate that the warm dust-thermal radio correlation is stronger than the cold dust-nonthermal radio correlation at scales smaller than 4kpc. A perfect 3.6cm-H$α$ correlation implies that extinction has no significant effect on H$α$ emitting structures.

astro-ph↗

arXiv2006

Magnetic fields and ionized gas in nearby late type galaxies

In order to analyze the importance of the star formation rate in generating and amplifying magnetic fields in the interstellar medium we obtained deep total power and polarization maps at 4.85 GHz of three late type spiral galaxies NGC4236, NGC4656 and IC2574 using the 100-m Effelsberg radio telescope. This was accompanied by imaging in the Halpha line. We also observed these objects at 1.4 GHz to obtain their integrated fluxes at this frequency and to determine their radio spectra. All galaxies were found to possess weak but detectable total power emission at 4.85 GHz, coincident with regions of recent star formation as traced by bright Halpha regions. The surface brightness of the radio-strongest object of our sample (NGC4656) is comparable to the radio-weakest objects in a sample of more than 50 normally star-forming spiral galaxies for which measurements at 4.8 GHz with the Effelsberg radio telescope are available. The fractional polarization of the 2 galaxies of our sample is less than 2%, significantly lower than for spiral galaxies of intermediate types, suggesting that the magnetic fields are not only weaker, but also less ordered than in spiral galaxies. The radio spectra of galaxies in our small sample are indicative of a substantial fraction of thermal emission, with a higher thermal fraction than in spirals with high star formation rates (SFR), while the nonthermal emission in our sample is relatively weak compared to spiral galaxies. We propose an equipartition model where the nonthermal emission increases ~SFR^(~1.4) and the ratio of nonthermal to thermal emission increases ~SFR^(~0.4). The objects of our sample still follow the radio-FIR correlation of surface brightness of the total emission, extending it towards the lowest values measured so far.

astro-ph↗

arXiv2005

Radio polarization and sub-mm observations of the Sombrero galaxy (NGC 4594) - Large-scale magnetic field configuration and dust emission

We observed the nearby early-type spiral galaxy NGC 4594 (M 104, Sombrero galaxy) with the Very Large Array at 4.86 GHz, with the Effelsberg 100-m telescope at 8.35 GHz as well as with the Heinrich Hertz Telescope at 345 GHz in radio continuum. The 4.86 and 8.35 GHz data contain polarization information and hence information about the magnetic fields: we detected a large-scale magnetic field which is to our knowledge the first detection of a large-scale magnetic field in an Sa galaxy in the radio range. The magnetic field orientation in M104 is predominantly parallel to the disk but has also vertical components at larger z-distances from the disk. This field configuration is typical for normal edge-on spiral galaxies. The 345 GHz data pertain to the cold dust content of the galaxy. Despite the optical appearance of the object with the huge dust lane, its dust content is smaller than that of more late-type spirals.

astro-ph↗