Stellar and gas dynamics of late-type barred-spiral galaxies: NGC 3359, a test case

We study the dynamics of a model for the late-type barred-spiral galaxy NGC 3359 by using both observational and numerical techniques. The results of our modelling are compared with photometric and kinematical data. The potential used is estimated directly from observations of the galaxy. It describes with a single potential function, a barred-spiral system with an extended spiral structure. Thus, the study of the dynamics in this potential has an interest by itself. We apply orbital theory and response models for the study of the stellar component, and smoothed particle hydrodynamics for modelling the gas. In particular, we examine the pattern speed of the system and the orbital character (chaotic or ordered) of the spiral arms. We conclude that the spiral pattern rotates slowly, in the sense that its corotation is close to or even beyond the end of the arms. Although a single, slow pattern speed could, under certain assumptions, characterize the whole disc, the comparison with the observational data indicates that probably the bar and the spirals have different angular velocities. In our two pattern speeds model, the best fit is obtained with a bar ending close to its 4:1 resonance and a more slowly rotating spiral. Assuming an 11 Mpc distance to the galaxy, a match of our models with the observed data indicates a pattern speed of about  39 km s⁻¹ kpc⁻¹  for the bar and about  15 km s⁻¹ kpc⁻¹  for the spiral. We do not find any indication for a chaotic character of the arms in this barred-spiral system. The flow in the region of the spirals can best be described as a regular 'precessing-ellipses flow'.     

Do high-velocity clouds form by thermal instability?

We examine the proposal that the H  i 'high-velocity' clouds (HVCs) surrounding the Milky Way and other disc galaxies form by condensation of the hot galactic corona via thermal instability. Under the assumption that the galactic corona is well represented by a non-rotating, stratified atmosphere, we find that for this formation mechanism to work the corona must have an almost perfectly flat entropy profile. In all other cases, the growth of thermal perturbations is suppressed by a combination of buoyancy and thermal conduction. Even if the entropy profile were nearly flat, cold clouds with sizes smaller than  10 kpc  could form in the corona of the Milky Way only at radii larger than  100 kpc  , in contradiction with the determined distances of the largest HVC complexes. Clouds with sizes of a few kpc can form in the inner halo only in low-mass systems. We conclude that unless even slow rotation qualitatively changes the dynamics of a corona, thermal instability is unlikely to be a viable mechanism for formation of cold clouds around disc galaxies.     

Disc–halo interaction – I. Three-dimensional evolution of the Galactic disc

The results of a three-dimensional model for disc–halo interaction are presented here. The model considers explicitly the input of energy and mass by isolated and correlated supernovae in the disc. Once disrupted by the explosions, the disc never returns to its initial state. Instead it approaches a state where a thin H  i disc is formed in the Galactic plane, overlaid by thick H  i and H  ii gas discs with scaleheights of 500 pc and 1–1.5 kpc, respectively. The upper parts of the thick H  ii disc (the diffuse ionized medium) act as a disc–halo interface, and its formation and stability are directly correlated to the supernova rate per unit area in the simulated disc.     

Vertical distribution of Galactic disc stars and gas constrained by a molecular cloud complex

We investigate the dynamical effects of a molecular cloud complex with a mass ∼ 10⁷ M_⊙ and a size ∼ a few 100 pc on the vertical distribution of stars and atomic hydrogen gas in a spiral galactic disc. Such massive complexes have now been observed in a number of spiral galaxies. The extended mass distribution in a complex, with an average mass density 6 times higher than the Oort limit, is shown to dominate the local gravitational field. This results in a significant redistribution or clustering of the surrounding disc components towards the mid-plane, with a resulting decrease in their vertical scaleheights.
The modified, self-consistent stellar density distribution is obtained by solving the combined Poisson equation and the force equation along the z -direction for an isothermal stellar disc on which the complex is imposed. The effect of the complex is strongest at its centre, where the stellar mid-plane density increases by a factor of 2.6 and the vertical scaleheight decreases by a factor of 3.4 compared with the undisturbed stellar disc. A surprising result is the large radial distance of ∼ 500 pc from the complex centre over which the complex influences the disc; this is due to the extended mass distribution in a complex. The complex has a comparable effect on the vertical distribution of the atomic hydrogen gas in the galactic disc. This 'pinching' or constraining effect should be detectable in the nearby spiral galaxies, as for example has been done for NGC 2403 by Sicking. Thus the gravitational field of a complex results in local corrugations of the stellar and H  i vertical scaleheights, and the galactic disc potential is highly non-uniform on scales of the intercomplex separation of ∼ 1 kpc.     

Galactic H i on the 50-au scale in the direction of three extragalactic sources observed with MERLIN

We present MERLIN observations of Galactic 21-cm H  i absorption at an angular resolution of  ∼0.1–0.2  arcsec and a velocity resolution of 0.5 km s⁻¹, in the direction of three moderately low latitude  (−8° < b < −12°)  extragalactic radio sources, 3C 111, 3C 123 and 3C 161, all of which are heavily reddened. H  i absorption is observed against resolved background emission sources up to ∼2 arcsec in extent and we distinguish details of the opacity distribution within 1–1.5 arcsec regions towards 3C 123 and 3C 161. This study is the second MERLIN investigation of small-scale structure in interstellar H  i (earlier work probed Galactic H  i in the directions of the compact sources 3C 138 and 3C 147). The 0.1-arcsec scale is intermediate between H  i absorption studies made with other fixed element interferometers with resolution of 1–10 arcsec and very long baseline interferometry studies with resolutions of 10–20 mas. At a scale of 1 arcsec (about 500 au), prominent changes in Galactic H  i opacity in excess of 1–1.5 are determined in the direction of 3C 161 with a signal-to-noise ratio of at least 10σ. Possible fluctuations in the H  i opacity at the level of about 1 are detected at the  2.5–3σ  level in the direction of 3C 123.     

Morphology and star formation in nearby low surface brightness galaxies

We present observations ( B, R, K , Hα and H  i ) of six nearby low surface brightness galaxies (LSBGs). They show an astonishing amount of variety; while some systems appear smooth and featureless, others resolve into loose assemblies of gas clouds. We have derived rotation curves, gas surface density profiles and star formation thresholds for three of the galaxies.
The results have been used to test two ideas describing their star formation: one in which star formation depends solely on the H  i gas surface density, and one that depends on differential rotation. We find that a critical H  i surface density criterion in the range  2.6–12.6 × 10²⁰ cm⁻² (2.1–10.1 M_⊙ pc⁻²)  best describes the star-forming ability of these galaxies on local and global scales. A critical gas surface density based on the rotation of the gas is also able to describe the results on a global scale for two of the three galaxies for which we were able to derive rotation curves.     

Star formation rates and the kinematics of gas in the spiral arms of NGC 628

The relations between star formation rates along the spiral arms and the velocities of gas inflow into the arms in the grand‐design galaxy NGC 628 were studied. We found that the radial distribution of the average star formation rate in individual star formation regions in regular spiral arms correlates with the velocity of gas inflow into the spiral arms. Both distributions have maxima at a galactocentric distance of 4.5–5 kpc. There are no correlations between the radial distributions of the average star formation rate in star formation regions in spiral arms and outside spiral arms in the main disc. We also did not find a correlation between the radial distribution of the average star formation rate in star formation regions in spiral arms and the H I column density. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the influence of cooling and heating processes on the Parker instability – II. Numerical simulations

We investigate the Parker instability (PI) influenced by thermal processes in a non-adiabatic, gravitationally stratified interstellar medium and discuss a model including the photoionization heating together with the supplemental heating mechanisms postulated by Reynolds, Haffner and Tufte. A cooling rate due to radiative losses is described by an approximation to the realistic cooling function of Dalgarno and McCray for ionized interstellar gas. An unperturbed initial state of the system simultaneously represents both a magnetohydrostatic and thermal equilibrium, and is thermally stable. We perform a set of 3D numerical magnetohydrodynamic simulations using the zeusmp code. We find that PI developing in the presence of non-adiabatic effects promotes a transition of gas in magnetic valleys to a thermally unstable regime. We find that the region of initially enhanced density due to PI starts to condense more as the result of thermal instability action. The density in this region rises above the classical isothermal limit of two times the equilibrium value at the mid-plane. The maximum density in an evolved system reaches 10–40 times the equilibrium value at the mid-plane, and the structures so formed attain oval shapes. These results lead to the conclusion that PI, operating in the presence of realistic cooling and heating processes, can trigger the formation of dense clouds, which may give rise to giant molecular complexes.     

An Analytic Model of Galactic Winds and Mass Outflows

Galactic winds and mass outflows are observed both in nearby starburst galaxies and in high-redshift star-forming galaxies. We develop a simple analytic model to understand the observed superwind phenomenon with a discussion of the model uncertainties. Our model is built upon the model of McKee & Ostriker for the interstellar medium. It allows one to predict how properties of a superwind, such as wind velocity and mass outflow rate, are related to properties of its starforming host galaxy, such as size, gas density and star formation rate. The model predicts a threshold of star formation rate density for the generation of observable galactic winds. Galaxies with more concentrated star formation activities produce superwinds with higher velocities. The predicted mass outflow rates are comparable to (or slightly larger than) the corresponding star formation rates. We apply our model to both local starburst galaxies and high-redshift Lyman break galaxies, and find its predictions to be in good agreement with current observations. Our model is simple and so can be easily incorporated into numerical simulations and semi-analytical models of galaxy formation.     

High-order 2MASS galaxy correlation functions: probing the Gaussianity of the primordial density field

We study Parker instability (PI) operating in a non-adiabatic, gravitationally stratified, interstellar medium. We discuss models with two kinds of heating mechanisms. The first one results from photoionization models. The other, relying on supplemental sources, has been postulated by Reynolds, Haffner & Tufte. The cooling rate, corresponding to radiative losses, is an approximation to the one given by Dalgarno & McCray. An unperturbed state of the system represents a magnetohydrostatic and thermal equilibrium. We perform linear stability analysis by solving the boundary value problem. We find that the maximum growth rate of PI rises for increasing magnitudes of non-adiabatic effects. In the pure photoionization model, the maximum growth rate of the general non-adiabatic case coincides with the isothermal limit. Adding other sources of heat leads to a maximum growth rate that is larger than the one corresponding to the isothermal limit. We find that the influence of the supplemental heating on PI also leads to a decrease in temperature in magnetic valleys. Finally, we conclude that the initial gas cooling due to the action of PI may promote a subsequent onset of thermal instability in magnetic valleys and formation of giant molecular clouds.     

The formation of galaxy discs

Galaxy disc formation must incorporate the multiphase nature of the interstellar medium. The resulting two-phase structure is generated and maintained by gravitational instability and supernova energy input, which yield a source of turbulent viscosity that is able to compete effectively in the protodisc phase with early angular momentum loss of the baryonic component via dynamical friction in the dark halo. Provided that star formation occurs on the viscous drag time-scale, this mechanism provides a means of accounting for disc sizes and radial profiles. The star formation feedback is self-regulated by turbulent gas pressure limited percolation of the supernova remnant heated hot phase, but can run away in gas-rich protodiscs to generate compact starbursts. A simple analytic model is derived for a Schmidt-like global star formation law in terms of the cold gas volume density.     

Gemini GMOS/integral field unit spectroscopy of NGC 1569 – II. Mapping the roots of the galactic outflow

We present a set of four Gemini-North Multi-Object Spectrograph/integral field unit (IFU) observations of the central disturbed regions of the dwarf irregular starburst galaxy NGC 1569, surrounding the well-known superstar clusters A and B. This continues on directly from a companion paper, in which we describe the data reduction and analysis techniques employed and present the analysis of one of the IFU pointings. By decomposing the emission-line profiles across the IFU fields, we map out the properties of each individual component identified and identify a number of relationships and correlations that allow us to investigate in detail the state of the ionized interstellar medium (ISM). Our observations support and expand on the main findings from the analysis of the first IFU position, where we conclude that a broad (≲400 km s⁻¹) component underlying the bright nebular emission lines is produced in a turbulent mixing layer on the surface of cool gas knots, set up by the impact of the fast-flowing cluster winds. We discuss the kinematic, electron-density and excitation maps of each region in detail and compare our results to previous studies. Our analysis reveals a very complex environment with many overlapping and superimposed components, including dissolving gas knots, rapidly expanding shocked shells and embedded ionizing sources, but no evidence for organized bulk motions. We conclude that the four IFU positions presented here lie well within the starburst region where energy is injected, and, from the lack of substantial ordered gas flows, within the quasi-hydrostatic zone of the wind interior to the sonic point. The net outflow occurs at radii beyond 100–200 pc, but our data imply that mass-loading of the hot ISM is active even at the roots of the wind.