We present the results of deep, high velocity resolution (~1.6 km s−1) Giant Meterwave Radio Telescope (GMRT) HI 21 cm observations of extremely faint (Mb > −12.5) dwarf irregular galaxies. We find that all of our sample galaxies show systematic large scale velocity gradients, unlike earlier studies which found chaotic velocity fields for such faint galaxies. For some of the sample galaxies the velocity fields are completely consistent with ordered rotation, though the peak circular velocities are comparable to the observed random motions. These are the faintest known galaxies with such regular kinematics. We present (“asymmetric drift” corrected) rotation curves and mass models (including fits for Isothermal and NFW halos) for some of these galaxies and discuss the implications for hierarchical models of galaxy formation.

We present observations and analysis of rotation curves and dark matter halo density profiles in the central regions of four nearby dwarf galaxies. This observing program has been designed to overcome some of the limitations of other rotation curve studies that rely mostly on longslit spectra. We find that these objects exhibit the full range of central density profiles between ρ ∝ r0 (constant density) and ρ ∝ r–1 (NFW halo). This result suggests that there is a distribution of central density slopes rather than a unique halo density profile.

The results of a large radial velocity survey of the Draco and Ursa Minor dwarf spheroidal galaxies are presented. the velocity dispersion profiles of both objects are very similar: initially the dispersion increases with radius, while at radii approaching the outer limit of the stellar distribution we observe a sharp fall-off. We present the results of mass modeling based on these new data – the mass to light ratios are found to be about 400M⊙/L⊙. Evidence is also presented that the dark matter in the central regions of UMi has a core-like distribution. Finally, we demonstrate that the inner regions of UMi have not been affected by tides and discuss the implications of our data for MOdified Newtonian Dynamics (MOND).

A simple and natural explanation for the dynamics and morphology of the Local Group Dwarf Spheroidal galaxies, Draco (Dra) and Ursa Minor (UMi), is that they are weakly unbound stellar systems with no significant dark matter component. A gentle, but persistent, Milky Way (MW) tide has left them in their current kinematic and morphological state (the “parametric tidal excitation”). A new test of a dark matter dominated dS potential follows from a careful observation of the “clumpiness” of the dS stellar surface density.

We have measured precise (± 3 km/s) radial velocities for 180 stars in the Fornax dwarf spheroidal galaxy, spanning the region R=0 to just beyond the nominal tidal radius. We perform a “classical” analysis of the resulting velocity dispersion profile. A mass-follows-light King model is ruled out, while a constant velocity dispersion model remains a good fit out to the limits of our dataset. For the constant velocity dispersion case, we calculate a velocity dispersion of 11.1 ± 0.7 km/s, which implies a central M/L/[M/L]⊙ ratio of 7.6 ± 1.0.

We present here preliminary results of a multi-wavelength investigation into high HI mass-to-light ratio dwarf galaxies identified in the HIPASS Bright Galaxy Catalog. Here we focus on the rotational properties of some of the galaxies.

We derive inner dark matter halo density profile slopes for a sample of 200 dwarf galaxies by inverting rotation curves obtained from high-quality, long-slit optical spectra. Using simulations to assess the impact of long-slit observing and data processing errors on our measurements, we conclude that our observations are consistent with the cuspy halos predicted by the CDM paradigm.

We present a high-resolution rotation curve of the Local Group dwarf irregular NGC 6822, obtained with the Australia Telescope Compact Array. the best curve has a resolution of 8″ or 20 pc and contains some 250 independent points. NGC 6822 is a very dark matter-dominated galaxy. There is no evidence for the presence of a steep density cusp down to scales of ~ 20 pc, in conflict with the predictions of CDM.

Published mass models fitted to galaxy rotation curves are used to study the systematic properties of dark matter (DM) halos in late-type and dwarf spheroidal (dSph) galaxies. Halo parameters are derived by fitting non-singular isothermals to (V2 – V2vis)1/2, where V(r) is the observed rotation curve and Vvis is the rotation curve of the visible matter. the latter is calculated from the surface brightness assuming that the mass-to-light ratio M/L is constant with radius. “Maximum disk” values of M/L are adjusted to fit as much of the inner rotation curve as possible without making the halo have a hollow core. Rotation curve decomposition becomes impossible fainter than absolute magnitude Mb ≃ −14, where V becomes comparable to the velocity dispersion of the gas. To increase the luminosity range further, we include dSph galaxies, which are physically related to spiral and irregular galaxies. Combining the data, we find that DM halos satisfy well defined scaling laws analogous to the “fundamental plane” relations for elliptical galaxies. Halos in less luminous galaxies have smaller core radii rc, higher central densities ρ0, and smaller central velocity dispersions σ. Scaling laws provide new and detailed constraints on the nature of DM and on galaxy formation and evolution. Some simple implications include:

1 – A single, continuous physical sequence of increasing mass extends from dSph galaxies with Mb ≃ −7.6 to Sc I galaxies with Mb ≃ −22.4.

2 – the high DM densities in dSph galaxies are normal for such tiny galaxies. Since virialised density depends on collapse redshift zcoll, ρ0 ∝ (1 + zcoll)3, the smallest dwarfs formed at least Δzcoll ≃ 7 earlier than the biggest spirals.

3 – the high DM densities of dSphs implies that they are real galaxies formed from primordial density fluctuations. They are not tidal fragments. Tidal dwarfs cannot retain even the low DM densities of their giant-galaxy progenitors. in contrast, dSphs have higher DM densities than do giant-galaxy progenitors.

4 – the fact that, as luminosity decreases, dwarf galaxies become much more numerous and also more nearly dominated by DM raises the possibility that there exists a large population of objects that are completely dark. Such objects are a canonical prediction of cold DM theory. If they exist, “empty halos” are likely to be small and dense -that is, darker versions of Draco and UMi.

5 – the slopes of the DM parameter correlations provide a measure on galactic mass scales of the slope n of the power spectrum |δk|2 ∝ kn of primordial density fluctuations. Our preliminary results, not yet corrected for baryonic compression of DM, give n ≃ –1.9 ± 0.2. This is consistent with cold DM theory.

We present the results of a study to determine the co-evolution of the virial and stellar masses for a sample of 83 disk galaxies between redshifts z = 0.2 − 1.2. the virial masses of these disks are computed using measured maximum rotational velocities from Keck spectroscopy and scale lengths from Hubble Space Telescope imaging. We compute stellar masses based on stellar population synthesis model fits to spectral energy distributions including K(2.2μm) band magnitudes. We find no apparent evolution with redshift from z = 0.2 − 1.2 in the relationship between stellar masses and maximum rotational velocities through the stellar mass Tully-Fisher relationship. We also find no evolution when comparing disk stellar and virial masses. Massive disk galaxies therefore appear to be already in place, in terms of their virial and stellar masses, out to the highest redshifts where they can be morphologically identified.

We have investigated the Tully-Fisher relation for Polar Ring Galaxies (PRGs), based on near infrared, optical and Hi data available for a sample of these peculiar objects. The total K-band luminosity, which mainly comes from the central host galaxy, and the measured Hi linewidth at 20% of the peak line flux density, which traces the potential in the polar plane, place most polar rings of the sample far from the Tully-Fisher relation defined for spiral galaxies, with many PRGs showing larger Hi line-widths than expected for the observed K band luminosity. This result is confirmed by a larger sample of objects, based on B-band data. This observational evidence may be related to the dark halo shape and orientation in these systems, which we study by numerical modeling of PRG formation and dynamics: the larger rotation velocities observed in PRGs can be explained by a flattened polar halo, aligned with the polar ring.

The Tully-Fisher relation is of interest because of its use as a secondary distance measure and the constraints it places on the physics of rotationally supported galaxies. We use data from the HI Parkes All-Sky Survey Catalogue to study and apply the Tully-Fisher relation on a sample of galaxies selected on their HI properties. the issues of third parameter dependencies and intrinsic scatter of the Tully-Fisher relation are investigated.

We investigate the properties of the B-band Tully-Fisher (TF) relation for 25 compact group galaxies, using Vmax derived from 2-D velocity maps. Our main result is that the majority of the Hickson Compact Group (HCG) galaxies lie on the TF relation, although with large scatter. However, 20% of the galaxies, including the lowest-mass systems, seem to have higher B luminosities, for a given mass, or alternatively, a mass which is too low for their luminosities. We favour the scenario of brightening of the outliers due to either enhanced star formation or merging, rather than truncation of the dark halo due to interactions, to explain the position of the outliers on the TF relation.

We present the Tully-Fisher relation in cluster Cl0024+1654 at z=0.4. We find that our sample of 15 distant cluster members are ⋐ 0.5 mag overluminous as compared to local galaxies, and they exhibit slightly more Tully-Fisher scatter. This scatter is correlated with galaxy colours and sizes such that the smallest and bluest Cl0024 members in our sample are most overluminous.

The secular evolution process which slowly transforms the morphology of a given galaxy over its lifetime through mostly internal dynamical mechanisms could naturally account for most of the observed properties of physical galaxies (Zhang 2003a). As an emerging paradigm for galaxy evolution, its dynamical foundations had been established in the past few years (Zhang 1996, 1998, 1999). in this paper, we explore further implications of the secular morphological evolution process in reproducing the well-known scaling relations of galaxies.

Dark matter halos formed in CDM simulations are known to have triaxial shapes, with substantial distortions from spherical symmetry. However, conflicting claims have been made with respect to radial variations of halo shape, or trends with mass. We propose a new, more robust method to determine halo shapes based on the gravitational potential, thereby avoiding measurement uncertainties that may be the cause of these discrepancies. We find a strong preference towards prolate halo shapes, with mean minor-to-major axis-ratios close to ~ 0.5. Spherical halos are generally very rare, but there is a trend for smaller mass systems to be less elongated. We also compare dark halo shapes found in hydrodynamical simulations that include radiative cooling, star formation, and feedback processes, with those measured in equivalent collisionless simulations. Dissipation makes dark halos substantially rounder at small radii, to the extent that their shape distribution matches that of elliptical galaxies.

This paper reviews the available information on the central density distribution and shape of the Milky Way's halo. At present, there is no strong evidence that the Milky Way's halo properties conflict with the predictions of cold dark matter (CDM): a primordial central power law cusp can be accommodated by the observations, and the current constraints on flattening are also consistent with the predictions of the theory. If you want to pick a fight with CDM, then the Milky Way is probably not the place to do it.

We present the results of a study of the average mass profile around galaxies using weak gravitational lensing. We use 45.5 deg2 of RC band imaging data from the Red-Sequence Cluster Survey (RCS) and define a sample of ~ 1.2 × 105 lenses with 19.5 < RC < 21, and a sample of ~ 1.5 × 106 background galaxies with 21.5 < R < 24.

We constrain the power law scaling relations between the B-band luminosity and the mass and size of the halo, and find that the results are in excellent agreement with observed luminosity–line-width relations. Under the assumption that the luminosity does not evolve with redshift, the best fit NFW model yields a mass M200 = (8.8±0.7) × 1011h–1M⊙ and a scale radius rs = I6.7+3.7–3.0h–1 kpc for a galaxy with a fiducial luminosity of Lb = 1010h–2LB⊙. the latter result is in excellent agreement with predictions from numerical simulations for a halo of this mass. We also observe a signficant anisotropy of the lensing signal around the lenses, implying that the halos are flattened and aligned with the light distribution. We find an average (projected) halo ellipticity of 〈ehalo〉 = 0.20+0.04–0.05, in fair agreement with results from numerical simulations of CDM. Alternative theories of gravity (without dark matter) predict an isotropic lensing signal, which is excluded with 99.5% confidence. Hence, our results provide strong support for the existence of dark matter.

After looking at the difference in the mass distribution between massive spiral and dwarf irregular (dIrr) and low surface brightness (LSB) galaxies, the central Dark Matter (DM) concentration (flat vs cuspy) in dwarf and LSB galaxies, derived from observations, will be examined. We will then present what kind of observational constraints can be put on the total mass and total extent of DM halos from the studies of individual galaxies, small groups, satellites' dynamics and tidal tails of interacting systems. Finally, we will discuss how limits on the physical parameters of DM halos could be set by deriving extended rotation curves beyond the HI radius (r > rHI), using either Lyα absorption or Hα emission observations.

We present a detailed non-spherical modeling of dark matter halos on the basis of a careful analysis of state-of-the art N-body simulations. the fitting formulae presented here form a complete and accurate description of the triaxial density profiles of halos in Cold Dark Matter (CDM) models. This modeling allows us to quantitatively discuss implications for shape observations of galactic and cluster halos. the predictions of the concordance ACDM model are confronted with the shape observations from the Milky Way to X-ray clusters.