Microlensing experiments have suggested that a significant part of the dark halo of the Milky Way could be composed of matter in the form of massive compact halo objects (MACHOs). Cool ancient white dwarfs (WDs) are the natural candidates. Here we present a new survey of halo WDs and evaluate the local space density using an accurate kinematic analysis. A comparison to a revaluation of the Oppenheimer et al. result is also provided. the local space density estimated for the two independent samples is about ~ 10–5M⊙ pc–3.

We increase the number of remote halo tracers by using blue horizontal branch (BHB) stars out to Galactocentric distances of 130 kpc. We use SDSS EDR photometry and the VLT to detect 16 BHB stars at Galactocentric distances 70 <r < 130 kpc, and to measure their radial velocities. We find the mass of the Milky Way is M = 1.7+3.0–0.6 × 1012M⊙. When completed this survey will: (i) substantially reduce the errors in the total mass and extent of the Milky Way halo, and (ii) map the velocity space in a hitherto unexplored region of the halo.

We present a database of galaxies rotation curve (from Hα observations), including our Galaxy.

We present a new estimate of the mass of the Milky Way based on the escape velocity of a sample of distant stars, about 12 kpc from the Galactic centre and about 5 kpc from the plane of the Galaxy. Our sample is very different from previous escape-velocity studies, being compiled from an all-object spectroscopic survey of a region of sky. the derived mass within 12 kpc of the Galactic centre is (1.3 ±0.3) × 1011M⊙.

New measurements of the velocity dispersion of the globular cluster ω Centauri allow us to trace its gravitational potential down to an acceleration of 8 × 10–9 cm s–2. It is found that the dispersion profile remains flat well inside the tidal radius as soon as the acceleration of gravity approaches a0, a result that finds its simplest explanation within the context of MOND. A similar behaviour is observed in the globular cluster M15 showing this is not a peculiar feature of ω Centauri. This result is surprising and suggestive of a failure of Newton's law at low accelerations.

The ACDM scenario to form galaxies encounters many problems when confronted with observations, namely the prediction of dark matter cusps in all galaxies, and in particular in dwarf irregulars, dominated by dark matter, or the low angular momentum and consequent small size of galaxy disks, or the high predicted number of small systems. We will consider the hypothesis that the baryonic dark matter could be in the form of gas and could be continuously involved in the galaxy formation scenario. By accreting cold gas throughout a galaxy's life, angular momentum could be increased, galaxy disks could be more dominated by baryons than previously thought, and small systems could merge more frequently.

The cosmic baryon budget is revisited using modern observations that have become available since our first publication. I also present an estimate for the heavy element abundance. An increased accuracy in the accounting of the baryon budget reveals ‘missing baryons’, which amount to ≈ 35% of the total. This would provide an interesting test for models of the cosmic structure formation.

In the inner parts of spiral galaxies, of high or low surface brightness, there is a close correlation between rotation curve shape and light distribution. For any feature in the luminosity profile there is a corresponding feature in the rotation curve and vice versa. This implies that the gravitational potential is strongly correlated with the distribution of luminosity: either the luminous mass dominates or there is a close coupling between luminous and dark matter. in a similar way, the declining rotation curves observed in the outer parts of high luminosity systems are a clear signature of the stellar disk which either dominates or traces the distribution of mass.

The notion that the baryons are dynamically important in the centres of galaxies, including LSBs, undermines the whole controversy over the cusps in CDM halos and the comparison with the observations. If the baryons dominate in the central regions of all spirals, including LSBs, how can the CDM profiles be compared with the observations? Alternatively, if the baryons do not dominate but simply trace the DM distribution, why, in systems of comparable luminosity, are some DM halos cuspy (like the light) and others (also like the light) are not?

We review the different cold dark gas models that have been proposed in the literature, as well as a new variant which addresses their principal stability problems by taking into account the property of molecular hydrogen to become solid or liquid below 33 K and at sufficiently high pressure. This new physical ingredient provides the possibility to stabilise cold gas globules by a core of condensed molecular hydrogen. Such loosely bound cold globules behave in a galaxy as a collisionless ensemble of matter, and form a reservoir of gas easily liberated through, e.g., UV excitation. the cold condensed cores survive the longest, of order a Gyr in the solar neighbourhood radiation field, and much longer in spiral outer HI disks.

In the last few years new evidence has been presented for the presence of ongoing massive star formation in the outer HI disks of galaxies. These discoveries strongly suggest that precursor molecular gas must also be present in some physical state which is escaping detection by the usual means (CO(1-0), IR, etc.). We present a model for such a gas in a framework which views the HI as the result of an ongoing “photodissociation ↔ dust grain reformation” equilibrium in a cold, clumpy molecular medium with a small area filling factor.

We report here an example of a series of computer simulations of the formation of disk galaxies, including a standard cold dark matter component, gas, and star formation, which result in objects which closely resemble observed giant galaxies, with a population of long-lived dwarf satellites which contain little or no non–baryonic dark matter.

Isolated barred galaxies evolve by redistributing their internal angular momentum, which is emitted mainly at the inner disc resonances and absorbed mainly at the resonances in the outer disc and the halo. This causes the bar to grow stronger and its pattern speed to decrease with time. A massive, responsive halo enhances this process. I show correlations and trends between the angular momentum absorbed by the halo and the bar strength, pattern speed and morphology. It is thus possible to explain why some disc galaxies are strongly barred, while others have no bar, or only a short bar or an oval. in some cases, a bar is found also in the halo component. This “halo bar” is triaxial, but more prolate-like, is shorter than the disc bar and rotates with roughly the same pattern speed. I finally discuss whether bars can modify the density cusps found in cosmological CDM simulations of dark matter haloes.

We discuss measurements of disk mass from non-circular streaming motions of gas in the barred galaxies NGC 3095 and NGC 4123. in these galaxies with strong shocks and non-circular motions, the inner regions must be disk-dominated to reproduce the shocks. This requires dark matter halos of low central density and low concentration, compared to LCDM halo predictions. in addition, the baryonic collapse to a disk should have compressed the halo and increased the dark matter density, which sharpens the disagreement. One possible resolution is a substantial amount of angular momentum transfer from disk to halo, but this is not particularly attractive nor elegant.

We measured the bar pattern speed in a sample of 7 SB0 galaxies using the Tremaine-Weinberg method. This represents the largest sample of galaxies for which the bar pattern speed has been measured this way. All the observed bars are as rapidly rotating as they can be. We compared this result with recent high-resolution N-body simulations of bars in cosmologically-motivated dark matter halos, and conclude that these bars are not located inside centrally concentrated halos.

We study the conditions that favour boxiness of isodensities in the face-on views of orbital 3D models for barred galaxies. in particular, we consider several cases with different mass values for the bar, disk and bulge components of the potential, as well as different values of the bar pattern speed. Using orbital weighted profiles of the basic stable periodic orbits of the x1 tree for every model, we show that boxiness is in general a composite effect due to the presence of stable orbits belonging to several 2D and 3D families.

In a 3D analytic potential we find the families of periodic orbits that support the formation of inner rings. These are families at high energies, between the inner radial ultraharmonic 4:1 (iUHR) resonance and corotation, influenced by the 4:1, 6:1 and 8:1 resonances. the inner rings they support are mainly ovals and polygons with ‘corners’ on the bar minor axis, on its sides, which correspond to morphologies often seen in real galaxies like NGC 6782 and IC 4290. We also investigate the conditions under which less probable shapes of rings may be supported by orbits at the region. Such rings include pentagonal features (NGC 3367) and hexagons with cusps on the major axis of the bar and two sides parallel to it (NGC 7020).

Two-dimensional velocity fields have been used to determine the dark matter properties of a sample of barred galaxies taken from the BIMA Survey of Nearby Galaxies (SONG). Preliminary results indicate that the maximal disk model is not appropriate in several galaxies in our sample, but higher resolution results will be needed to confirm this.

We present a simple technique to estimate mass-to-light (M/L) ratios of stellar populations based on two broadband photometry measurements, i.e. a colour-M/L relation. We apply the colour-M/L relation to galaxy rotation curves, using a large set of galaxies that span a great range in Hubble type, luminosity and scale size and that have accurately measured HI and/or Hα rotation curves. Using the colour-M/L relation, we construct stellar mass models of the galaxies and derive the dark matter contribution to the rotation curves.

We compare our dark matter rotation curves with adiabatically contracted Navarro, Frenk, & White (1997, NFW hereafter) dark matter halos. We find that before adiabatic contraction most high surface brightness galaxies and some low surface brightness galaxies are well fit by a NFW dark matter profile. However, after adiabatic contraction, most galaxies are poorly fit in the central few kpc. the observed angular momentum distribution in the baryonic component is poorly matched by ACDM model predictions, indicating that the angular momentum distribution is not conserved during the galaxy assembly process. We find that in most galaxies the dark matter distribution can be derived by scaling up the HI gas contribution. However, we find no consistent value for the scaling factor among all the galaxies.

We are studying the mass distribution in a sample of 50 early type spiral galaxies, with morphological type betweens SO and Sab and absolute magnitudes Mb between −18 and −22; they form the massive and high-surface brightness extreme of the disk galaxy population. Our study is designed to investigate the relation between dark and luminous matter in these systems, of which very little yet is known.

From a combination of WSRT Hi observations and long-slit optical spectra, we have obtained high-quality rotation curves. the rotation velocities always rise very fast in the center; in the outer regions, they are often declining, with the outermost measured velocity 10-25% lower than the maximum.

We decompose the rotation curves into contributions from the luminous (stellar and gaseous) and dark matter. the stellar disks and bulges always dominate the rotation curves within the inner few disk scale lengths, and are responsible for the decline in the outer parts. As an example, we present here the decompositions for UGC 9133. We are able to put tight upper and lower limits on the stellar mass-to-light ratios.

NGC 2915 is a blue compact dwarf galaxy embedded in an extended, low surface brightness HI disk with a bar and two-armed spiral structure. Common mechanisms are unable to explain those patterns and disk dark matter or a rotating triaxial dark halo were proposed as alternatives. Hydrodynamical simulations were run for each case and compared to observations using customized column density and kinematic constraints. the spiral structure can be accounted for by an unseen bar or triaxial halo, but the large bar mass or halo pattern frequency required make it unlikely that the spiral is driven by an external perturber. in particular, the spin parameter lambda is much higher than predicted by current CDM structure formation scenarios. Massive disk models show that when the gas surface density is scaled up by a factor of about 10, the disk develops a spiral structure matching the observed one in perturbed density as well as velocity. This suggests that the disk of NGC 2915 contains much more mass than is visible, tightly linked to the neutral hydrogen. A classic (quasi-)spherical halo is nevertheless still required, as increasing the disk mass further to fit the circular velocity curve would make the disk violently unstable