arXiv subjects

Herwig Dejonghe

Publications and source records attributed to Herwig Dejonghe.

34 records · Page 2Linked to original sources

arXiv2004

Observational evidence for a connection between SMBHs and dark matter haloes

We investigate the relation between circular velocity vc and bulge velocity dispersion sigma in spiral galaxies, based on literature data and new spectroscopic observations. We find a strong, nearly linear vc-sigma correlation with a negligible intrinsic scatter, and a striking agreement with the corresponding relation for elliptical galaxies. The least massive galaxies (sigma < 80 km/s) significantly deviate from this relation. We combine this vc-sigma correlation with the well-known MBH-sigma relation to obtain a tight correlation between circular velocity and supermassive black hole mass, and interpret this as observational evidence for a close link between supermassive black holes and the dark matter haloes in which they presumably formed. Apart from being an important ingredient for theoretical models of galaxy formation and evolution, the relation between MBH and circular velocity has the potential to become an important practical tool in estimating supermassive black hole masses in spiral galaxies.

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arXiv2004

A completely analytical family of dynamical models for spherical galaxies and bulges with a central black hole

We present a family of spherical models for elliptical galaxies and bulges consisting of a stellar component and a central black hole. All models in this family share the same stellar density profile, which has a steep central cusp. The gravitational potential of each models is a linear combination of the potential generated selfconsistently by the stars and the potential of a central black hole. The relative importance of these two contributions is a free parameter in the models. Assuming an isotropic dynamical structure, almost all kinematical properties of these models can be calculated analytically. In particular, they form the first simple dynamical models for galaxies with a central black hole where the distribution function and differential energy distribution can be written completely in terms of elementary functions only. We also present various extensions of this family to models with anisotropic orbital structures. Also for these dynamical models, the distribution function and its moments can be expressed completely in terms of elementary functions. This family is useful for a large range of applications, in particular to generate initial conditions for N-body and hydrodynamical simulations to model galactic nuclei with a central black hole.

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arXiv2003

Tracing the relation between black holes and dark haloes

We present new velocity dispersion measurements for a set of 12 spiral galaxies and use them to derive a more accurate V_c - sigma relation which holds for a wide morphological range of galaxies. Combined with the M_BH - sigma relation, this relation can be used as a tool to estimate supermassive black hole (SMBH) masses by means of the asymptotic circular velocity. Together with the Tully-Fisher relation, it serves as a constraint for galaxy formation and evolution models.

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arXiv2003

Radiative transfer in disc galaxies -- III. The observed kinematics of dusty disc galaxies

We present SKIRT (Stellar Kinematics Including Radiative Transfer), a new Monte Carlo radiative transfer code that allows the calculation of the observed stellar kinematics of a dusty galaxy. The code incorporates the effects of both absorption and scattering by interstellar dust grains, and calculates the Doppler shift of the emerging radiation exactly by taking into account the velocities of the emitting stars and the individual scattering dust grains. The code supports arbitrary distributions of dust through a cellular approach, whereby the integration through the dust is optimized by means of a novel efficient trilinear interpolation technique. We apply our modelling technique to calculate the observed kinematics of realistic models for dusty disc galaxies. We find that the effects of dust on the mean projected velocity and projected velocity dispersion are severe for edge-on galaxies. For galaxies which deviate more than a few degrees from exactly edge-on, the effects are already strongly reduced. As a consequence, dust attenuation cannot serve as a possible way to reconcile the discrepancy between the observed shallow slopes of the inner rotation curves of LSB galaxies and the predictions of CDM cosmological models. For face-on galaxies, the velocity dispersion increases with increasing dust mass due to scattering, but the effects are limited, even for extended dust distributions. Finally, we show that serious errors can be made when the individual velocities of the dust grains are neglected in the calculations.

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arXiv2003

Observational evidence for a connection between supermassive black holes and dark matter haloes

We present new velocity dispersion measurements of sample of 12 spiral galaxies for which extended rotation curves are available. These data are used to refine a recently discovered correlation between the circular velocity and the central velocity dispersion of spiral galaxies. We find a slightly steeper slope for our larger sample, we confirm the negligible intrinsic scatter on this correlation, and we find a striking agreement with a corresponding relation for elliptical galaxies. We combine this correlation with the well-known MBH-sigma relation to obtain a tight correlation between the circular velocities of galaxies and the masses of the supermassive black holes they host. This correlation is the observational evidence for an intimate link between dark matter haloes and supermassive black holes. Apart from being an important ingredient for theoretical models of galaxy formation and evolution, the relation between MBH and circular velocity can serve as a practical tool to estimate black hole masses in spiral galaxies.

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arXiv2002

The Hernquist model revisited: completely analytical anisotropic dynamical models

Simple analytical models, such as the Hernquist model, are very useful tools to investigate the dynamical structure of galaxies. Unfortunately, most of the analytical distribution functions are either isotropic or of the Osipkov-Merritt type, and hence basically one-dimensional. We present three different families of anisotropic distribution functions that self-consistently generate the Hernquist potential-density pair. These families have constant, increasing and decreasing anisotropy profiles respectively, and can hence represent a wide variety of orbital structures. For all of the models presented, the distribution function and the velocity dispersions can be written in terms of elementary functions. These models are ideal tools for a wide range of applications, in particular to generate the initial conditions for N-body or Monte Carlo simulations.

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arXiv2002

Kinematics of elliptical galaxies with a diffuse dust component - III. A Monte Carlo approach to include the effects of scattering

This paper is the third one in a series, intended to investigate how the observed kinematics of elliptical galaxies are affected by dust attenuation. In Paper I and Paper II, we investigated the effects of dust absorption; here we extend our modelling in order to include the effects of scattering. We describe how kinematical information can be combined with the radiative transfer equation, and present a Monte Carlo code that can handle kinematical information in an elegant way. Compared to the case where only absorption is taken into account, we find that dust attenuation considerably affects the observed kinematics when scattering is included. For the central lines of sight, dust can either decrease or increase the central observed velocity dispersion. The most important effect of dust attenuation, however, is found at large projected radii. The kinematics at these lines of sight are strongly affected by photons scattered into these lines of sight, which were emitted by high-velocity stars in the central regions of the galaxy. These photons bias the LOSVDs towards high line-of-sight velocities, and significantly increase the observed velocity dispersion and LOSVD shape parameters. These effects are similar to the expected kinematical signature of a dark matter halo, such that dust attenuation may form an alternative explanation for the usual stellar kinematical evidence for dark matter halos around elliptical galaxies. We apply our results to discuss several other topics in galactic dynamics, where we feel dust attenuation should be taken into account. In particular, we argue that the kinematics observed at various wavelengths can help to constrain the spatial distribution of dust in elliptical galaxies.

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arXiv2002

The influence of binary stars on the kinematics of low-mass galaxies

In this paper, the influence of binary stars on the measured kinematics of dwarf galaxies is investigated. Using realistic distributions of the orbital parameters (semi-major axis, eccentricity, ...), analytical expressions are derived for the changes induced by the presence of binary stars in the measured velocity moments of low-mass galaxies (such as the projected velocity dispersion and the 4th order Gauss-Hermite coefficient h4). It is shown that there is a noticeable change in the observed velocity dispersion if the intrinsic velocity dispersion of a galaxy is of the same order as the binary velocity dispersion. The kurtosis of the line-of-sight velocity distribution (LOSVD) is affected even at higher values of the intrinsic velocity dispersion.

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arXiv2001

Dark matter halos around elliptical galaxies: how reliable is the stellar kinematical evidence ?

Hierarchical models of galaxy formation and various observational evidence suggest that elliptical galaxies are, like disk galaxies, embedded in massive dark matter halos. Stellar kinematics are considered the most important tracer for this dark halo at a few effective radii. Using detailed modeling techniques, several authors have recently presented stellar kinematical evidence of a dark halo for a number of elliptical galaxies. In these modeling techniques, dust attenuation (absorption and scattering of starlight by dust grains) has not been taken into account. Nevertheless, elliptical galaxies contain a significant amount of interstellar dust, which affects all observable quantities, including the observed kinematics. We constructed a set of dynamical models for elliptical galaxies, in which dust attenuation is included through a Monte Carlo technique. We find that a dust component, shallower than the stellar distribution and with an optical depth of order unity, affects the observed kinematics significantly, in the way that it mimics the presence of a dark halo. If such dust distributions are realistic in elliptical galaxies, we are faced with a new mass-dust degeneracy. Taking dust into account in dynamical modeling procedures will hence reduce or may even eliminate the need for a dark matter halo at a few effective radii.

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arXiv2001

Radiative transfer in disc galaxies I - A comparison of four methods to solve the transfer equation in plane-parallel geometry

Accurate photometric and kinematic modelling of disc galaxies requires the inclusion of radiative transfer models. Due to the complexity of the radiative transfer equation (RTE), sophisticated techniques are required. Various techniques have been employed for the attenuation in disc galaxies, but a quantitative comparison of them is difficult, because of the differing assumptions, approximations and accuracy requirements which are adopted in the literature. In this paper, we present an unbiased comparison of four methods to solve the RTE, in terms of accuracy, efficiency and flexibility. We apply them all on one problem that can serve as a first approximation of large portions of disc galaxies: a one-dimensional plane-parallel geometry, with both absorption and multiple scattering taken into account, with an arbitrary vertical distributions of stars and dust and an arbitrary angular redistribution of the scattering. We find that the spherical harmonics method is by far the most efficient way to solve the RTE, whereas both Monte Carlo simulations and the iteration method, which are straightforward to extend to more complex geometries, have a cost which is about 170 times larger.

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arXiv2001

Radiative transfer in disc galaxies II - The influence of scattering and geometry on the attenuation curve

We investigate the influence of scattering and geometry on the attenuation curve in disc galaxies. We investigate both qualitatively and quantitatively which errors are made by either neglecting or approximating scattering, and which uncertainties are introduced due to a simplification of the star-dust geometry. We find that the magnitude of these errors depends on the inclination of the galaxy, and in particular that for face-on galaxies, the errors due to an improper treatment of scattering dominate those due to an imprecise star-dust geometry. Therefore we argue that in all methods which aim at determining the opacity of disc galaxies, scattering should be taken into account in a proper way.

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arXiv2000

Dynamical models for dusty disk galaxies

Disk galaxies contain a large amount of interstellar dust, which affects the projection of kinematic quantities. We investigate in detail the effects of dust extinction on the mean projected velocity and the projected velocity dispersion. We use our results to construct a general strategy to determine the dynamical structure of disk galaxies, with the aim to constrain their mass distribution and dynamical history.

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arXiv2000

Kinematics of elliptical galaxies with a diffuse dust component - II. Dust effects on kinematic modelling

Observations have demonstrated the presence of substantial amounts of interstellar dust in elliptical galaxies, most of which is believed to be distributed diffusely over the galaxy. Absorption by dust grains has a major impact on the transfer of stellar radiation, and affects the projection of each physical (and kinematic) quantity. In a previous paper, we have investigated the effects of a diffusely distributed dust component on the observed kinematics of spherical galaxies. In this paper we investigate the effect of not taking dust into account in dynamical modelling procedures. We use a set of semi-analytical dusty galaxy models to create synthetic LOSVD data sets, which we then model as if no dust were present. We find some considerable differences between the best-fitting models and the input models, and we find that these differences are dependent on the orbital structure of the input galaxy. For radial and isotropic models on the one hand, we find that the dynamical mass of the models decreases nearly linearly with optical depth, with an amount of 5 per cent per optical depth unit, whereas their orbital structure is hardly affected. For tangential models on the other hand, the dynamical mass decrease is smaller, but their orbital structure is affected: their distribution functions appears less tangentially anisotropic. For all models the mass-to-light ratio will be underestimated, by a factor of around 20 per cent per optical depth unit. We discuss these results in the light of the limited effects of dust extinction on the LOSVDs, as obtained in paper I, and conclude that the determination of the dynamical mass and the kinematic structure of galaxies... (for full abstract, see paper)

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arXiv1999

Kinematics of elliptical galaxies with a diffuse dust component

Observations show that early-type galaxies contain a considerable amount of interstellar dust, most of which is believed to exist as a diffusely distributed component. We construct a four-parameter elliptical galaxy model in order to investigate the effects of such a smooth absorbing component on the projection of kinematic quantities, such as the line profiles and their moments. We investigate the dependence on the optical depth and on the dust geometry. Our calculations show that both the amplitude and the morphology of these quantities can be significantly affected. Dust effects should therefore be taken in consideration when interpreting photometric and kinematic properties, and correlations that utilize these quantities.

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arXiv1998

Dynamical models of NGC 3115

We present new dynamical models of the S0 galaxy N3115, making use of the available published photometry and kinematics as well as of two-dimensional TIGER spectrography. We first examined the kinematics in the central 40 arcsec in the light of two integral f(E,J) models. Jeans equations were used to constrain the mass to light ratio, and the central dark mass whose existence was suggested by previous studies. The even part of the distribution function was then retrieved via the Hunter & Qian formalism. We thus confirmed that the velocity and dispersion profiles in the central region could be well fit with a two-integral model, given the presence of a central dark mass of ~10^9 Msun. However, no two integral model could fit the h_3 profile around a radius of 25 arcsec where the outer disc dominates the surface brightness distribution. Three integral analytical models were therefore built using a Quadratic Programming technique. These models showed that three integral components do indeed provide a reasonable fit to the kinematics, including the higher Gauss-Hermite moments. Again, models without a central dark mass failed to reproduce the observed kinematics in the central arcseconds. This clearly supports the presence of a nuclear black hole of at least 6.5 10^8 Msun in the centre of NGC 3115. These models were finally used to estimate the importance of the dark matter in the outer part of NGC 3115, suggested by the flat stellar rotation curve observed by Capaccioli et al. (1993).

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arXiv1998

The Three-Dimensional Mass Distribution in NGC 1700

A variety of modeling techniques is used with surface photometry from the literature and recently acquired high-accuracy stellar kinematic data to constrain the three-dimensional mass distribution in the luminous cuspy elliptical galaxy NGC 1700. First, we model the radial velocity field and photometry, and, using a Bayesian technique, estimate the triaxiality T and short-to-long axis ratio c in five concentric annuli between approximately 1 and 3 effective radii. The results are completely consistent with T being constant inside about 2.5 r_e (36 arcsec; 6.7/h kpc). Adding an assumption of constant T as prior information gives an upper limit of T < 0.16 (95% confidence); this relaxes to T < 0.22 if it is also assumed that there is perfect alignment between the angular momentum and the galaxy's intrinsic short axis. Near axisymmetry permits us then to use axisymmetric models to constrain the radial mass profile. Using the Jeans (moment) equations, we demonstrate that 2-integral, constant-M/L models cannot fit the data; but a 2-integral model in which the cumulative enclosed M/L increases by a factor of roughly 2 from the center out to 12/h kpc can. Three-integral models constructed by quadratic programming show that, in fact, no constant-M/L model is consistent with the kinematics. Anisotropic 3-integral models with variable M/L, while not uniquely establishing a minimum acceptable halo mass, imply, as do the moment models, a cumulative M/L_B approximately 10 h at 12/h kpc. We conclude that NGC 1700 represents the best stellar dynamical evidence to date for dark matter in elliptical galaxies.

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