arXiv subjects

B. Fuchs

Publications and source records attributed to B. Fuchs.

At least 55 records · Page 3Linked to original sources

arXiv2007

Can Massive Dark Haloes Destroy the Disks of Dwarf Galaxies?

Recent high-resolution simulations together with theoretical studies of the dynamical evolution of galactic disks have shown that contrary to wide-held beliefs a `live', dynamically responsive, dark halo surrounding a disk does not stabilize the disk against dynamical instabilities. We generalize Toomre's Q stability parameter for a disk-halo system and show that if a disk, which would be otherwise stable, is embedded in a halo, which is too massive and cold, the combined disk-halo system can become locally Jeans unstable. The good news is, on the other hand, that this will not happen in real dark haloes, which are in radial hydrostatic equilibrium. Even very low-mass disks are not prone to such dynamical instabilities.

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arXiv2007

Signatures of star streams in the phase space distribution of nearby halo stars

We have analyzed the phase space distribution of a sample of about 900 non--kinematically selected low metallicity stars in the solar vicinity. The stars primarily represent the thick disk and halo populations of the Milky Way. We aim to identify overdensely populated regions in phase space, which we interpret as signatures of star streams passing close to the Sun. The search was conducted in a space constructed from the angular momenta and eccentricities of the stellar orbits. Besides recovering all well known star streams in the thick disk, we isolated four statistically significant phase space overdensities amongst halo stars. One of them is associated with a previously known halo star stream, but three of them are novel features, which we propose be also considered as genuine halo streams.

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arXiv2007

Dynamical friction force exerted on spherical bodies

We present a rigorous calculation of the dynamical friction force exerted on a spherical massive perturber moving through an infinite homogenous system of field stars. By calculating the shape and mass of the polarization cloud induced by the perturber in the background system, which decelerates the motion of the perturber, we recover Chandrasekhar's drag force law with a modified Coulomb logarithm. As concrete examples we calculate the drag force exerted on a Plummer sphere or a sphere with the density distribution of a Hernquist profile. It is shown that the shape of the perturber affects only the exact form of the Coulomb logarithm. The latter converges on small scales, because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm this way earlier results based on the impulse approximation of small angle scatterings.

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arXiv2007

Jeans instability of a galactic disk embedded in a live dark halo

We investigate the Jeans instability of a galactic disk embedded in a dynamically responsive dark halo. It is shown that the disk-halo system becomes nominally Jeans unstable. On small scales the instability is suppressed, if the Toomre stability index Q_T is higher than a certain threshold, but on large scales the Jeans instability sets invariably in. However, using a simple self-consistent disk-halo model it is demonstrated that this occurs on scales which are much larger than the system so that this is indeed only a nominal effect. From a practical point of view the Jeans instability of galactic disks is not affected by a live dark halo.

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arXiv2006

The search for the origin of the Local Bubble redivivus

We present a new unbiased search and analysis of all B stars in the solar neighbourhood (within a volume of 400 pc diameter) using the Arivel data base to track down the remains of the OB associations, which hosted the supernovae responsible for the Local Bubble in the interstellar gas. We find after careful dereddening and by comparison with theoretical isochrones, that besides the Upper Scorpius the Upper Centaurus Lupus and Lower Centaurus Crux subgroups are the youngest stellar associations in the solar neighbourhood with ages of 20 to 30 Myr, in agreement with previous work. In search for the ``smoking gun'' of the origin of the Local Bubble, we have traced the paths of the associations back into the past and found that they entered the present bubble region 10 to 15 Myr ago. We argue that the Local Bubble began to form then and estimate that 14 to 20 supernovae have gone off since. It is shown that the implied energy input is sufficient to excavate a bubble of the presently observed size.

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arXiv2006

CADIS has seen the Virgo overdensity and parts of the Monoceros and `Orphan' streams in retrospect

We reanalyze deep star counts in five CADIS fields. The data are presented as vertical density distributions of stars perpendicular to the Galactic plane. In three fields the profiles are consistent with each other, while in two fields significant overdensities of stars are found. The overdensity in one field can be associated with the Virgo overdensity which can be traced right into the disk of the Milky Way. Using this detection we estimate the mass of the Virgo overdensity and show that this is equivalent to the stellar content of a Local Group dwarf spheroidal galaxy. The overdensity in the second field is more difficult to associate with a previously known overdensity. We suggest that it is related both to the Monoceros stream and the recently discovered Orphan stream.

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arXiv2006

Scalar field haloes as gravitational lenses

A non-topological soliton model with a repulsive scalar self-interaction of the Emden type provides a constant density core,similarly as the empirical Burkert profile of dark matter haloes. As a further test, we derive the gravitational lens properties of our model, in particular, the demarcation curves between `weak' and `strong' lensing. Accordingly, strong lensing with typically three images is almost three times more probable for our solitonic model than for the Burkert fit. Moreover, some prospective consequences of a possible flattening of dark matter haloes are indicated.

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arXiv2005

Fine structure in the phase space distribution of nearby subdwarfs

We analysed the fine structure of the phase space distribution function of nearby subdwarfs using data extracted from various catalogues. Applying a new search strategy based on Dekker's theory of galactic orbits, we found four overdensely populated regions in phase space. Three of them were correlated with previously known star streams: the Hyades-Pleiades and Hercules streams in the thin disk of the Milky Way and the Arcturus stream in the thick disk. In addition we find evidence for another stream in the thick disk, which resembles closely the Arcturus stream and probably has the same extragalactic origin.

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arXiv2005

Interaction between a galactic disk and a live dark halo with an anisotropic velocity distribution

We have extended previous analytical studies of the interaction of dark halos with galactic disks by introducing for the halo particles anisotropic distribution functions in phase space. For this purpose we have employed the shearing sheet model of a patch of a galactic disk embedded in a homogeneous halo. We find that velocity anisotropy increases considerably the maximum growth factor of perturbations in the disk.

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arXiv2005

Density waves in the shearing sheet V. Feedback cycle for swing amplification by non-linear effects

Non-linear effects in the dynamical evolution of a shearing sheet made of stars are studied. First the implications of hitherto neglected non-linearities of the Boltzmann equation for the dynamical evolution of the shearing sheet are investigated. Using a formalism developed previously on the basis of the linearized Boltzmann equation it is demonstrated that the inclusion of the non-linear term leads to a feedback cycle for swing amplified density waves in the unbounded shearing sheet. Such a feedback is unique to star disks and is not known for gas disks. In order to present concrete examples of the non-linear feedback cycle a SCF code was developed and numerical simulations of the dynamical evolution of the shearing sheet are performed. The numerical results seem to confirm the theoretical predictions. The evolution of the shearing sheet resembles closely and might actually explain the recurrent spiral instabilities found in large-scale numerical simulations of the dynamical evolution of galactic disks.

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arXiv2005

Galactic structure from the Calar Alto Deep Imaging Survey (CADIS)

We used 1627 faint (15.5< R<23) stars in five fields of the Calar Alto Deep Imaging Survey (CADIS) to estimate the structure parameters of the Galaxy. The results were derived by applying two complementary methods: first by fitting the density distribution function to the measured density of stars perpendicular to the Galactic plane, and second by modelling the observed colors and apparent magnitudes of the stars in the field, using Monte Carlo simulations. The best-fitting model of the Galaxy is then determined by minimising the C-statistic, a modified chisquared. Our model includes a double exponential for the stellar disk with scaleheights h_1 and h_2 and a power law halo with exponent alpha. 24480 different parameter combinations have been simulated. Both methods yield consistent results: the best fitting parameter combination is alpha=3.0 (or alpha=2.5, if we allow for a flattening of the halo with an axial ratio of (c/a)=0.6), h_1=300 pc, h_2=900 pc, and the contribution of thick disk stars to the disk stars in the solar neighbourhood is found to be between 4 and 10%.

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arXiv2005

Kinematics of Nearby Subdwarf Stars

We present an analysis of the space motions of 742 subdwarf stars based on the sample of Carney et al. (1994, CLLA). Hipparcos parallaxes, TYC2+HIP proper motions and Tycho2 proper motions were combined with radial velocities and metallicities from CLLA. The kinematical behavior is discussed in particular in relation to their metallicities. The majority of these sample stars have metal abundances of [Fe/H] >-1 and represent the thick disk population. The halo component, with [Fe/H] <-1.6, is characterized by a low mean rotation velocity and a radially elongated velocity ellipsoid. In the intermediate metallicity range (-1.6 < [Fe/H] <-1), we find a significant number of subdwarfs with disklike kinematics. We interpret this population of stars as a metal-weak thick disk population.

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arXiv2004

Weighing stellar - mass black holes with Gaia

Stellar-mass black holes have been detected by radial-velocity observations in star/black hole binaries. These allow only the determination of the mass function. Tracking the astrometric orbits of the visible components of the star/black hole binaries would allow the full determination of the black hole masses, which would be of great astrophysical interest. We investigate the possibilities to do this with Gaia. A very promising object seems to be Cyg X-1. The donor star is an O9.7Iab supergiant having a mass of 25 M_Sun, and the mass of the black hole is estimated to be at least 13 M_Sun masses. The period of 5.6 days implies a semi-major axis of 1.01 micropc of the binary orbit. At the distance of Cyg X-1 (2.5kpc) this translates into a semi-major axis of about 28 microarcsec for the visible component, which can be detected by Gaia. We discuss further candidates.

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arXiv2004

Density waves in the shearing sheet IV. Interaction with a live dark halo

It is shown that if the self-gravitating shearing sheet, a model of a patch of a galactic disk, is embedded in a live dark halo, this has a strong effect on the dynamics of density waves in the sheet. I describe how the density waves and the halo interact via halo particles either on orbits in resonance with the wave or on non-resonant orbits. Contrary to expectation the presence of the halo leads to a very considerable enhancement of the amplitudes of the density waves in the shearing sheet. This effect appears to be the equivalent of the recently reported enhanced growth of bars in numerically simulated stellar disks embedded in live dark halos. Finally I discuss the transfer of linear momentum from a density wave in the sheet to the halo and show that it is mediated only by halo particles on resonant orbits.

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arXiv2004

Scaling behaviour of a scalar field model of dark matter halos

Galactic dark matter is modelled by a scalar field. In particular, it is shown that an analytically solvable toy model with a non-linear self-interaction potential U(Phi) leads to dark halo models which have the form of quasi-isothermal spheres. We argue that these fit better the observed rotation curves of galaxies than the centrally cusped halos of standard cold dark matter. The scalar field model predicts a proportionality between the central densities of the dark halos and the inverse of their core radii. We test this prediction successfully against a set of rotation curves of low surface brightness galaxies and nearby bright galaxies.

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arXiv2002

Constraints on the Decomposition of the Rotation Curves of Spiral Galaxies

I discuss anew how arguments about the internal dynamics of galactic disks set constraints on the otherwise ambiguous decomposition of the rotation curves of spiral galaxies into the contributions by the various constituents of the galaxies. Analyzing the two sample galaxies NGC 3198 and NGC 2985 I conclude from the multiplicities of the spiral arms and the values of the Q disk stability parameters that the disks of both galaxies are `maximum disks'.

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arXiv2002

Massive Disks in Low Surface Brightness Galaxies

An update of the set of low surface brightness galaxies is presented which can be used to set constraints on the otherwise ambiguous decompositions of their rotation curves into contributions due to the various components of the galaxies. The selected galaxies show all clear spiral structure and arguments of density wave theory of galactic spiral arms are used to estimate the masses of the galactic disks. Again these estimates seem to indicate that the disks of low surface brightness galaxies might be much more massive than currently thought. This puzzling result contradicts stellar population synthesis models. This would mean also that low surface brightness galaxies are not dominated by dark matter in their inner parts.

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arXiv2002

Dim Matter in the Disks of Low Surface Brightness Galaxies

An attempt is made to set constraints on the otherwise ambiguous decomposition of the rotation curves of low surface brightness galaxies into contributions due to the various components of the galaxies. For this purpose galaxies are selected which show clear spiral structure. Arguments of density wave theory of galactic spiral arms are then used to estimate the masses of the galactic disks. These estimates seem to indicate that the disks of low surface brightness galaxies might be much more massive than currently thought. This unexpected result contradicts stellar population synthesis models. This would also mean that low surface brightness galaxies are not dominated by dark matter in their inner parts.

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