Comparison between Disk-like Objects Formed in Hierarchical Hydrodynamical Simulations and Observations of Spiral Galaxies

We analyze the structural and dynamical properties of disk-like objects formed in fully consistent cosmological simulations which include inefficient star formation. Comparison with data of similar observable properties of spiral galaxies gives satisfactory agreement, in contrast with previous findings using other codes. This suggests that the stellar formation implementation used has allowed the formation of disks as well as guaranteed their stability

Rotation Curves of Spiral Galaxies

Rotation curves of spiral galaxies are the major tool for determining the distribution of mass in spiral galaxies. They provide fundamental information for understanding the dynamics, evolution and formation of spiral galaxies. We describe various methods to derive rotation curves, and review the results obtained. We discuss the basic characteristics of observed rotation curves in relation to various galaxy properties, such as Hubble type, structure, activity, and environment.Comment: 40 pages, 6 gif figures; Ann. Rev. Astron. Astrophys. Vol. 39, p.137, 200

Cold gas accretion in galaxies

Evidence for the accretion of cold gas in galaxies has been rapidly accumulating in the past years. HI observations of galaxies and their environment have brought to light new facts and phenomena which are evidence of ongoing or recent accretion: 1) A large number of galaxies are accompanied by gas-rich dwarfs or are surrounded by HI cloud complexes, tails and filaments. It may be regarded as direct evidence of cold gas accretion in the local universe. It is probably the same kind of phenomenon of material infall as the stellar streams observed in the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI have been found in nearby spiral galaxies. While a large fraction of this gas is produced by galactic fountains, it is likely that a part of it is of extragalactic origin. 3) Spirals are known to have extended and warped outer layers of HI. It is not clear how these have formed, and how and for how long the warps can be sustained. Gas infall has been proposed as the origin. 4) The majority of galactic disks are lopsided in their morphology as well as in their kinematics. Also here recent accretion has been advocated as a possible cause. In our view, accretion takes place both through the arrival and merging of gas-rich satellites and through gas infall from the intergalactic medium (IGM). The infall may have observable effects on the disk such as bursts of star formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates needed to sustain the observed star formation (~1 Msol/yr), additional infall of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages. Full-resolution version available at http://www.astron.nl/~oosterlo/accretionRevie

THE MASS-DISTRIBUTION OF THE DWARF SPIRAL NGC-1560

H I synthesis observations with the WSRT and optical surface photometry of the dwarf spiral galaxy NGC 1560 are presented. This galaxy has an absolute luminosity of M(B) = -15.87. The observations show that the galaxy is gas rich, with an M(HI)/L(B) of 2.4. We obtained a very detailed rotation curve with more than four independent points per kiloparsec, extending to about twice the de Vaucouleurs radius. The curve shows remarkable structure. A mass model using a constant M/L stellar disk, and a thin gas disk as observable components has been constructed. These components alone cannot explain the observed rotation curve. The discrepancy between dynamical and observable mass is noticeable at a radius of about 2 kpc, well within the optical part of NGC 1560. At the last measured point of the rotation curve the global mass-to-light ratio is about 35, corresponding to a mass discrepancy of a factor 5, taking the gas mass into account. Models have been constructed, consisting of the two observable components and a dark, spherically symmetric halo. The parameter range allowed for the halo is in agreement with halo parameters determined for more massive galaxies. The neutral hydrogen distribution seems to be a quite precise tracer of the dark matter distribution. By adding a dominant mass component with a smooth density distribution (i.e. a dark halo) to the observable disk components, the fine structure in the rotation curve due to fluctuations in the gas surface density distribution is diluted. One way to match the observed shape of the rotation curve in detail using the disk-halo models is to increase the distance to NGC 1560 by at least a factor two. The models obtained by using modified dynamics (MOND) fit the observed rotation curve with all its detailed structure very well. This study shows that very detailed rotation curves can place severe limits on the distribution of luminous and non-luminous matter. Dark material in a dominant halo with a smooth density distribution could be ruled out if more rotation curves with prominent features become available

DEEP CCD PHOTOMETRY OF SPIRAL GALAXIES

Deep optical surface photometry in the R-band is presented for the three spiral galaxies NGC 1003, NGC 5533 and NGC 6674. The luminosity profiles, derived by fitting ellipses to the isophotes, have been decomposed into bulge and disk contributions using the method proposed by Kent (1986). In forthcoming papers the profiles will be combined with kinematical data from HI radio synthesis observations, to study the mass distribution of a larger sample of spiral galaxies

A SEARCH FOR SPIRAL GALAXIES WITH EXTENDED HI DISKS

We present short 21-cm line observations of about 50 spiral galaxies, made with the Westerbork Synthesis Radio Telescope. They form the first stage of a two-stage project to study the relation between the shape of extended rotation curves and galaxy properties, such as luminosity and morphological type. In this first stage, one-dimensional information has been obtained about the kinematics and spatial distribution of the neutral hydrogen (H I) along the major axis of each galaxy. The primary goal of these observations was to find galaxies with extended neutral hydrogen disks that have regular spatial and velocity distributions. In the second stage (Broeils, in preparation, see also Broeils 1992a,b), H I synthesis observations of twelve regular systems thus selected are analysed, in order to obtain full two-dimensional information on the H I distributions and velocity fields. The results of the first stage ace presented in Fig. A1, as position-velocity maps of the neutral hydrogen distribution of all galaxies in our sample. Global H I profiles and H I strip integrals are formed by integrating these maps, respectively in the position and in the velocity directions, The strip integrals are deprojected to face-on radial surface density distributions. For each galaxy a number of global H I properties, like diameters, fluxes, hydrogen masses, profile widths and systemic velocities are determined, In a future paper these properties and their relation to optical properties will be discussed, and representative rotation velocities from the position-velocity maps will be derived in order to investigate the influence of using these velocities instead of the profile widths on the scatter and slope of the Tully-Fisher relation