Rotation curves of luminous spiral galaxies

We have investigated the stellar light distribution and the rotation curves of high‐luminosity spiral galaxies in the local Universe. The sample contains 30 high‐quality extended Hα and H I rotation curves. The stellar disk scale‐length of these objects was measured or taken from the literature. We find that in the outermost parts of the stellar disks of these massive objects, the rotation curves agree with the Universal Rotation Curve (Salucci et al. 2007), however a few rotation curves of the sample show a divergence (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The coalescence of invariant manifolds and the spiral structure of barred galaxies

In a previous paper (Voglis et al., Paper I), we demonstrated that, in a rotating galaxy with a strong bar, the unstable asymptotic manifolds of the short-period family of unstable periodic orbits around the Lagrangian points L ₁ or L ₂ create correlations among the apocentric positions of many chaotic orbits, thus supporting a spiral structure beyond the bar. In this paper, we present evidence that the unstable manifolds of all the families of unstable periodic orbits near and beyond corotation contribute to the same phenomenon. Our results refer to a N -body simulation, a number of drawbacks of which, as well as the reasons why these do not significantly affect the main results, are discussed. We explain the dynamical importance of the invariant manifolds as due to the fact that they produce a phenomenon of 'stickiness' slowing down the rate of chaotic escape in an otherwise non-compact region of the phase space. We find a stickiness time of the order of 100 dynamical periods, which is sufficient to support a long-living spiral structure. Manifolds of different families become important at different ranges of values of the Jacobi constant. The projections of the manifolds of all the different families in the configuration space produce a pattern due to the 'coalescence' of the invariant manifolds. This follows closely the maxima of the observed   m = 2  component near and beyond corotation. Thus, the manifolds support both the outer edge of the bar and the spiral arms.     

Accretion of gas on to nearby spiral galaxies

We present evidence for cosmological gas accretion on to spiral galaxies in the local universe. The accretion is seen through its effects on the dynamics of the extraplanar neutral gas. The accretion rates that we estimate for two nearby spiral galaxies are of the order of their star formation rates. Our model shows that most of the extraplanar gas is produced by supernova feedback (galactic fountain) and only 10–20 per cent comes from accretion. The accreting material must have low specific angular momentum about the disc's spin axis, although the magnitude of the specific angular momentum vector can be higher. We also explore the effects of a hot corona on the dynamics of the extraplanar gas and find that it is unlikely to be responsible for the observed kinematical pattern and the source of accreted gas. However, the interaction with the fountain flow should profoundly affect the hydrodynamics of the corona.     

The onset of warps in Spitzer observations of edge-on spiral galaxies

We analyse warps in the nearby edge-on spiral galaxies observed in the Spitzer /Infrared Array Camera (IRAC) 4.5-μm band. In our sample of 24 galaxies, we find evidence of warp in 14 galaxies. We estimate the observed onset radii for the warps in a subsample of 10 galaxies. The dark matter distribution in each of these galaxies are calculated using the mass distribution derived from the observed light distribution and the observed rotation curves. The theoretical predictions of the onset radii for the warps are then derived by applying a self-consistent linear response theory to the obtained mass models for six galaxies with rotation curves in the literature. By comparing the observed onset radii to the theoretical ones, we find that discs with constant thickness can not explain the observations; moderately flaring discs are needed. The required flaring is consistent with the observations. Our analysis shows that the onset of warp is not symmetric in our sample of galaxies. We define a new quantity called the onset-asymmetry index and study its dependence on galaxy properties. The onset asymmetries in warps tend to be larger in galaxies with smaller disc scalelengths. We also define and quantify the global asymmetry in the stellar light distribution, that we call the edge-on asymmetry in edge-on galaxies. It is shown that in most cases the onset asymmetry in warp is actually anticorrelated with the measured edge-on asymmetry in our sample of edge-on galaxies and this could plausibly indicate that the surrounding dark matter distribution is asymmetric.     

Invariant manifolds, phase correlations of chaotic orbits and the spiral structure of galaxies

In the presence of a strong   m = 2  component in a rotating galaxy, the phase-space structure near corotation is shaped to a large extent by the invariant manifolds of the short-period family of unstable periodic orbits terminating at L ₁ or L ₂. The main effect of these manifolds is to create robust phase correlations among a number of chaotic orbits large enough to support a spiral density wave outside corotation. The phenomenon is described theoretically by soliton-like solutions of a Sine–Gordon equation. Numerical examples are given in an N -body simulation of a barred spiral galaxy. In these examples, we demonstrate how the projection of unstable manifolds in configuration space reproduces essentially the entire observed bar–spiral pattern.     

The disc mass of spiral galaxies

We derive the disc masses of 18 spiral galaxies of different luminosity and Hubble type, both by mass modelling their rotation curves and by fitting their spectral energy distribution with spectrophotometric models. The good agreement of the estimates obtained from these two different methods allows us to quantify the reliability of their performance and to derive very accurate stellar mass-to-light ratio versus colour (and stellar mass) relationships.     

Ram pressure stripping of spiral galaxies in clusters

We use three-dimensional SPH/ N -body simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters. We find that the analytic expectation of Gunn & Gott, relating the gravitational restoring force provided by the disc to the ram pressure force, provides a good approximation to the radius at which gas will be stripped from a galaxy. However, at small radii it is also important to consider the potential provided by the bulge component. A spiral galaxy passing through the core of a rich cluster, such as Coma, will have its gaseous disc truncated to ∼4 kpc, thus losing ∼80 per cent of its diffuse gas mass. The time-scale for this to occur is a fraction of a crossing time ∼10⁷ yr. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intracluster medium, will lose significantly less gas. We conclude that ram pressure alone is insufficient to account for the rapid and widespread truncation of star formation observed in cluster galaxies, or the morphological transformation of Sabs to S0s that is necessary to explain the Butcher–Oemler effect.     

The origin of the light distribution in spiral galaxies

We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy simulation, to study the source of the double-exponential light profiles seen in many stellar discs, and the effects of stellar radial migration upon the spatiotemporal evolution of both the disc age and metallicity distributions. We find a 'break' in the pure exponential stellar surface brightness profile, and trace its origin to a sharp decrease in the star formation per unit surface area, itself produced by a decrease in the gas volume density due to a warping of the gas disc. Star formation in the disc continues well beyond the break. We find that the break is more pronounced in bluer wavebands. By contrast, we find little or no break in the mass density profile. This is, in part, due to the net radial migration of stars towards the external parts of the disc. Beyond the break radius, we find that ∼60 per cent of the resident stars migrated from the inner disc, while ∼25 per cent formed in situ . Our simulated galaxy also has a minimum in the age profile at the break radius but, in disagreement with some previous studies, migration is not the main mechanism producing this shape. In our simulation, the disc metallicity gradient flattens with time, consistent with an 'inside-out' formation scenario. We do not find any difference in the intensity or the position of the break with inclination, suggesting that perhaps the differences found in empirical studies are driven by dust extinction.     

A dynamical model for the extraplanar gas in spiral galaxies

Recent H  i observations reveal that the discs of spiral galaxies are surrounded by extended gaseous haloes. This extraplanar gas reaches large distances (several kiloparsecs) from the disc and shows peculiar kinematics (low rotation and inflow). We have modelled the extraplanar gas as a continuous flow of material from the disc of a spiral galaxy into its halo region. The output of our models is pseudo data cubes that can be directly compared to the H  i data. We have applied these models to two spiral galaxies (NGC 891 and NGC 2403) known to have a substantial amount of extraplanar gas. Our models are able to reproduce accurately the vertical distribution of extraplanar gas for an energy input corresponding to a small fraction (<4 per cent) of the energy released by supernovae. However, they fail in two important aspects: (1) they do not reproduce the right gradient in rotation velocity; (2) they predict a general outflow of the extraplanar gas, contrary to what is observed. We show that neither of these difficulties can be removed if clouds are ionized and invisible at 21 cm as they leave the disc but become visible at some point on their orbits. We speculate that these failures indicate the need for accreted material from the intergalactic medium that could provide the low angular momentum and inflow required.     

The radial Tully–Fisher relation for spiral galaxies – I

We find a new Tully–Fisher-like relation for spiral galaxies holding at different galactocentric radii. This radial Tully–Fisher relation allows us to investigate the distribution of matter in the optical regions of spiral galaxies. This relation, applied to three different samples of rotation curves of spiral galaxies, directly proves that: (i) the rotation velocity of spirals is a good measure of their gravitational potential and both the rotation curve's amplitudes and profiles are well predicted by galaxy luminosity, (ii) the existence of a dark component, less concentrated than the luminous one, and (iii) a scaling law, according to which, inside the disc optical size:   M _dark/ M _lum= 0.5( L _B /10¹¹ L _{B ⊙})^−0.7  .     

Dynamical stability and evolution of the discs of Sc galaxies

We examine the local stability of galactic discs against axisymmetric density perturbations with special attention to the different dynamics of the stellar and gaseous components. In particular, the discs of the Milky Way and of NGC 6946 are studied. The Milky Way is shown to be stable, whereas the inner parts of NGC 6946, a typical Sc galaxy from the Kennicutt sample, are dynamically unstable. The ensuing dynamical evolution of the composite disc is studied by numerical simulations. The evolution is so fierce that the stellar disc heats up dynamically on a short time-scale to such a high degree, which seems to contradict the morphological appearance of the galaxy. The star formation rate required to cool the disc dynamically is estimated. Even if the star formation rate in NGC 6946 is at present high enough to meet this requirement, it is argued that the discs of Sc galaxies cannot sustain such a high star formation rate for extended periods.     

Stability of power-law discs — II. The global spiral modes

This paper reports on the in-plane normal modes in the self-consistent and the cut-out power-law discs. Although the cut-out discs are remarkably stable to bisymmetric perturbations, they are very susceptible to one-armed modes. For this harmonic, there is no inner Lindblad resonance, thus removing a powerful stabilizing influence. A physical mechanism for the generation of the one-armed instabilities is put forward. Incoming trailing waves are reflected as leading waves at the inner cut-out, thus completing the feedback for the swing-amplifier. Growing three-armed and four-armed modes occur only at very low temperatures. However, neutral m  = 3 and m  = 4 modes are possible at higher temperatures for some discs. The rotation curve index β has a marked effect on stability. For all azimuthal wavenumbers, any unstable modes persist to higher temperatures and grow more vigorously if the rotation curve is rising (β < 0) than if the rotation curve is falling (β > 0). If the central regions or outer parts of the disc are carved out more abruptly, any instabilities become more virulent. The self-consistent power-law discs possess a number of unusual stability properties. There is no natural time-scale in the self-consistent disc. If a mode is admitted at some pattern speed and growth rate, then it must be present at all pattern speeds and growth rates. Our analysis — although falling short of a complete proof — suggests that such a two-dimensional continuum of non-axisymmetric modes does not occur and that the self-consistent power-law discs admit no global non-axisymmetric normal modes whatsoever. Without reflecting boundaries or cut-outs, there is no resonant cavity and no possibility of unstable growing modes. The self-consistent power-law discs certainly admit equi-angular spirals as neutral modes, together with a one-dimensional continuum of growing axisymmetric modes.