Frank C. van den Bosch

Dwarf galaxy rotation curves and the core problem of dark matter haloes

ABSTRA C T The standard cold dark matter (CDM) model has recently been challenged by the claim that dwarf galaxies have dark matter haloes with constant-density cores, whereas CDM predicts haloes with steeply cusped density distributions. Consequently, numerous alternative dark matter candidates have recently been proposed. In this paper we scrutinize the observational evidence for the incongruity between dwarf galaxies and the CDM model. To this end, we analyse the rotation curves of 20 late-type dwarf galaxies studied by Swaters. Taking the effects of beam smearing and adiabatic contraction into account, we fit mass models to these rotation curves with dark matter haloes with different cusp slopes, ranging from constantdensity cores to r 22 cusps. Even though the effects of beam smearing are small for these data, the uncertainties in the stellar mass-to-light ratio and the limited spatial sampling of the halo’s density distribution hamper a unique mass decomposition. Consequently, the rotation curves in our sample cannot be used to discriminate between dark haloes with constant-density cores and r 21 cusps. We show that the dwarf galaxies analysed here are consistent with CDM haloes in a LCDM cosmology, and that there is thus no need to abandon the idea that dark matter is cold and collisionless. However, the data are also consistent with any alternative dark matter model that produces dark matter haloes with central cusps less steep than r 21.5 . In fact, we argue that based on existing H I rotation curves alone, at best weak limits can be obtained on cosmological parameters and/or the nature of the dark matter. In order to make progress, rotation curves with higher spatial resolution and independent measurements of the mass-to-light ratio of the disc are required.

Constraints on the Structure of Dark Matter Halos from the Rotation Curves of Low Surface Brightness Galaxies

We reexamine the disk-halo decompositions of the rotation curves of low surface brightness (LSB) galaxies with Vmax ≥ 80 km s-1, taking full account of the effects of beam smearing. We show that the spatial resolution of the data is not sufficient to put any meaningful constraints on the density profiles of the dark halos, or on cosmological parameters. This is in strong contrast to claims made in the literature that these LSB rotation curves are only consistent with dark matter halos with shallow central cusps, and it has important implications regarding the halos of LSB galaxies, such as the self-similarity of their rotation curves, and their inconsistency with certain cosmological models or with cold dark matter altogether. Only in one case are the data of sufficient spatial resolution to obtain reliable constraints on the slope of the central density distribution of the dark matter halo. For this single case, we find a central cusp ρ ∝ r-α with 0.55 < α < 1.26 at the 99.73% confidence level. This contrasts strongly with the results for two dwarf galaxies (Vmax < 70 km s-1) that we analyze, which yield α < 0.5 at the same level of confidence. This possibly suggests that halos with constant-density cores are restricted to low-mass systems. We show that violent outflows of baryonic matter by supernova feedback can reproduce this mass dependence of halo cusp slopes.

WFPC2 Images of the Central Regions of Early-Type Galaxies. I. The Data

We present high-resolution R-band images of the central regions of 67 early-type galaxies obtained with the Wide Field and Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope (HST). This homogeneously selected sample roughly doubles the number of early-type galaxies that have now been imaged at HST resolution and complements similar data on the central regions of radio galaxies and the bulges of spiral galaxies. Our sample strikingly confirms the complex morphologies of the central regions of early-type galaxies which have become apparent from previous studies with HST. In particular, we detect dust, either in the form of nuclear disks or with a filamentary distribution, in 43% of all galaxies, in good agreement with previous estimates. In addition, we find evidence for embedded stellar disks in a remarkably large fraction of 51%. In 14 of those galaxies the disklike structures are misaligned with the main galaxy, suggesting that they correspond to stellar bars in S0 galaxies. We analyze the luminosity profiles of the galaxies in our sample and classify galaxies according to their central cusp slope. To a large extent we confirm the results from previous HST surveys in that early-type galaxies reveal a clear dichotomy: the bright ellipticals (MB -20.5) are generally boxy and have luminosity profiles that break from steep outer power laws to shallow inner cusps (referred to as core galaxies). The fainter ellipticals, on the other hand, typically have disky isophotes and luminosity profiles that lack a clear break and have a steep central cusp (referred to as power-law galaxies). The advantages and shortcomings of classification schemes utilizing the extrapolated central cusp slope γ are discussed, and it is shown that γ might be an inadequate representation for galaxies whose luminosity profile slope changes smoothly with radius rather than resembling a broken power law. Thus, we introduce a new, alternative parameter and show how this affects the classification. In fact, we find evidence for an intermediate class of galaxies that cannot unambiguously be classified as either core or power-law galaxies and that have central cusp slopes and absolute magnitudes intermediate between those of core and power-law galaxies. It is unclear at present, however, whether these galaxies make up a physically distinct class or whether distance and/or resolution effects cause them to lose their distinct core or power-law characteristics.

Hubble Space Telescope photometry of the central regions of Virgo cluster elliptical galaxies. 3: Brightness profiles

We have used the Planetary Camera on the Hubble Space Telescope (HST) to study the morphology and surface brightness parameters of a luminosity-limited sample of fourteen elliptical galaxies in the Virgo cluster. The total apparent blue magnitudes of the galaxies range between 9.4 and 13.4. In this paper, the core brightness profiles are presented, while the overall morphology and the isophotal shapes are discussed in two companion papers (Jaffe et al. (1994); van den Bosch et al. (1994)). We show that, in spite of the spherical aberration affecting the HST primary mirror, deconvolution techniques allow recovery of the brightness profile up to 0.2 arcsec from the center of the galaxies. We find that none of the galaxies has an isothermal core. On the basis of their morphological and photometrical properties, the galaxies can be divided in two physically distinct groups, referred to as Type I and Type II. All of the Type I galaxies are classified as E1 to E3 in the Revised Shapley Ames Catalog (Sandage & Tammann 1981), while Type II galaxies are classified as E5 to E7. The characteristics of Type II galaxies are explained by the presence of disks component on both the 1 arcsec and the 10 arcsec scales, while Type I galaxies correspond to the classical disk-free ellipticals.

The Formation of Disk-Bulge-Halo Systems and the Origin of the Hubble Sequence

We investigate the formation of disk-bulge-halo systems by including bulges in the Fall & Efstathiou theory of disk formation. This allows an investigation of bulge-dominated disk galaxies, such as S0s and disky ellipticals. These latter systems, which consist of an elliptical spheroid with an embedded disk with a scale length of typically a few hundred parsecs, seem to form a smooth sequence with spirals and S0s toward a lower disk-to-bulge ratio. The aim of this paper is to examine whether spirals, S0s, and disky ellipticals all can be incorporated in one simple galaxy-formation scenario. We investigate an inside-out formation scenario in which subsequent layers of gas cool and form stars inside a virialized dark halo. The inner, low angular momentum material is assumed to form the bulge. Stability arguments are used to suggest that this bulge formation is a self-regulating process in which the bulge grows until it is massive enough to allow the remaining gas to form a stable disk component. We assume that the baryons that build the disk do not lose their specific angular momentum, and we search for the parameters and physical processes that determine the disk-to-bulge ratio and therewith explain to a large extent the origin of the Hubble sequence. The spread in halo angular momenta coupled with a spread in the formation redshifts can explain the observed spread in disk properties and disk-to-bulge ratios from spirals to S0s. If galaxy formation is efficient, and all available baryons are transformed into the disk-bulge system, cosmologies with Ω0 0.3 can be excluded since stable spiral disks would not be allowed to form. If we assume, however, that the efficiency with which galaxies form depends on the formation redshift, as suggested by the small amount of scatter in the observed Tully-Fisher relation, and we assume that the probability for a certain baryon ultimately to end up in the disk or bulge is independent of its specific angular momentum, spirals are allowed to form, but only at small formation redshifts (z 1). At higher formation redshifts, stability arguments suggest the formation of systems with smaller disk-to-bulge ratios, such as S0s. Since density perturbations in clusters will generally collapse earlier than those in the field, this scenario naturally predicts a density-morphology relation, the amplitude of which depends on the baryon fraction of the universe. Disky ellipticals are too compact to be incorporated in this scenario, and thus they do not form a continuous sequence with spirals and S0s, at least not in the sense of the galaxy-formation scenario envisioned in this paper. Alternative formation scenarios for the disky ellipticals, such as gas-rich mergers or an internal mass-loss origin for the embedded disks, are much more viable.

Semianalytical Models for the Formation of Disk Galaxies. I. Constraints from the Tully-Fisher Relation

We present new semianalytical models for the formation of disk galaxies with the purpose of investigating the origin of the near-infrared Tully-Fisher (TF) relation. The models assume that disks are formed by cooling of the baryons inside dark halos with realistic density profiles and that the baryons conserve their specific angular momentum. Adiabatic contraction of the dark halo is taken into account, as well as a recipe for bulge formation based on a self-regulating mechanism that ensures disks to be stable. Only gas with densities above the critical density given by Toomres stability criterion is considered eligible for star formation. A Schmidt law is assumed to prescribe the rate at which this gas is transformed into stars. The introduction of the star formation threshold density proves an essential ingredient of our models and yields gas mass fractions that are in excellent agreement with observations. Finally, a simple recipe for supernovae feedback is included. We emphasize the importance of extracting the proper luminosity and velocity measures from the models, something that has often been ignored in the past. We use the zero point of the K-band TF relation to place stringent constraints on cosmological parameters. In particular, we rule out a standard cold dark matter universe, in which disk galaxies are too faint to be consistent with observations. The TF zero point, in combination with nucleosynthesis constraints on the baryon density, and with constraints on the normalization of the power spectrum, requires a matter density Ω0 0.3. The observed K-band TF relation has a slope that is steeper than simple predictions based on dynamical arguments suggest. Taking the stability related star formation threshold densities into account steepens the TF relation and decreases its scatter. However, in order for the slope to be as steep as observed, further physics are required. We argue that the characteristics of the observed near-infrared TF relation do not reflect systematic variations in stellar populations, or cosmological initial conditions. In fact, feedback seems an essential ingredient in order to explain the observed slope of the K-band TF relation. Finally, we show that our models provide a natural explanation for the small amount of scatter that makes the TF relation useful as a cosmological distance indicator.

Substructure in Dark Halos: Orbital Eccentricities and Dynamical Friction

The virialized regions of galaxies and clusters contain significant amounts of substructure; clusters have hundreds to thousands of galaxies, and satellite systems and globular clusters orbit the halos of individual galaxies. These orbits can decay owing to dynamical friction. Depending on their orbits and their masses, the substructures either merge, are disrupted, or survive to the present day. We examine the distributions of eccentricities of orbits within mass distributions similar to those we see for galaxies and clusters. A comprehensive understanding of these orbital properties is essential to calculate the rates of physical processes relevant to the formation and evolution of galaxies and clusters. We derive the orbital eccentricity distributions for a number of spherical potentials. These distributions depend strongly on the velocity anisotropy, but only slightly on the shape of the potential. The eccentricity distributions in the case of an isotropic distribution function are strongly skewed toward high eccentricities, with a median value of typically ~0.6, corresponding to an apocenter-to-pericenter ratio of 4.0. We also present high-resolution N-body simulations of the orbital decay of satellite systems on eccentric orbits in an isothermal halo. The dynamical friction timescales are found to decrease with increasing orbital eccentricity because of the dominating deceleration at the orbits pericenter. The orbital eccentricity stays remarkably constant throughout the decay; although the eccentricity decreases near pericenter, it increases again near apocenter, such that there is no net circularization. We briefly discuss several applications for our derived distributions of orbital eccentricities and the resulting decay rates from dynamical friction. We compare the theoretical eccentricity distributions to those of globular clusters and galactic satellites for which all six phase-space coordinates (and therewith their orbits) have been determined. We find that the globular clusters are consistent with a close-to-isotropic velocity distribution, and they show large orbital eccentricities because of this (not in spite of this, as has been previously asserted). In addition, we find that the limited data on the Galactic system of satellites appears to be different and warrants further investigation as a clue to the formation and evolution of our Milky Way and its halo substructure.

Evidence for a Massive Black Hole in the S0 Galaxy NGC 4342

We present axisymmetric dynamical models of the edge-on S0 galaxy NGC 4342. This small low-luminosity galaxy harbors, in addition to its outer disk, a bright nuclear stellar disk. A combination of observations from the ground and with the Hubble Space Telescope (HST) has shown that NGC 4342 rotates rapidly and has a strong central increase in velocity dispersion. We construct simple two-integral Jeans models as well as fully general, three-integral models. The latter are built using a modified version of Schwarzschilds orbit-superposition technique developed by Rix et al. and Cretton et al. These models allow us to reproduce the full line-of-sight velocity distributions, or velocity profiles (VPs), which we parameterize by a Gauss-Hermite series. The modeling takes seeing convolution and pixel binning into account. The two-integral Jeans models suggest a black hole (BH) mass between 3 and 6×108 M☉, depending on the data set used to constrain the model, but they fail to fit the details of the observed kinematics. The three-integral models can fit all ground-based and HST data simultaneously, but only when a central BH is included. Models without BHs are ruled out to a confidence level of better than 99.73%. We determine a BH mass of 3.0+1.7−1.0×108 M☉, where the errors are the formal 68.3% confidence levels. This corresponds to 2.6% of the total mass of the bulge, making NGC 4342 one of the galaxies with the highest BH mass to bulge mass ratio currently known. The models that best fit the data do not have a two-integral phase-space distribution function. They have rather complex dynamical structures: the velocity anisotropies are strong functions of radius reflecting the multicomponent structure of this galaxy. When no central BH is included, the best-fit model tries to fit the high central velocity dispersion by placing stars on radial orbits. The high rotation velocities measured, however, restrict the amount of radial anisotropy such that the central velocity dispersion measured with the HST can be fit only when a massive BH is included in the models.

Hubble Space Telescope photometry of the central regions of Virgo cluster elliptical galaxies. II: Isophote shapes.

The isophotal shapes of a magnitude limited sample of Virgo ellipticals are presented. These are derived from high resolution Hubble Space Telescope (HST) photometry. The absence of atmospheric seeing and accurate knowledge of the Point Spread Function (PSF) allows us to perform an accurate deconvolution. Model galaxies were constructed to test the deconvolution algorithms used, and showed that we can accurately recover isophotal shape parameters down to 0.5 sec. From the isophotal parameters we can classify the galaxies in two subsamples: disky and non-disky galaxies. In three of these disky galaxies we found evidence for a nuclear stellar disk in the inner 1.5 sec. In addition these galaxies also have an outer disk, that seems to break up inside 2 sec - 3 sec. In the two galaxies for which there is kinematic evidence from the literature of a decoupled core, we found no indication for such subsystem from the isophotal shape analysis. In 80% of these early type galaxies there are indications for dust. For eight of these galaxies the dust has not been detected before.

Nuclear stellar discs in early‐type galaxies — II. Photometric properties

Hubble Space Telescope images of two early-type galaxies harboring both nuclear and outer stellar discs are studied in detail. By means of a photometric decomposition, the images of NGC 4342 and NGC 4570 are analyzed and the photometric properties of the nuclear discs investigated. We find a continuity of properties in the parameter space defined by the central surface brightness and the scalelength of discs in spirals, S0s and embedded discs in ellipticals, in the sense that the nuclear discs extend the observed disc properties even further towards smaller scalelengths and brighter central surface brightnesses. When including the nuclear discs, disc properties span more than four orders of magnitude in both scalelength and central surface brightness. The nuclear discs studied here are the smallest and brightest stellar discs known, and as such, they are as extreme in their photometric properties as Malin I, when compared to typical galactic discs that obey Freemans law. We discuss a possible formation scenario in which the double-disc structure observed in these galaxies has been shaped by now dissolved bars. Based on the fact that the black holes known to exist in some of these galaxies have masses comparable to those of the nuclear discs, we explore a possible link between the black holes and the nuclear discs.