arXiv subjects

P. Tim de Zeeuw

Publications and source records attributed to P. Tim de Zeeuw.

47 records · Page 3Linked to original sources

arXiv2001

The SAURON project. I. The panoramic integral-field spectrograph

A new integral-field spectrograph, SAURON, is described. It is based on the Tiger principle, and uses a lenslet array. SAURON has a large field of view and high throughput, and allows simultaneous sky subtraction. Its design is optimized for studies of the stellar kinematics, gas kinematics, and line-strength distributions of nearby early-type galaxies. The instrument design and specifications are described, as well as the extensive analysis software which was developed to obtain fully calibrated spectra, and the associated kinematic and line-strength measurements. A companion paper reports on the first results obtained with SAURON on the William Herschel Telescope.

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arXiv2000

A Hipparcos study of the Hyades open cluster: Improved colour-absolute magnitude and Hertzsprung-Russell diagrams

Hipparcos parallaxes fix distances to individual stars in the Hyades cluster with an accuracy of 6%. We use the Hipparcos (and Tycho-2) proper motions, which have a larger relative precision than the trigonometric parallaxes, to derive ~3 times more precise distance estimates, by assuming that all members share the same space motion. The improved parallaxes as a set are statistically consistent with the Hipparcos parallaxes. The new parallaxes confirm that the Hipparcos measurements are correlated on small angular scales, consistent with the limits specified in the Hipparcos Catalogue, though with significantly smaller `amplitudes' than claimed by Narayanan & Gould. The colour-absolute magnitude diagram of the cluster based on the new paral- laxes shows a well-defined main sequence with two gaps/turn-offs. These features provide the first direct observational support of Boehm-Vitense's prediction that (the onset of) surface convection in stars affects their B-V colours. We present and discuss the theoretical HRD for an objectively defined set of 88 high-fidelity members of the cluster as well as the delta Scuti star theta^2 Tau, the giants delta^1, theta^1, epsilon, and gamma Tau, and the white dwarfs V471 Tau and HD 27483 (all of which are also members). The precision with which the new parallaxes place individual Hyades members in the Hertz- sprung-Russell diagram is limited by (systematic) uncertainties related to the transformations from observed colours and absolute magnitudes to effective temperatures and luminosities. The new parallaxes provide stringent constraints on the calibration of such transformations when combined with theoretical stellar evolutionary modelling, tailored to the chemical composition and age of the Hyades, over the large stellar mass range probed by Hipparcos.

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arXiv2000

Galaxy mapping with the SAURON integral-field spectrograph: The star formation history of NGC 4365

We report the first wide-field mapping of the kinematics and stellar populations in the E3 galaxy NGC 4365. The velocity maps extend previous long-slit work. They show two independent kinematic subsystems: the central 300 x 700 pc rotates about the projected minor axis, and the main body of the galaxy, 3 x 4 kpc, rotates almost at right angles to this. The line-strength maps show that the metallicity of the stellar population decreases from a central value greater than solar, to one-half solar at a radius of 2 kpc. The decoupled core and main body of the galaxy have the same luminosity-weighted age, of ~14 Gyr, and the same elevated magnesium-to-iron ratio. The two kinematically distinct components have thus shared a common star formation history. We infer that the galaxy underwent a sequence of mergers associated with dissipative star formation that ended >12 Gyr ago. The misalignment between the photometric and kinematic axes of the main body is unambiguous evidence of triaxiality. The similarity of the stellar populations in the two components suggests that the observed kinematic structure has not changed substantially in 12 Gyr.

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arXiv2000

Two Boehm-Vitense gaps in the main sequence of the Hyades

Hipparcos proper motions and trigonometric parallaxes allow the derivation of secular parallaxes which fix the distances to individual stars in the Hyades cluster to an accuracy of \sim 2 percent. The resulting color-absolute magnitude diagram for 92 high-fidelity single members of the cluster displays a very narrow main sequence, with two turn-offs and associated gaps. These occur at the locations where the onset of surface convection affects the B-V colors, as predicted by Boehm-Vitense thirty years ago. The new distances provide stringent constraints on the transformations of colors and absolute magnitudes to effective temperatures and luminosities, and on models of stellar interiors.

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arXiv2000

Mapping the substructure in the Galactic halo with the next generation of astrometric satellites

We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to build up the entire stellar halo, in order to investigate what the next generation of astrometric satellites will reveal by observing the halo of the Milky Way. We generate artificial DIVA, FAME and GAIA halo catalogues, in which we look for the signatures left by the accreted satellites. We develop a method based on the standard Friends-of-Friends algorithm applied to the space of integrals of motion. We find this simple method can recover about 50% of the different accretion events, when the observational uncertainties expected for GAIA are taken into account, even when the exact form of the Galactic potential is unknown. The recovery rate for DIVA and FAME is much smaller, but these missions, like GAIA, should be able to test the hierarchical formation paradigm on our Galaxy by measuring the amount of halo substructure in the form of nearby kinematically cold streams with for example, a two-point correlation function in velocity space.

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arXiv1999

Debris streams in the solar neighbourhood as relicts from the formation of the Milky Way

It is now generally believed that galaxies were built up through gravitational amplification of primordial fluctuations and the subsequent merging of smaller precursor structures. The stars of the structures that assembled to form the Milky Way should now make up much or all of its bulge and halo, in which case one hopes to find "fossil" evidence for those precursor structures in the present distribution of halo stars. Confirmation that this process is continuing came with the discovery of the Sagittarius dwarf galaxy, which is being disrupted by the Milky Way, but direct evidence that this process provided the bulk of the Milky Way's population of old stars has so far been lacking. Here we show that about ten per cent of the metal--poor stars in the halo of the Milky Way, outside the radius of the Sun's orbit, come from a single coherent structure that was disrupted during or soon after the Galaxy's formation. This object had a highly inclined orbit about the Milky Way at a maximum distance of $\sim$ 16 kpc, and it probably resembled the Fornax and Sagittarius dwarf spheroidal galaxies.

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arXiv1999

Axisymmetric Three-Integral Models for Galaxies

We describe an improved, practical method for constructing galaxy models that match an arbitrary set of observational constraints, without prior assumptions about the phase-space distribution function (DF). Our method is an extension of Schwarzschild's orbit superposition technique. As in Schwarzschild's original implementation, we compute a representative library of orbits in a given potential. We then project each orbit onto the space of observables, consisting of position on the sky and line-of-sight velocity, while properly taking into account seeing convolution and pixel binning. We find the combination of orbits that produces a dynamical model that best fits the observed photometry and kinematics of the galaxy. A key new element of this work is the ability to predict and match to the data the full line-of-sight velocity profile shapes. A dark component (such as a black hole and/or a dark halo) can easily be included in the models. We have tested our method, by using it to reconstruct the properties of a two-integral model built with independent software. The test model is reproduced satisfactorily, either with the regular orbits, or with the two-integral components. This paper mainly deals with the technical aspects of the method, while applications to the galaxies M32 and NGC 4342 are described elsewhere (van der Marel et al., Cretton & van den Bosch). (abridged)

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arXiv1998

Detecting Halo Streams with GAIA

We investigate what the proposed ESA astrometric satellite GAIA will reveal by observing the halo of the Milky Way. Specifically, we look for halo streams which are the signatures left by the merging/accretion events experienced by a typical galaxy like the Milky Way. We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to generate artificial GAIA halo catalogs. We recover the streams by searching for peaks in angular momentum space.

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arXiv1998

The microlensing rate and mass function vs. dynamics of the Galactic bar

With the steady increase of the sample size of observed microlenses towards the central regions of the Galaxy, the main source of the uncertainty in the lens mass will shift from the simple Poisson noise to the intrinsic non-uniqueness of our dynamical models of the inner Galaxy, particularly, the Galactic bar. We use a set of simple self-consistent bar models to investigate how the microlensing event rate varies as a function of axis ratio, bar angle and velocity distribution. The non-uniqueness of the velocity distribution of the bar model adds a significant uncertainty (by about a factor of 1.5) to any prediction of the lens mass. Kinematic data and self-consistent models are critical to lift the non-uniqueness. We discuss the implications of these results for the interpretation of microlensing observations of the Galactic bulge. In particular we show that Freeman bar models scaled to the mass of the Galactic bulge/bar imply a typical lens mass of around $0.8M_\odot$, a factor of 3-5 times larger than the value from other models.

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arXiv1996

Self-Consistent, Axisymmetric Two_Integral Models of Elliptical Galaxies with embedded Nuclear Discs

Recently, observations with the Hubble Space Telescope have revealed small stellar discs embedded in the nuclei of a number of ellipticals and S0s. In this paper we construct two-integral axisymmetric models for such systems. We calculate the even part of the phase-space distribution function, and specify the odd part by means of a simple parameterization. We investigate the photometric as well as the kinematic signatures of nuclear discs, including their velocity profiles (VPs), and study the influence of seeing convolution. The rotation curve of a nuclear disc gives an excellent measure of the central mass-to-light ratio whenever the VPs clearly reveal the narrow, rapidly rotating component associated with the nuclear disc. Steep cusps and seeing convolution both result in central VPs that are dominated by the bulge light, and these VPs barely show the presence of the nuclear disc, impeding measurements of the central rotation velocities of the disc stars. However, if a massive BH is present, the disc component of the VP can be seen in the wing of the bulge part, and measurements of its mean rotation provide a clear signature of the presence of the BH. This signature is insensitive to the uncertainties in the velocity anisotropy, which often lead to ambiguity in the interpretation of a central rise in velocity dispersion as due to a central BH.

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arXiv1996

Scale-free dynamical models for galaxies: flattened densities in spherical potentials

This paper presents two families of phase-space distribution functions (DFs) that generate scale-free spheroidal mass densities in scale-free spherical potentials. The `case I' DFs are anisotropic generalizations of the flattened f(E,L_z) model, which they include as a special case. The `case II' DFs generate flattened constant-anisotropy models, in which the constant ratio of rms tangential to radial motion is characterized by Binney's parameter beta. The models can describe the outer parts of galaxies and the density cusp structure near a central black hole, but also provide general insight into the dynamical properties of flattened systems. The dependence of the intrinsic and projected properties on the model parameters and the inclination is described. The observed ratio of the rms projected line-of-sight velocities on the projected major and minor axes of elliptical galaxies is best fit by the case II models with beta > 0. These models also predict non-Gaussian velocity profile shapes consistent with existing observations. The distribution functions are used to model the galaxies NGC 2434 (E1) and NGC 3706 (E4), for which stellar kinematical measurements out to two effective radii indicate the presence of dark halos (Carollo et al.). The velocity profile shapes of both galaxies can be well fit by radially anisotropic case II models with a spherical logarithmic potential. This contrasts with the f(E,L_z) models studied previously, which require flattened dark halos to fit the data.

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