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B. Fuchs

Publications and source records attributed to B. Fuchs.

71 records · Page 4Linked to original sources
arXiv2001

Density waves in the shearing sheet III. Disc heating

The problem of dynamical heating of galactic discs by spiral density waves is discussed using the shearing sheet model. The secular evolution of the disc is described quantitatively by a diffusion equation for the distribution function of stars in the space spanned by integrals of motion of the stars, in particular the radial action integral and an integral related to the angular momentum. Specifically, disc heating by a succession of transient, `swing amplified' density waves is studied. It is shown that such density waves lead predominantly to diffusion of stars in radial action space. The stochastical changes of angular momenta of the stars and the corresponding stochastic changes of the guiding centre radii of the stellar orbits induced by this process are much smaller.

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arXiv2001

Implications of modes of star formation for the overall dynamics of galactic disks

One of the present concepts for the onset of massive star formation is the Kennicutt criterion. This relates the onset of massive star formation to a general gravitational instability of the gas disks of spiral galaxies. It is often overlooked, however that such gravitational instabilities of the gas disks have severe implications for the overall stability of the gas and star disks of spiral galaxies. I show by numerical simulations of the evolution of a combined gas and star disk that the violation of the stability condition induces violent dynamical evolution of the combined system. In particular the star disk heats up on time scales less than a Gyr to unrealistic high values of the Toomre stability parameter Q. The morphologies of both the star and gas disk resemble then no longer observed morphologies of spiral galaxies. Star formation of stars on low velocity dispersion orbits would lead to dynamical cooling of the disks to more realistic states. However, the required star formation rate is extremely high.

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arXiv2001

Density waves in the shearing sheet I. Swing amplification

The shearing sheet model of a galactic disk is studied anew. The theoretical description of its dynamics is based on three building blocks: Stellar orbits, which are described here in epicyclic approximation, the collisionless Boltzmann equation determining the distribution function of stars in phase space, and the Poisson equation in order to take account of the self-gravity of the disk. Using these tools I develop a new formalism to describe perturbations of the shearing sheet. Applying this to the unbounded shearing sheet model I demonstrate again how the disturbances of the disk evolve always into `swing amplified' density waves, i.e. spiral-arm like, shearing density enhancements, which grow and decay while the wave crests swing by from leading to trailing orientation. Several examples are given how such `swing amplification' events are incited in the shearing sheet.

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arXiv2000

The Amount of Dark Matter in Spiral Galaxies

The `maximum' disk hypothesis of galactic disks imbedded in dark matter halos is examined. First, decompositions of the rotation curves of NGC 2613, 3198, 6503, and 7184 are analyzed. For these galaxies the radial velocity dispersions of the stars have been measured. If the parameters of the decompositions are chosen according to the `maximum' disk hypothesis, the Toomre Q stability parameter is sytematically less than one, which is a strong argument against the `maximum' disk hypothesis. Next, density wave theory arguments are used to describe the morphology of the spiral arms of NGC 3223, 157, and 7083. It is shown that the `maximum' disk hypothesis is not consistent with the observed morphologies of the galaxies.

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arXiv2000

Kinematics of Metal-Poor Stars in the Galaxy. II. Proper Motions for a Large Non-Kinematically Selected Sample

We present a revised catalog of 2106 Galactic stars, selected without kinematic bias, and with available radial velocities, distance estimates, and metal abundances in the range 0.0 <= [Fe/H] <= -4.0. This update of the Beers and Sommer-Larsen (1995) catalog includes newly-derived homogeneous photometric distance estimates, revised radial velocities for a number of stars with recently obtained high-resolution spectra, and refined metallicities for stars originally identified in the HK objective-prism survey (which account for nearly half of the catalog) based on a recent re-calibration. A subset of 1258 stars in this catalog have available proper motions, based on measurements obtained with the Hipparcos astrometry satellite, or taken from the updated Astrographic Catalogue (AC 2000; second epoch positions from either the Hubble Space Telescope Guide Star Catalog or the Tycho Catalogue), the Yale/San Juan Southern Proper Motion (SPM) Catalog 2.0, and the Lick Northern Proper Motion (NPM1) Catalog. Our present catalog includes 388 RR Lyrae variables (182 of which are newly added), 38 variables of other types, and 1680 non-variables, with distances in the range 0.1 to 40 kpc.

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arXiv2000

CADIS deep star counts: Galactic structure and the stellar luminosity function

In this paper we present the first results of deep star counts carried out within the Calar Alto Deep Imaging Survey, CADIS (Meisenheimer 1998). Although CADIS was designed as an extragalactic survey, it also attempts to identify the stars in the fields in order to avoid confusion with quasars and compact galaxies. We have identified a sample of about 300 faint stars 15.5< R < 23), which are well suited to study the structure of the Galaxy. The stars lie in two fields, hereafter 16h and 9h field, respectively. The stars have been separated from galaxies by a classification scheme based on photometric spectra and morphological criteria. Distances were derived by photometric parallaxes. We are able to find stars up to distances of approximately 25 kpc above the Galactic plane. The vertical density distribution of the stars shows the contribution of the thin disk, the stellar halo and the ``thick disk'' of the Galaxy. We give quantitative descriptions of the components in terms of exponential disks and a de Vaucouleurs spheroid. For the disk stars we derive the luminosity function. It is equal within the errors to the local luminosity function and continues to rise out to at least M_V = 13. Implications for the mass function are briefly discussed.

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arXiv1999

No nearby counterparts to the moving objects in the Hubble Deep Field

Ibata et al (1999) have recently discovered faint, moving objects in the Hubble Deep Field. The quantity, magnitudes and proper motions of these objects are consistent with old white dwarfs making up the Galactic dark halo. We review a number of ground-based proper motion surveys in which nearby dark halo white dwarfs might be present, if they have the colours and absolute magnitudes proposed. No such objects have been found, whereas we argue here that several times more would be expected than in the Hubble Deep Field. We conclude it is unlikely that hydrogen atmosphere white dwarfs make up a significant fraction of the halo dark matter. No limits can be placed yet on helium atmosphere dwarfs from optical searches.

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arXiv1998

A Comparison of ground-based Cepheid P-L Relations with HIPPARCOS Parallaxes

The statistical test described by Wielen et al.(1994) is used to derive new zero-points of ground-based Cepheid period-luminosity (PL) and period-luminosity-colour (PLC) relations. Eleven relations are compared with the Hipparcos data. Our results argue for a typical increase of the adopted distance scale by about 8% +- 8%. Our zero-point for the PL relation of Caldwell & Laney (1991) is in agreement with that of Feast & Catchpole (1997).

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arXiv1998

NGC2613, 3198, 6503, 7184: Case studies against `maximum' disks

Decompositions of the rotation curves of NGC2613, 3198, 6505, and 7184 are analysed. For these galaxies the radial velocity dispersions of the stars have been measured and their morphology is clearly discernible. If the parameters of the decompositions are chosen according to the `maximum' disk hypothesis, the Toomre Q stability parameter is systematically less than one and the multiplicities of the spiral arms as expected from density wave theory are inconsitent with the observed morphologies of the galaxies. The apparent Q<1 instability, in particular, is a strong argument against the `maximum' disk hypothesis.

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arXiv1998

Kinematics of nearby subdwarfs

A sample of subdwarfs with accurate space velocities and standarized metallicities is presented. This was constructed by combining Hipparcos parallaxes ond proper motions with radial velocities and metallicities from Carney et al. (1994; CLLA). The accurate Hipparcos parallaxes lead to an - upward - correction factor of 11% of the photometric distance scale of CLLA. The kinematical behaviour of the subdwarfs is discussed in particular in relation to their metallicities. Most of the stars turn out to be thick disk stars, but the sample contains also many genuine halo stars. While the extreme metal poor halo does not rotate, a population of subdwarfs with metallicities in the range -1.6 < [Fe/H] < -1.0 dex appears to rotate around the galactic center with a mean rotation speed of about 100 km/s.

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arXiv1998

The contribution of brown dwarfs to the local mass budget of the Galaxy

Based on the recent discoveries of free floating brown dwarfs we derive estimates of the local mass density of this population of objects. Mass density estimates from various surveys span the range 0.03 to 0.005 M_Sun/pc^3. These estimates are compared with the local mass densities of the other constituents of the galactic disk and, in particular, with the dynamically determined total local mass density. We argue that brown dwarfs might indeed contribute significantly to the local mass budget, but that a local mass density as high as 0.03 M_Sun/pc^3 as suggested by Ruiz et al. (1997) is rather unlikely.

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arXiv1998

Decomposition of the rotation curves of distant field galaxies

We present decompositions of the rotation curves of distant spiral galaxies into contributions due to their bulges, disks, and putative dark haloes. In order to set constraints on the ambiguities of the decompositions we interpret the morphology of the spiral structures quantitatively in the framework of density wave theory. Galaxy models constrained in such a way show that the distant galaxies, which are much younger than nearby galaxies, are indeed also imbedded in dark haloes as expected from contemporary theories of the cosmonogy of galaxies.

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arXiv1997

Halo stars in the immediate solar neighbourhood

We have searched the Forth Catalogue of Nearby Stars for halo stars and identified 15 subdwarfs and a high velocity white dwarf in the solar neighbourhood. This search was motivated mainly by the recent determinations of MACHO masses of about 0.5 M_sun, which are typical for halo dwarfs. The local mass density of the stars is 1.5 10^{-4} M_sun pc^{-3}, which is only 3% of the current estimate of the local mass density of the MACHO population. We compare the local density of subdwarfs with constraints set by HST observations of distant red dwarfs. Using models of the stellar halo with density laws that fall off like r^{-α}, α= 3.5 to 4, we find that the HST constraints can only be matched, if we assume that the stellar halo is flattened with an axial ratio of about 0.6. The non-detection of the analogues of MACHOs in the solar neighbourhood allows to set an upper limit to the luminosity of MACHOs of M_B > 21 magnitudes.

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arXiv1997

Dynamical stability and evolution of the discs of Sc galaxies

We examine the local stability of galactic discs against axisymmetric density perturbations with special attention to the different dynamics of the stellar and gaseous components. In particular the discs of the Milky Way and of NGC 6946 are studied. The Milky Way is shown to be stable, whereas the inner parts of NGC 6946, a typical Sc galaxy from the Kennicutt (1989) sample, are dynamically unstable. The ensuing dynamical evolution of the composite disc is studied by numerical simulations. The evolution is so fierce that the stellar disc heats up dynamically on a short time scale to such a degree, which seems to contradict the morphological appearance of the galaxy. The star formation rate required to cool the disc dynamically is estimated. Even if the star formation rate in NGC 6946 is at present high enough to meet this requirement, it is argued that the discs of Sc galaxies cannot sustain such a high star formation rate for longer periods.

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arXiv1996

MACHOs in the solar neighbourhood

We rediscuss the question of what fraction of the population of MACHOs in the halo of the Milky Way might be in the form of halo dwarfs, because recent determinations indicate masses of the MACHOs, which are typical for such stars. For this purpose we have analyzed the Third Catalogue of Nearby Stars and identified the subdwarfs and a high velocity white dwarf in the solar neighbourhood. The local mass density of these stars is 1.5*10^{-4} M_sun/pc^3, which is only 3% of the current estimate of the local mass density of the MACHO population. We compare the local density of subdwarfs with constraints set by HST observations of distant red dwarfs. Using models of the stellar halo with density laws that fall off like r^{-alpha}, alpha = 3.5 to 4, we find that the HST constraints can only be matched, if we assume that the stellar halo is flattened with an axial ratio of about 0.6. The non-detection of the analogs of MACHOs in the solar neighbourhood allows to set an upper limit to the luminosity of MACHOs of M_B > 21 magnitudes.

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arXiv1996

Derivation of the Galactic Rotation Curve using space velocities

We present rotation curves of the Galaxy based on the space-velocities of 197 OB stars and 144 classical cepheids, respectively, which range over a galactocentric distance interval of about 6 to 12 kpc. No significant differences between these rotation curves and rotation curves based solely on radial velocities assuming circular rotation are found. We derive an angular velocity of the LSR of Omega_0 = 5.5 +/- 0.4 mas/a (OB stars) and Omega_0 = 5.4 +/- 0.5 mas/a (cepheids), which is in agreement with the IAU 1985 value of Omega_0 = 5.5 mas/a. If we correct for probable rotations of the FK5 system, the corresponding angular velocities are Omega_0 = 6.0 mas/a (OB stars) and Omega_0 = 6.2 mas/a (cepheids). These values agree better with the value of Omega_0 = 6.4 mas/a derived from the VLA measurement of the proper motion of SgrA*.

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