Ivanio Puerari

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.

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.

THE STRUCTURAL PROPERTIES OF ISOLATED GALAXIES, SPIRAL-SPIRAL PAIRS, AND MERGERS: THE ROBUSTNESS OF GALAXY MORPHOLOGY DURING SECULAR EVOLUTION

We present a structural analysis of nearby galaxies in spiral-spiral pairs in optical BV RI bands and compare with the structures of isolated spiral galaxies and galaxies in ongoing mergers. We use these comparisons to determine how galaxy structure changes during galaxy interactions and mergers. We analyze light concentration (C), asymmetry (A), and clumpiness (S) parameters, and use the projections of CAS parameter space to compare these samples. We find that the CAS parameters of paired galaxies are correlated with the projected separations of the pair. For the widest and closest pairs, the CAS parameters tend to be similar to those of isolated and ongoing major mergers (ULIRGs), respectively. Our results imply that galaxy CAS morphology is a robust quantity that only changes significantly during a strong interaction or major merger. The typical time-scale for this change in our paired sample, based on dynamical friction arguments, is short, � ≈ 0.1 −0.5 Gyr. We find average enhancement factors for the spiral pair asymmetries and clumpiness values of ∼ 2.2 and 1.5. The S parameter, which is related to star formation activity, has a moderate level of enhancement suggesting that this activity in modern spirals depends more on internal processes than on external conditions. We furthermore test the statistical criterion for picking up interacting galaxies in an automated way by using the A − S projection plane. The diversity of our spiral pair sample in the CAS space suggests that structural/SF/morphological properties of interacting galaxies change abruptly only when the interaction becomes very strong and that the criteria for finding galaxies involved in major mergers from Conselice (2003) is effective. Subject headings: Galaxies: spiral – Galaxies: structure – Galaxies: photometry – Galaxies: interactions – Galaxies: fundamental parameters– Galaxies: morphology – Galaxies: general

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.

Non-axisymmetric Structure in the Satellite Dwarf Galaxy NGC?2976: Implications for its Dark/Bright Mass Distribution and Evolution

We present the result of an extensive search for non-axisymmetric structures in the dwarf satellite galaxy of M81, NGC 2976, using multiwavelength archival observations. The galaxy is known to present kinematic evidence for a bisymmetric distortion; however, the stellar bar presence is controversial. This controversy motivated the possible interpretation of NGC 2976 as presenting an elliptical disk triggered by a prolate dark matter halo. We applied diagnostics used in spiral galaxies in order to detect stellar bars or spiral arms. The m = 2 Fourier phase has a jump around 60 arcsec, consistent with a central bar and bisymmetric arms. The CO, 3.6 μm surface brightness, and the dust lanes are consistent with a gas-rich central bar and possibly with gaseous spiral arms. The bar-like feature is offset close to 20° from the disk position angle, in agreement with kinematic estimations. The kinematic jumps related to the dust lanes suggest that the bar perturbation in the disk kinematics is non-negligible and the reported non-circular motions, the central gas excess, and the nuclear X-ray source (active galactic nucleus/starburst) might be produced by the central bar. Smoothed particle hydrodynamics simulations of disks inside triaxial dark halos suggest that the two symmetric spots at 130 arcsec and the narrow arms may be produced by gas at turning points in an elliptical disk, or, alternatively, the potential ellipticity can be produced by a tidally induced strong stellar bar/arms; in both cases the rotation curve interpretation is, importantly, biased. The M81 group is a natural candidate to trigger the bisymmetric distortion and the related evolution as suggested by the H I tidal bridge detected by Chynoweth et al. We conclude that both mechanisms, the gas-rich bar and spiral arms triggered by the environment (tidal stirring) and primordial halo triaxiality, can explain most of the NGC 2976 non-circular motions, mass redistribution, and nuclear activity. Distinguishing between them requires detailed modeling of environmental effects. A similar analysis to ours may reveal such structures in other nearby dwarf satellite galaxies; if this is confirmed, the same evolutionary scenario will be applicable to them. This implies biases in constraining their dark matter distribution and also in making comparisons against theoretical predictions for isolated galaxies.

Keck spectroscopy and Spitzer space telescope analysis of the outer disk of the Triangulum spiral galaxy M 33

In an earlier study of the spiral galaxy M 33, we photometrically identified arcs or outer spiral arms of intermediate age (0.6-2 Gyr) carbon stars precisely at the commencement of the HI-warp. Stars in the arcs were unresolved, but were likely thermally-pulsing asymptotic giant branch carbon stars. Here we present Keck I spectroscopy of seven intrinsically bright and red target stars in the outer, northern arc in M 33. The target stars have estimated visual magnitudes as faint as V~25^(m). Absorption bands of CN are seen in all seven spectra reported here, confirming their carbon star status. In addition, we present Keck II spectra of a small area 0.5 degree away from the centre of M 33; the target stars there are also identified as carbon stars. We also study the non-stellar PAH dust morphology of M 33 secured using IRAC on board the Spitzer Space Telescope. The Spitzer 8 µm image attests to a change of spiral phase at the start of the HI warp. The Keck spectra confirm that carbon stars may safely be identified on the basis of their red J-K_s colours in the outer, low metallicity disk of M 33. We propose that the enhanced number of carbon stars in the outer arms are an indicator of recent star formation, fueled by gas accretion from the HI-warp reservoir.

Detection of Star Streams and Turbulence in Nearby Galaxies: Power Spectrum Analysis of Spitzer Images

Fourier transform power spectra of azimuthal scans in 33 galaxies imaged with the Infrared Array Camera on the Spitzer Space Telescope show an approximate power-law structure over a wide range of wavenumbers with a gradual steepening from 3.6 or 4.5 μm to 8.0 μm, in the order of an increasing contribution from gas and dust. At radii with active star formation, the average of the slopes of the 8 μm power spectra at intermediate spatial frequencies is about the same for all galaxies, independent of spiral arm morphology. These power spectra are reminiscent of turbulence, although the 8 μm slopes, averaging –2.0, are slightly steeper than expected (–1.7). Reconstructed images using only these intermediate spatial frequencies at 4.5 μm show aging star complexes distorted by shear. These sheared features illustrate the transition from a hierarchical structure during star formation into azimuthal star streams like the Pleiades moving group in the Solar neighborhood. This is the first time that young star streams have been observed in spiral galaxies other than the Milky Way.

The Large Magellanic Cloud: A Power Spectral Analysis of Spitzer Images

We present a power spectral analysis of Spitzer images of the Large Magellanic Cloud.The power spectra of the FIR emission show two different power laws.At larger scales (kpc) the slope is ~ −1.6, while at smaller ones (tens to few hundreds of parsecs) the slope is steeper, with avalue ~ −2.9.The break occurs at a scale ~ 100 − 200 pc. We interpret this break as the scale height of the dust disk of the LMC. We perform high-resolution simulations with and without stellar feedback.Our AMR hydrodynamic simulations of model galaxies using the LMC mass and rotation curve confirm that they have similar two-component power laws for projected density – and that the break does indeed occur at the disk thickness. Power spectral analysis of velocities betrays a single power law for in-plane components.The vertical component of the velocity shows a flat behavior for large structures and a power law similar to the in-plane velocities at small scales. The motions are highly anisotropic at large scales,with in-plane velocities being much more important than vertical ones.In contrast, at small scales, the motions become more isotropic.

Bar and Spiral Torques in the Triangulum Galaxy M33

The Triangulum Spiral (M33=NGC 598) presents a daunting task to near-infrared observers, due to its huge spatial extent. From the deepest near-infrared (JHKS) images of M33 yet secured, we identify a bar and calculate the gravitational torque, the bar torque as well as the spiral arm torque. Our methodology reveals the presence of a “class 2” bar, meaning that the tangential force field reaches a maximum of ~ 20% relative to the axisymmetric radial force field. Furthermore, we are able for the first time to quantify the Reynolds (1927) spiral arm index for M33 (described as ‘broad’: low arm-interarm contrast), and estimate that the spiral arm torque reaches a maximum of only 15%.

Dust Penetrated Morphology in the High Redshift Universe

Images from the Hubble Deep Field (HDF) North and South show a large percentage of dusty, high redshift galaxies whose appearance falls outside traditional classification systems. The nature of these objects is not yet fully understood. Since the HDF preferentially samples restframe UV light, HDF morphologies are not dust or ‘mask’ penetrated. The appearance of high redshift galaxies at near-infrared restframes remains a challenge for the New Millennium. The Next Generation Space Telescope (NGST) could routinely provide us with such images. In this contribution, we quantitatively determine the dust-penetrated structures of high redshift galaxies such as NGC 922 in their near-infrared rest-frames. We show that such optically peculiar objects may readily be classified using the dust penetrated z ~ 0 templates of Block and Puerari (1999), Block and Puerari (1999) and Buta and Block (2001).