Regularities in the distribution of star/gas complexes in the spiral arms of our galaxy and M31

The fragmentation of gaseous spiral arms in the outer Galaxy into superclouds has been studied using recently published data on the HI distribution in the Galactic disk. Regular chains of superclouds have been found or confirmed in the Cygnus (Outer) and Carina arms, with the spacings between the superclouds being concentrated near 0.1 and 0.2 of the solar Galactocentric distance. The star complexes in the northwestern arm of the galaxy M31 are spaced, on average, 1.2 kpc apart, with the most distinct chain of complexes being located in the arm region where Beck et al. (1989) detected a strong and wavy (along the arm) magnetic field. Its wavelength turns out to be related to the spacing between the complexes. In this arm, the HII regions lie inside the star complexes, which, in turn, are located inside the gas-dust lane. In contrast, the southwestern arm of M31 is split into a gas-dust lane and a dense stellar arm, which is not fragmented into star complexes. Here, the HII regions are located along the boundary between the gas-dust and stellar components of the arm; other evidence for the presence of a spiral shock wave triggering star formation is also observed, which is probably attributable to the large pitch angle of this segment of the southwestern arm. It may be suggested that the shock wave rapidly leads to star formation everywhere in this arm, while in the northwestern arm, where the shock wave is absent, star formation begins in the superclouds formed along the arm by the magneto-gravitational instability. This is how the chains of star complexes in the northwestern arm of M31 and, obviously, the chains of superclouds in the Carina and Cygnus arms of our Galaxy have been formed. The detection of a regularmagnetic field in the corresponding segments of these arms can be predicted.     

Chemical and dynamical evolution of spiral galaxies

We present the very first results of a new 3D numerical model for the formation and evolution of spiral galaxies along the Hubble sequence. We take into account the hydrodynamical properties of the gas with an SPH method while we use a tree code for the gravitational forces of the dark matter and stars. The chemical evolution is also fully included, with both SNe Ia and SNe II explosions being followed, and this will allows us to predict abundances of various chemical species, abundance ratios and their radial distributions. This revised version was published online in September 2006 with corrections to the Cover Date.     

Chemical and dynamical evolution of spiral galaxies

We investigate the consequences of the hypothesis of the secular evolution (growth of the bulge from disc material via a bar and temporal evolution of the Hubble sequence) on the chemical evolution of a galaxy. We present the first dynamical and chemical results of our 3D tree-SPH simulations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Exploring the evolution of spiral galaxies

We have constructed a family of simple models for spiral galaxy evolution to allow us to investigate observational trends in star formation history with galaxy parameters. The models are used to generate broad-band colours from which ages and metallicities are derived in the same way as the data. We generate a grid of model galaxies and select only those that lie in regions of parameter space covered by the sample. The data are consistent with the proposition that the star formation history of a region within a galaxy depends primarily on the local surface density of the gas but that one or two additional ingredients are required to explain the observational data fully. The observed age gradients appear steeper than those produced by the density dependent star formation law, indicating that the star formation law or infall history must vary with galactocentric radius. Furthermore, the metallicity–magnitude and age–magnitude correlations are not reproduced by a local density dependence alone. These correlations require one or both of the following: (i) a combination of mass dependent infall and metal enriched outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these possibilities on the basis of current data is extremely difficult.     

Star formation history in the central region of the barred galaxy NGC 4245

We have investigated the gas and stellar kinematics and the stellar population properties at the center of the early-type galaxy NGC 4245 with a large-scale bar by the method of two-dimensional spectroscopy. The galaxy has been found to possess a pronounced chemically decoupled compact stellar nucleus, which is at least a factor of 2.5 richer in metals than the stellar population of the bulge, and a ring of young stars with a radius of 300 pc. Star formation goes on in the ring even now; its location corresponds to the inner Lindblad resonance of the large-scale bar. According to Hubble Space Telescope data, the mean stellar age in the chemically decoupled nucleus is significantly younger than that within 0″.25 of the center. It may be concluded that we take the former ultracompact star formation ring with a radius of no more than 100 pc located at the inner Lindblad resonance of the now disappeared nuclear bar as the chemically decoupled nucleus. On the whole, the picture of star formation at the center of this gas-poor galaxy is consistent with theoretical predictions of the consequences of the secular evolution of a stellar-gaseous disk under the action of a bar or bars.     

Long-term evolution of spheroidal galaxies with locally isotropic velocity distribution

The short-term evolution of spheroidal galaxies has been explored by S. Chandrasekhar et al. and G. S. Sunder et al. In this paper, we study their long-term evolution with Laskar's method of frequency analysis. The main new results are as follows: (1) There exists a unique equilibrium, which is spherically symmetric. This equilibrium has a critical linear stability. (2) Generally speaking, the semi-axes exhibit quasiperiodic or nearly quasi-periodic (in a time scale longer than a Hubble time) oscillations around the radius of the above-mentioned equilibrium, so the equilibrium is practically stable. (3) There are cases in which one of the semi-axes tends fast to zero while the other to some finite value. The limit state is generally planar rather than linear, i.e. it is the symmetric semi-axis that tends to zero. This implies that some disk galaxies may have originated from spheroidal pregalaxy material.     

Chemo-spectrophotometric evolution of spiral galaxies – IV. Star formation efficiency and effective ages of spirals

We study the star formation history of normal spirals by using a large and homogeneous data sample of local galaxies. For our analysis we utilize detailed models of chemical and spectrophotometric galactic evolution, calibrated on the Milky Way disc. We find that star formation efficiency is independent of galactic mass, while massive discs have, on average, lower gas fractions and are redder than their low-mass counterparts; put together, these findings convincingly suggest that massive spirals are older than low-mass ones. We evaluate the effective ages of the galaxies of our sample and we find that massive spirals must be several Gyr older than low-mass ones. We also show that these galaxies (having rotational velocities in the 80–400 km s⁻¹ range) cannot have suffered extensive mass losses, i.e. they cannot have lost during their lifetime an amount of mass much larger than their current content of gas+stars.     

Toward a more consistent evolution model for the local galactic disk

Infall models for the evolution of the local galactic disk were studied and confronted with a large number of observational constraints from the solar vicinity, inclusive of the white dwarf luminosity function. The models are characterized as follows: 1. The key-functions (SFR, IMF, gas infall rate) are not prescribed by simple laws, but are directly derived from observational constraints. 2. A scatter in the metallicity at fixed age is considered which partly reflects inhomogeous chemical evolution. 3. Special attention is drawn to the internal consistency of the models. 4. In addition to infall of low-metallicity gas, metal-enriched outflows are allowed. The “best” model is characterized by a disk age of ≈︁ 12 Gyr, a SFR which is decreasing over the first half and is nearly constant over the second half of the disk evolution, and by a similar temporal run of the gas infall rate. Moderate metal-enriched outflow can not be excluded.     

On the properties of elliptical and spiral galaxies in a CDM scenario

This paper aims to investigate what kind of density perturbations did lead to elliptical galaxies and what kind to spiral galaxies, in the context of a CDM scenario. Previous work by HP (Heavens and Peacock, 1988) is reviewed and extended; more particularly: (i) a theoreticalJM relation is derived for virialized configurations, with a slope increasing with the mass, in the range 5/3<<2, and compared with its counterpart deduced by HP for expanding configurations; (ii) an non-dimensional angular momentum =J/(G ² M ⁵ t _ff )^1/3; witht _ff free-fall time at turnaround, is calculated explicitly and compared with the usual spin parameter =J(–E) ^1/2/(GM ^5/2), in the special case of polytropic spheres and for different peak heights; (iii) a model is built up where the ellipsoidal density perturbations described by HP are approximated as spherical density perturbations with the same mass and the same rate of acquisition of angular momentum, and the contribution of the latter to the expansion is also taken into account. The calculations are limited to the special case of Einstein-de Sitter universes (of dust only), in the whole range of HP distributions of angular momenta. If a massM=10¹² m is typical for galaxies, the results are consistent with both an inferred difference roughly in a factor of six times between angular momenta of ellipticals and spirals, and a continuous transition from the former to the latter, for each peak height. It is argued that star formation together with angular momentum, instead of peak height alone, makes the fate of a given proto-object; more specifically, if star formation is high enough and/or angular momentum is low enough, the system will be frozen as an elliptical galaxy; if, on the other hand, the rate of star formation is low enough, and/or angular momentum is high enough, the system will be frozen as a spiral galaxy. In addition, the results lead to an obvious trend: more ellipticals are associated with higher peak heights and more spirals with lower peak heights.     

Formation and evolution of disk galaxies

I review several of the current issues in the theory of disk galaxy formation. There is still much to be done, observationally and theoretically, before we can expect to approach an understanding of disk galaxies that is reliable enough to make robust predictions about the high redshift universe. This revised version was published online in August 2006 with corrections to the Cover Date.     

Massive Star Mergers: Induced Mixing and Nucleosynthesis

We study the nucleosynthesis and the induced mixing during the merging of massive stars inside a common envelope. The systems of interest are close binaries, initially consisting of a massive red supergiant and a main-sequence companion of a few solar masses. We apply parameterized results based on hydrodynamical simulations to model the stream-core interaction and the response of the star in a standard stellar-evolution code. Preliminary results are presented illustrating the possibility of unusual nucleosynthesis and post-merging dredge-up which can cause composition anomalies in the supergiant's envelope. This revised version was published online in September 2006 with corrections to the Cover Date.     

Asteroseismic Investigation of 44 Tau, A Slowly Rotating δ Scuti Star

Evolutionary model calculations of 44 Tau, a δ-Scuti star, have been carried out. The star in question is a slow rotator (vsini = 6.8 kms^-1).However small it may be, the effect of rotation on the oscillation properties of the star was thought to be worth of studying. Models were evolved with uniform rotation. Radial and non-radial adiabatic oscillation frequencies were calculated. In determining oscillation frequencies rotation has been treated as a perturbation. First order effect has been considered. The possible rotational splittings, due to two rotation speeds, have been calculated. This revised version was published online in July 2006 with corrections to the Cover Date.     

On a simple model for self-regulating star formation in the galactic disk

Sternentstehung in Galaxien ist ein Prozeß mit Rückkopplung zum interstellaren Medium (ISM) und möglicherweise ein Teil eines selbstregu-lierenden Zyklus. DOPITA (1985) hat ein Modell vorgeschlagen, in dem Sternentstehung in Spiral- und irregulären Galaxien über den Druck im ISM selbstregulierend wirkt. In der vorliegenden Arbeit wird gezeigt, daß die verfügbaren Daten für die radialen Verteilungen von Gas, Gesamtmasse und Lymankontinuumphotonenfluß in der Scheibe unserer Galaxis dieses einfache Modell nicht stützen. Verschiedene mögliche Ursachen werden diskutiert.     

The evolution of barred spiral galaxies in the Hubble Deep Fields North and South

The frequency of barred spiral galaxies as a function of redshift contains important information on the gravitational influence of stellar discs in their dark matter haloes and may also distinguish between contemporary theories for the origin of galactic bulges. In this paper we present a new quantitative method for determining the strength of barred spiral structure, and verify its robustness to redshift-dependent effects. By combining galaxy samples from the Hubble Deep Field North with newly available data from the Hubble Deep Field South, we are able to define a statistical sample of 46 low-inclination spiral systems with I _814 W<23.2 mag. Analysing the proportion of barred spiral galaxies seen as a function of redshift, we find a significant decline in the fraction of barred spirals with redshift. The redshift distribution of 22 barred and 24 non-barred spirals with suitable inclinations is inconsistent with their being drawn from the same distribution at the 99 per cent confidence level. The physical significance of this effect remains unclear, but several possibilities include dynamically hotter (or increasingly dark-matter-dominated) high-redshift discs, or an enhanced efficiency in bar destruction at high redshifts. By investigating the formation of the 'orthogonal' axis of Hubble's classification tuning fork, our result complements studies of evolution in the early–late sequence, and pushes to later epochs the redshift at which the Hubble classification sequence is observed to be in place.     

Chemo-spectrophotometric evolution of spiral galaxies – II. Main properties of present-day disc galaxies

We study the chemical and spectrophotometric evolution of galactic discs with detailed models calibrated on the Milky Way and using simple scaling relations, based on currently popular semi-analytic models of galaxy formation. We compare our results with a large body of observational data on present-day galactic discs, including disc sizes and central surface brightness, Tully–Fisher relations in various wavelength bands, colour–colour and colour–magnitude relations, gas fractions versus magnitudes and colours and abundances versus local and integrated properties, as well as spectra for different galactic rotational velocities. Despite the extremely simple nature of our models, we find satisfactory agreement with all those observables, provided that the time-scale for star formation in low-mass discs is longer than for more massive ones. This assumption is apparently in contradiction with the standard picture of hierarchical cosmology. We find, however, that it is extremely successful in reproducing major features of present-day discs, like the change in the slope of the Tully–Fisher relation with wavelength, the fact that more massive galaxies are on average 'redder' than low-mass ones (a generic problem of standard hierarchical models) and the metallicity–luminosity relation for spirals. It is concluded that, on a purely empirical basis, this new picture is at least as successful as the standard one. Observations at high redshifts could help to distinguish between the two possibilities.     

Combined color indices and photometric structures of the galaxies NGC 834 and NGC 1134

Surface BVRI photometry is presented for two spiral galaxies with a complex photometric structure: NGC 834 and NGC 1134. We propose to introduce the combined color indices Q _BVI and Q _VRI to investigate the photometric structure of the galaxies. These color indices depend only slightly on selective absorption, which allows them to be used to study the photometric structure of “dusty” galaxies. Evolutionary stellar-population models show that Q _BVI is most sensitive to the presence of blue stars, while Q _VRI depends on local Hα equivalent width. A ring with active star formation manifests itself on the Q _BVI map for NGC 834 at a distance of ～15 from its center, and a spiral structure shows up on the Q _VRI map for NGC 1134 in its inner region. The Q _BVI –Q _VRI diagram can provide information about the current stage of a star's formation in various galactic regions. A comparison of the color indices for the galaxies with their model values allows us to estimate the color excesses and extinction in various galactic regions.     

AGB星的超星风与热脉动的研究

至今国际上对ＡＧＢ星的超星风、热脉动及振动的理论研究工作中都忽略了湍流压的存在，然而对于一个整个外壳都处于完全对流的ＡＧＢ星，湍流压是存在的，并且由于湍流压的存在将对气体压强、密度、温度以及各热力学量产生影响，因而对ＡＧＢ星的超星风、热脉动及振动产生影响。本文以恒星结构演化理论中常用的混合程对流理论为基础，给出了湍流压以及考虑湍流压情况下恒星内部物态方程和各热力学量的表达式，并给出了判别恒星外壳产生动力学非稳定性的判据，并在此基础上研究了质量为２．８Ｍ⊙和７Ｍ⊙的恒星从主序到ＡＧＢ星的演化。目的是研究有湍流压情况下ＡＧＢ星的超星风及热脉动的机理。结果表明，对于７Ｍ⊙恒星，在红巨星及ＡＧＢ阶段，湍流压可以达到总压强的３０％，并且对恒星的结构产生重要影响，对于２．８Ｍ⊙恒星，在红巨星及ＡＧＢ阶段影响较小。由于湍流压的作用，２．８Ｍ⊙和７Ｍ⊙恒星在ＡＧＢ阶段和红巨星阶段，靠近表面的区域会出现动力学非稳定性，造成物质向外喷流，这就是ＡＧＢ星和红巨星产生强大星风的物质损失的原因。研究还表明，湍流压效应造成７Ｍ⊙ＡＧＢ星的氢燃烧壳层产生主周期为５０年的脉动现象，同时使氦燃烧壳层的热脉动提前发生和周期变短     

A grid of chemical evolution models as a tool to interpret spiral and irregular galaxies data

We present a generalization of the multiphase chemical evolution model (CEM) applied to a wide set of theoretical galaxies with different masses and evolutionary rates. This generalized set of models has been computed using the so-called universal rotation curve from Persic, Salucci & Steel to calculate the radial mass distribution of 44 theoretical protogalaxies. This distribution is a fundamental input which, besides its own effect on the galaxy evolution, defines the characteristic collapse time-scale or gas infall rate on to the disc. We have adopted 10 sets of values, between 0 and 1, for the molecular cloud and star formation efficiencies, as corresponding to their probability nature, for each one of the radial distributions of total mass. Thus, we have constructed a biparametric grid of models, depending on those efficiency sets and on the rotation velocity, whose results are valid in principle for any spiral or irregular galaxy. The model results provide the time-evolution of different regions of the disc and the halo along galactocentric distance, measured by the gas (atomic and molecular) and stellar masses, the star formation rate (SFR) and chemical abundances of 14 elements, for a total of 440 models. This grid may be used to estimate the evolution of a given galaxy for which only present time information, such as radial distributions of elemental abundances, gas densities and/or star formation, which are the usual observational constraints of chemical evolution models (CEMs), is available.