David L. Block

ISM properties in hydrodynamic galaxy simulations: turbulence cascades, cloud formation, role of gravity and feedback

We study the properties of ISM substructure and turbulence in hydrodynamic (AMR) galaxy simulations with resolutions up to 0.8 pc and 5 � 10 3 M� . We analyse the power spectrum of the density distribution, and various components of the velocity field. We show that the disk thickness is about the average Jeans scale length, and is mainly regulated by gravitational instabilities. From this scale of energy injection, a turbulence cascade towards small-scale is observed, with almost isotropic small-scale motions. On scales larger than the disk thickness, density waves are observed, but there is also a full range of substructures with chaotic and strongly non-isotropic gas velocity dispersions. The power spectrum of vorticity in an LMC-sized model suggests that an inverse cascade of turbulence might be present, although energy input over a wide range of scales in the coupled gaseous+stellar fluid could also explain this quasi-2D regime on scales larger than the disk scale height. Similar regimes of gas turbulence are also found in massive high-redshift disks with high gas fractions. Disk properties and ISM turbulence appear to be mainly regulated by gravitational processes, both on large scales and inside dense clouds. Star formation feedback is however essential to maintain the ISM in a steady state by balancing a systematic gas dissipation into dense and small clumps. Our galaxy simulations employ a thermal model based on a barotropic Equation of State (EoS) aimed at modelling the equilibrium of gas between various heating and cooling processes. Denser gas is typically colder in this approach, which is shown to correctly reproduce the density structures of a star-forming, turbulent, unstable and cloudy ISM down to scales of a few parsecs.

New Extragalactic Perspectives in the New South Africa

What is the nature and composition of the dust grains responsible for the visual extinction in our Galaxy and in other galaxies beyond? What are the ranges in temperature of dust grains? Can these be less than 2.7K? Can the distribution of cold grains be studied optically at unprecedented arcsecond resolution? How does the presence of dust affect the morphology of a galaxy? Is this new dust-penetrated view bringing us to the verge of a breakthrough in understanding the connection between galaxy morphology and the underlying physics of galaxies? How large are the amounts of cold molecular hydrogen gas and cold dust in galactic disks? These are some of the key issues addressed in this book, which takes the postgraduate reader and professional researcher to the cutting edge of this rapidly developing field. Unique features of the book include fourteen in-depth invited review papers and twenty-six pages of discussion transcribed from a television tape. The contributions reflect the entire proceedings of an intensive one week International Conference on cold dust and galaxy morphology held in Johannesburg, South Africa, during January 1996.

Morphology of 15 Southern Early-Type Disk Galaxies

Structural analysis has been performed for a sample of 15 southern early-type disk galaxies, mainly S0 galaxies, using high-resolution Ks-band images. The galaxies are mostly barred, and many of them show multiple structures including bars and ovals, typical for S0 galaxies. The new images are of sufficient quality to reveal new detail of the morphology of the galaxies. For example, we report a hitherto undetected nuclear ring in NGC 1387, a nuclear bar in NGC 1326, and in the residual image a weak primary bar in NGC 1317. For the galaxies we (1) measure the radial profiles of the orientation parameters derived from the elliptical isophotes, (2) apply Fourier methods for calculating tangential forces, and, in particular, (3) apply structural decomposition methods. For galaxies with multiple structures a two-dimensional method is found to be superior to a one-dimensional method but only if in addition to the bulge and the disk at least one other component is taken into account. We find strong evidence of pseudobulges in S0 galaxies: 10 of the galaxies have a shape parameter of the bulge near n = 2, indicating that the bulges are more disklike than following the R1/4 law. Also, six of the galaxies have either nuclear rings, nuclear bars, or nuclear disks. In all nonelliptical galaxies in our sample the bulge-to-total (B/T) flux ratio is less than 0.4, as is typically found in galaxies having pseudobulges. In two of the galaxies the B/T flux ratio is as small as in typical Sc-type spiral galaxies. This might be the hitherto undiscovered link in the scenario in which spiral galaxies are transformed into S0 galaxies. Also, bars in S0 systems are found to be shorter and less massive and to have smaller bar torques than bars in S0/a-type galaxies.

Gravitational torques in spiral galaxies: Gas accretion as a driving mechanism of galactic evolution

The distribution of gravitational torques and bar strengths in the local Universe is derived from a detailed study of 163 galaxies observed in the near-infrared. The results are compared with numerical models for spiral galaxy evolution. It is found that the observed distribution of torques can be accounted for only with external accretion of gas onto spiral disks. Accretion is responsible for bar renewal - after the dissolution of primordial bars - as well as the maintenance of spiral structures. Models of isolated, non-accreting galaxies are ruled out. Moderate accretion rates do not explain the observational results: it is shown that galactic disks should double their mass in less than the Hubble time. The best fit is obtained if spiral galaxies are open systems, still forming today by continuous gas accretion, doubling their mass every 10 billion years.

Gravitational Bar and Spiral Arm Torques from Ks-band Observations and Implications for the Pattern Speeds

We have obtained deep near-infrared Ks-band William Herschel Telescope observations of a sample of 15 nearby spiral galaxies having a range of Hubble types and apparent bar strengths. The near-infrared light distributions are converted into gravitational potentials, and the maximum relative gravitational torques due to the bars and the spirals are estimated. We find that spiral strength, Qs, and bar strength, Qb, correlate well with other measures of spiral arm and bar amplitudes and that spiral and bar strengths also correlate well with each other. We also find a correlation between the position angle of the end of the bar and the position angle of the inner spiral. These correlations suggest that the bars and spirals grow together with the same rates and pattern speeds. We also show that the strongest bars tend to have the most open spiral patterns. Because open spirals imply high disk-to-halo mass ratios, bars and spirals most likely grow together as a combined disk instability. They stop growing for different reasons, however, giving the observed variation in bar-spiral morphologies. Bar growth stops because of saturation when most of the inner disk is in the bar, and spiral growth stops because of increased stability as the gas leaves and the outer disk heats up.

A Philosophical Perspective

The paper by Professor Ellis in this volume concludes the contributions from the world of science.

Dust penetrated arm classes: Insights from rising and falling rotation curves

We present near-infrared K-band images of 15 galaxies. We have performed a Fourier analysis on the spiral structure of these galaxies in order to determine their pitch angles and dust-penetrated arm classes. We have also obtained rotation curve data for these galaxies and calculated their shear rates. We show that there is a correlation between pitch angle and shear rate and conclude that the main determinant of pitch angle is the mass distribution within the galaxy. This correlation provides a physical basis for the dust-penetrated classification scheme of Block & Puerari (1999).

The gravitational torque of bars in optically unbarred and barred galaxies

The relative bar torques for 45 galaxies observed at K-band with the 4.2 m William Herschel Telescope are determined by transforming the light distributions into potentials and deriving the maximum ratios of the tangential forces relative to the radial forces. The results are combined with the bar torques for 30 other galaxies determined from our previous K-band survey (Buta & Block 2001). Relative bar torques determine the degree of spiral arm forcing, gas accretion, and bar evolution. They dier from other measures of bar strength, such as the relative amplitude of the bar determined photometrically, because they include the bulge and other disk light that contributes to the radial component of the total force. If the bulge is strong and the radial forcing large, then even a prominent bar can have a relatively weak influence on the azimuthal motions in the disk. Here we nd that the relative bar torque correlates only weakly with the optical bar type listed in the Revised Shapley-Ames and de Vaucouleurs systems. In fact, some classically barred galaxies have weaker relative bar torques than classically unbarred galaxies. The optical class is a poor measure of azimuthal disk forcing for two reasons: some infrared bars are not seen optically, and some bars with strong bulges have their azimuthal forces so strongly diluted by the average radial force that they exert only small torques on their disks. The Hubble classication scheme poorly recognizes the gravitational influence of bars. Applications of our bar torque method to the high-redshift universe are briefly discussed.

Variation of Galactic Bar Length with Amplitude and Density as Evidence for Bar Growth over a Hubble Time

Ks-band images of 20 barred galaxies show an increase in the peak amplitude of the normalized m = 2 Fourier component with the R25-normalized radius at this peak. This implies that longer bars have higher m = 2 amplitudes. The long bars also correlate with an increased density in the central parts of the disks, as measured by the luminosity inside 0.25R25 divided by the cube of this radius in kpc. Because denser galaxies evolve faster, these correlations suggest that bars grow in length and amplitude over a Hubble time, with the fastest evolution occurring in the densest galaxies. All but three of the sample have early-type flat bars; there is no clear correlation between the correlated quantities and the Hubble type.

A Two-component Power Law Covering Nearly Four Orders of Magnitude in the Power Spectrum of Spitzer Far-infrared Emission from the Large Magellanic Cloud

Power spectra of Large Magellanic Cloud (LMC) emission at 24, 70, and 160 ?m observed with the Spitzer Space Telescope have a two-component power-law structure with a shallow slope of ?1.6 at low wavenumber, k, and a steep slope of ?2.9 at high k. The break occurs at k ?1 ~ 100-200 pc, which is interpreted as the line-of-sight thickness of the LMC disk. The slopes are slightly steeper for longer wavelengths, suggesting the cooler dust emission is smoother than the hot emission. The power spectrum (PS) covers ~3.5 orders of magnitude, and the break in the slope is in the middle of this range on a logarithmic scale. Large-scale driving from galactic and extragalactic processes, including disk self-gravity, spiral waves, and bars, presumably causes the low-k structure in what is effectively a two-dimensional geometry. Small-scale driving from stellar processes and shocks causes the high-k structure in a three-dimensional geometry. This transition in dimensionality corresponds to the observed change in PS slope. A companion paper models the observed power law with a self-gravitating hydrodynamics simulation of a galaxy like the LMC.