We present a new deep 21-cm survey of the Andromeda galaxy, based on high resolution observations performed with the Synthesis Telescope and the 26-m antenna at DRAO. The Hi distribution and kinematics of the disc are analyzed and basic dynamical properties are given. The rotation curve is measured out to 38 kpc, showing a nuclear peak at 340 km s −1 , a dip at 202 km s −1 around 4 kpc, two distinct flat parts at 264 km s −1 and 230 km s −1 and an increase to 275 km s −1 in the outermost regions. Except for the innermost regions, the axisymmetry of the gas rotation is very good. A very strong warp of the Hi disc is evidenced. The central regions appear less inclined than the average disc inclination of 74 • , while the outer regions appear more inclined. Mass distribution models by ΛCDM Navarro-Frenk-White, Einasto or pseudo-isothermal dark matter halos with baryonic components are presented. They fail to reproduce the exact shape of the rotation curve. No significant differences are measured between the various shapes of halo. The dynamical mass of M31 enclosed within a radius of 38 kpc is (4.7 ± 0.5) × 10 11 M ⊙ . The dark matter component is almost 4 times more massive than the baryonic mass inside this radius. A total mass of 1.0 × 10 12 M ⊙ is derived inside the virial radius. New Hi structures are discovered in the datacube, like the detection of up to five Hi components per spectrum, which is very rarely seen in other galaxies. The most remarkable new Hi structures are thin Hi spurs and an external arm in the disc outskirts. A relationship between these spurs and outer stellar clumps is evidenced. The external arm is 32 kpc long, lies on the far side of the galaxy and has no obvious counterpart on the other side of the galaxy. Its kinematics clearly differs from the outer adjacent disc. Both these Hi perturbations could result from tidal interactions with galaxy companions.

The results obtained from a study of the mass distribution of 36 spiral galaxies are presented. The galaxies were observed using Fabry–Perot interferometry as part of the GHASP survey. The main aim of obtaining high‐resolution Hα 2D velocity fields is to define more accurately the rising part of the rotation curves which should allow to better constrain the parameters of the mass distribution. The Hα velocities were combined with low resolution H i data from the literature, when available. Combining the kinematical data with photometric data, mass models were derived from these rotation curves using two different functional forms for the halo: an isothermal sphere (ISO) and a Navarro–Frenk–White (NFW) profile. For the galaxies already modelled by other authors, the results tend to agree. Our results point at the existence of a constant density core in the centre of the dark matter haloes rather than a cuspy core, whatever the type of the galaxy from Sab to Im. This extends to all types the result already obtained by other authors studying dwarf and low surface brightness galaxies but would necessitate a larger sample of galaxies to conclude more strongly. Whatever model is used (ISO or NFW), small core radius haloes have higher central densities, again for all morphological types. We confirm different halo scaling laws, such as the correlations between the core radius and the central density of the halo with the absolute magnitude of a galaxy: low‐luminosity galaxies have small core radius and high central density. We find that the product of the central density with the core radius of the dark matter halo is nearly constant, whatever the model and whatever the absolute magnitude of the galaxy. This suggests that the halo surface density is independent from the galaxy type.

This paper presents Australia Telescope Compact Array H I synthesis observations of the weak blue compact dwarf (BCD) galaxy NGC 2915. It is shown that NGC 2915 has the H I properties of a late type spiral galaxy (Sd -Sm), including a double horn global profile, and H I spiral arms. The H I extends out to over five times the Holmberg radius, and 22 times the exponential scale length in the B band. The optical counterpart corresponds to a central H I bar. The H I distribution and kinematics are discussed in detail. A rotation curve is derived and fitted with a mass model consisting of a stellar disk, a neutral gas disk, and a dark matter (DM) halo. The DM halo dominates at nearly all radii. The total mass to blue light ratio, M T /L B = 76 within the last measured point. Thus NGC 2915 is one of the darkest disk galaxies known. The complex H I dynamics of the central region results in a high uncertainty of many of the fitted 1 parameters. Nevertheless it is clear that the core of the DM halo is unusually dense (ρ 0 ≈ 0.1 M ⊙ pc −3 ) and compact (R c ≈ 1 kpc). The neutral gas component, with mass M g = 1.27×10 9 M ⊙ is probably more massive than the stellar disk. Split and broad H I lines (velocity dispersion ≈ 35 km s −1 ) are seen in the central region. Pressure support is probably significant, and it is not clear whether the core is in equilibrium. Beyond the optical disk the average H I line of sight velocity dispersion is 8 km s −1 , which is normal for disk galaxies. NGC 2915 does not obey the Tully-Fisher (1977) relation, being underluminous for its V rot = 88 km s −1 by a factor of nine. It also does not obey the star formation threshold model of Kennicutt (1989), when only the neutral gas is considered. A simple H I surface density threshold of Σ HI,crit ≈ 10 21 cm −2 adequately describes the location of current star formation. Although the H I properties of NGC2915 are extreme relative to normal galaxies they appear less extreme in comparison to other BCDS, which have similar radial profiles of H I density and velocity dispersion, and H I extending well beyond the optical disk.

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