We study the properties of interstellar medium (ISM) substructure and turbulence in hydrodynamic [adaptive mesh refinement (AMR)] galaxy simulations with resolutions up to 0.8 pc and 5 × 103 M⊙. We analyse the power spectrum of the density distribution, and various components of the velocity field. We show that the disc thickness is about the average Jeans scalelength, 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 disc 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 a Large Magellanic Cloud 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‐two‐dimensional regime on scales larger than the disc scaleheight. Similar regimes of gas turbulence are also found in massive high‐redshift discs with high gas fractions. Disc 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 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.

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Abstract. The unusual morphology of the Andromeda Spiral (Messier 31, the closest spiral galaxy to the Milky Way) has long been an enigma. Although regarded for decades as showing little evidence of a violent history, M 31 has a well-known outer ring 1−7 of star formation at a radius of 10 kpc whose center is offset from the galaxy nucleus. In addition, the outer galaxy disk is warped as seen at both optical 8 and radio 9 wavelengths. The halo contains numerous loops and ripples. Here we report the discovery, based on analysis of previously-obtained data 10 , of a second, inner dust ring with projected dimensions 1.5 by 1 kpc and offset by ∼ 0.5 kpc from the center of the galaxy. The two rings appear to bedensity waves propagating in the disk.Numerical simulations offer a completely new interpretation for the morphology of M 31: both rings result from a companion galaxyplunging head-on through the center of the disk of M 31. The most likely interloper is M 32. Head-on collisions between galaxies are rare, but it appears nonetheless that one took place 210 million years ago in our Local Group of galaxies.Newly-acquired images 10 of M 31 secured by the Infrared Array Camera 11 (IRAC) onboard the Spitzer Space Telescope span the wavelength regime of 3.6 to 8.0 microns. These images offer unique probes of the morphologies of the stellar distribution and interstellar medium with no interference from extinction. Figure 1 shows the emission map of the interstellar medium at 8 microns, generated by subtracting a scaled 3.6 micron image (dominated by starlight) from the 8 micron image. The subtraction removes the contribution from stellar photospheres and leaves only the emission from dust grains 10 , which trace the interstellar medium of M 31. What is most striking in Figure 1 (and in the enlarged inset) is the presence of a complete though asymmetric inner ring of dust 6.9 by 4.4 arcmin in extent, translating to linear dimensions of about 1.5 by 1 kpc (assuming a distance 12 of 780 kpc). The inner ring lies between the two well known Baade spiral dust arms 13 , both of which are clearly seen in emission. The inner ring is elongated in a direction close to the minor axis and belongs to the central gas disk, which appears to be more face-on 14 . It is therefore not possible to know the inner rings precise ellipticity, but it is unlikely to be circular. IRAC imaging thus reveals two rings. The outer ring is offset by approximately 10 percent of its radius, while the inner ring is offset by about 40 percent or 0.5 kpc. The inner elliptical ring has been alluded to in earlier studies 15,16 , but all investigators have hitherto believed it to be a mini-spiral, related to a bar. Published Spitzer 24 micron images5 of M 31 show centrally-concentrated dust emission; the ring morphology is therefore disguised at these longer wavelengths. The IRAC images beautifully show the inner ring at high spatial resolution and furthermore confirm that this feature is a complete and continuous ring, even though offset and asymmetrical. Ther...

Abstract. 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.

Structural analysis has been performed for a sample of 15 Southern early-type disk galaxies, mainly S0s, using high resolution K s -band images. The galaxies are mostly barred and many of them show multiple structures including bars and ovals, typical for S0s. The new images are of sufficient quality to reveal new detail on 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 also 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 particularly, (3) apply structural decomposition methods. For galaxies with multiple structures a 2-dimensional method is found to be superior to a 1-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 pseudo-bulges in S0s: ten of the galaxies have the shape parameter of the bulge near to n = 2, indicating that the bulges are more disk-like than following the R 1/4 -law. Also, six of the galaxies have either nuclear rings, nuclear bars or nuclear disks. In all non-elliptical galaxies in our sample the B/T < 0.4, as typically found in galaxies having pseudo-bulges.In two of the galaxies the B/T flux ratio is as small as in typical Sc-type spirals.This might be the hitherto undiscovered link in the scenario in which spirals are transformed into S0s. Also, bars in S0s are found to be shorter and less massive, and have smaller bar torques than bars in S0/a-type galaxies.

We have obtained deep near-infrared K s -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, Q s , and bar strength, Q b , 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.

In the last decade, near‐infrared imaging has highlighted the decoupling of gaseous and old stellar discs: the morphologies of optical (Population I) tracers compared to the old stellar disc morphology, can be radically different. Galaxies which appear multi‐armed and even flocculent in the optical may show significant grand‐design spirals in the near‐infrared. Furthermore, the optically determined Hubble classification scheme does not provide a sound way of classifying dust‐penetrated stellar discs: spiral arm pitch angles (when measured in the near‐infrared) do not correlate with Hubble type. The dust‐penetrated classification scheme of Block & Puerari provides an alternative classification based on near‐infrared morphology, which is thus more closely linked to the dominant stellar mass component. Here we present near‐infrared K‐band images of 14 galaxies, on which we have performed a Fourier analysis of the spiral structure in order to determine their near‐infrared pitch angles and dust‐penetrated arm classes. We have also used the rotation curve data of Mathewson et al. to calculate the rates of shear in the stellar discs of these galaxies. We find a correlation between near‐infrared pitch angle and rate of shear: galaxies with wide open arms (the γ class) are found to have rising rotation curves, while those with falling rotation curves belong to the tightly wound α bin. The major determinant of near‐infrared spiral arm pitch angle is the distribution of matter within the galaxy concerned. The correlation reported in this study provides the physical basis underpinning spiral arm classes in the dust‐penetrated regime and underscores earlier spectroscopic findings by Burstein and Rubin that Hubble type and mass distributions are unrelated.

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