On the role of interstellar gas in spiral galaxies

This paper discusses the role played by the interstellar gas in spiral galaxies,using a two-disk model, one for the stars, one for the gas. The following conclusions are drawn. 1. When stars and gas have different velocity dispersions,the stellar and gaseous arm must separate. 2. Such a separation makes the spiral mode of density waves unstable. 3. The ratio η between the densities of gas and stars must be less than a certain value for density waves to be maintained.4.The smaller η is,the more tightly wound will be the arms.     

The interaction of the spiral density wave and the Sun's galactic orbit

The galactocentric radial motion of the Sun introduces another periodicity to the encounters between the Sun and the spiral density wave. We describe a model simulation of the effect and present the associated periods.     

The structure of spiral galaxies — II. Near-infrared properties of spiral arms

We have imaged a sample of 45 face-on spiral galaxies in the K band, to determine the morphology of the old stellar population, which dominates the mass in the disc. The K -band images of the spiral galaxies have been used to calculate different characteristics of the underlying density perturbation such as arm strengths, profiles and cross-sections, and spiral pitch angles. Contrary to expectations, no correlation was found between arm pitch angle and Hubble type, and combined with previous results this leads us to conclude that the morphology of the old stellar population bears little resemblance to the optical morphology used to classify galaxies. The arm properties of our galaxies seem inconsistent with predictions from the simplest density wave theories, and some observations, such as variations in pitch angle within galaxies, seem hard to reconcile even with more complex modal theories. Bars have no detectable effect on arm strengths for the present sample. We have also obtained B -band images of three of the galaxies. For these galaxies we have measured arm cross-sections and strengths, to investigate the effects of disc density perturbations on star formation in spiral discs. We find that B -band arms lead K -band arms and are narrower than K -band arms, apparently supporting predictions made by the large-scale shock scenario, although the effects of dust on B -band images may contribute towards these results.     

On the role of the magnetic field in a wave theory of the spiral structure of galaxies

The spiral waves in a model galaxy consisting of the differentially rotating interstellar gas and Population I are considered. The instability of spiral waves in the presence of differential rotation and magnetic field is found. This instability may lead as well as the Landau-instability found by Marochnik and Suchkov (1968, 1969) to the formation of an observable spiral pattern in a short time. It may play a definitive role in the spiral structure formation in galaxies with weak Population II.     

Leading wave as a component of the spiral pattern of the galaxy

The spiral pattern of the Galaxy, identified by analyzing the kinematics of young stars within 3 kpc of the Sun, is Fourier decomposed into spiral harmonics. The spiral pattern of the Galaxy is shown to be representable as a superposition of trailing and leading waves with interarm distances of λ = 1.8 ± 0.4 and 4 ± 2 kpc, respectively. Shock waves are probably present only in the portions of the trailing spiral pattern where it crosses the crest of the leading wave. The small interarm distance of the trailing spiral wave (λ = 1.8 kpc) can be explained by its evolution—by the decrease in the interarmd istance as the wave is displaced toward the inner Lindblad resonance. The Carina arm may be part of this resonance ring.     

The global structure of the interstellar medium

In the context of a review of work on the global structure of the interstellar medium, supernova remnant evolution, flows in multiphase media, cosmic ray moderation of flows, theories of the Galactic halo gas, and the nature of the local superbubble are considered. Speculations about the nature of a one parameter fully self-consistent model of the interstellar medium-supernova-radiation and cosmic ray background system are offered.     

Large-scale flow of interstellar gas in galactic spiral waves: Effects of thermal balance and self-gravitation

Using the recent observational data on atomic and molecular hydrogen in the Galaxy, we analyse the dynamics of the interstellar gas in a spiral density wave. Within the framework of Marochniket al.'s (1972) model of the galactic spiral structure, the gas flow is obtained, with self-gravitation and thermal processes taken into account.It is shown that: (1) the self-gravitation of gas does not practically affect the galactic shock if the dominant contribution into the gas density comes from atomic hydrogen; (2) the effects of self-gravitation could be essential for both the gas flow and the stellar spiral wave only if the density contribution of H₂ exceeded several times that ofHi, with molecular hydrogen as a continuous medium having the isothermal equation of state; (3) however, regardless of the estimates of H₂ abundance in the Galaxy, its reaction to the density wave is weak, since it forms a collisionless system not dragged by the intercloud gas.It has been found that, if we allow for thermal processes in the interstellar medium, new types of gas flow can develop alongside with a previously-known continuous flow and galactic shock. They are: (1) galactic shock with the phase transition leading to the formation of dense cold clouds; (2) a three-phase flow where hot cavities and dense cold clouds coexist with an initial, moderately dense and cold phase; (3) an accretion wave which is a specific type of nonlinear wave with an amplitude of 11/2 orders of magnitude larger than that of the isothermal galactic shock appearing under the same conditions, but without heating and cooling.     

The radio luminosity of pulsars and the distribution of interstellar electron density

The radio luminosities of pulsars are given as functions of their period and the time variation of the period. The parameters of that dependence are calculated and independent distances are determined for pulsars. The average electron densities toward the pulsars are determined from the known dispersion measures. The results obtained are used to study the large-scale electron density distribution in the Galaxy. The distribution maximum lies in the vicinity of the Sagittarius spiral arm. The electron density falls off exponentially in the regions between spiral arms. This revised version was published online in July 2006 with corrections to the Cover Date.     

A simplification of the Poisson integral in the theory of spiral density waves

We give here a method of simplifying the evaluation of the Poisson integral in the spiral density wave theory, which will save computer time in the evaluation of the kernal function by a factor of 84. A systematic check of the method has been made for a series of asymptotic density — gravitational potential relations.     

A predicted relation between the temperature and density profile of cluster hot gas

Based on the assumptions that a fraction of cluster dark matter is composed of degenerate neutrinos and they are in hydrostatic equilibrium with other matter, we predict a relation between the density profile and temperature of the cluster hot gas. The predicted relation agrees with observational data of 103 clusters.     

The interstellar medium:the key component in galactic evolution and modern cosmology

The gases of the interstellar medium(ISM) possess orders of magnitude more mass than those of all the stars combined and are thus the prime component of the baryonic Universe. With L-band surface sensitivity even better than the planned phase one of the Square Kilometre Array(SKA1), the Five-hundredmeter Aperture Spherical radio Telescope(FAST) promises unprecedented insights into two of the primary components of ISM, namely, atomic hydrogen(HI) and the hydroxyl molecule(OH). Here, we discuss the evolving landscape of our understanding of ISM, particularly, its complex phases, the magnetic fields within, the so-called dark molecular gas(DMG), high velocity clouds and the connection between local and distant ISM. We lay out, in broad strokes, several expected FAST projects, including an all northern-sky high-resolution HI survey(22 000 deg2, 3′FWHM beam, 0.2 km s~(-1)), targeted OH mapping, searching for absorption and maser signals, etc. Currently under commissioning, the commensal observing mode of FAST will be capable of simultaneously obtaining HI and pulsar data streams, making large-scale surveys in both science areas more efficient.     

Similarity theory of stellar models and the structure of very massive stars

The similarity theory of stellar models is used to study the properties of very massive stars when all the opacity sources, except the Thomson scattering, can be disregarded. The dimensionless internal structure of such stars is essentially independent of the energy generation law. It is shown that the mass-luminosity relation can be fitted by an analytical expression that is virtually universal with regard to the chemical composition and the energy generation law. A detailed comparison with the Eddington standard model is made. The application of the results obtained to the observations of massive stars is briefly discussed.     

The excitation of spiral density waves through turbulent fluctuations in accretion discs – I. WKBJ theory

We study and elucidate the mechanism of spiral density wave excitation in a differentially rotating flow with turbulence which could result from the magneto-rotational instability. We formulate a set of wave equations with sources that are only non-zero in the presence of turbulent fluctuations. We solve these in a shearing box domain, subject to the boundary conditions of periodicity in shearing coordinates, using a WKBJ method. It is found that, for a particular azimuthal wavelength, the wave excitation occurs through a sequence of regularly spaced swings during which the wave changes from leading to trailing form. This is a generic process that is expected to occur in shearing discs with turbulence. Trailing waves of equal amplitude propagating in opposite directions are produced, both of which produce an outward angular momentum flux that we give expressions for as functions of the disc parameters and azimuthal wavelength.
By solving the wave amplitude equations numerically, we justify the WKBJ approach for a Keplerian rotation law for all parameter regimes of interest. In order to quantify the wave excitation completely, the important wave source terms need to be specified. Assuming conditions of weak non-linearity, these can be identified and are associated with a quantity related to the potential vorticity, being the only survivors in the linear regime. Under the additional assumption that the source has a flat power spectrum at long azimuthal wavelengths, the optimal azimuthal wavelength produced is found to be determined solely by the WKBJ response and is estimated to be  2π H   , with H being the nominal disc scaleheight. In a following paper by Heinemann & Papaloizou, we perform direct three-dimensional simulations and compare results manifesting the wave excitation process and its source with the assumptions made and the theory developed here in detail, finding excellent agreement.     

Time-dependent nucleation theory and the formation of interstellar grains

The theory of homogeneous nucleation is discussed and applied to the formation of interstellar grains. Special attention is given to the thermodynamics of very small clusters (N10); it is found that, at least for carbon and metals, the free energies ofN-mers may be conveniently expressed in terms of a parameter _N which is nearly independent ofN. Estimates of _N from the Lothe-Pound theory disagree with the experimental observations forN<10; it is preferable to simply approximate _N 1/2 ( is known from bulk properties). The kinetic process of nucleation and cluster growth is essentially just a function of two dimensionless parameters; /T and . The domain in which nucleation theory should be valid is delineated. Approximate formulae are given for the critical supercooling, the critical cluster size, and the mean final cluster size; more accurate numerical results are presented graphically. Nucleation of grains in planetary nebulae, novae, and red giant outflows is discussed. The origin of the Pt-rich nuggets found in the Allende meteorite is considered; it is shown that the nuggets could have been formed by homogeneous nucleation in the primordial solar nebula, whereas they couldnot have formed in a supernova explosion.Invited contribution to the Proceedings of a Workshop onThermodynamics and Kinetics of Dust Formation in the Space Medium, held at the Lunar and Planetary Institute, Houston, 6–8 September, 1978.     

The spiral structure of the Milky Way

The morphology and kinematics of the spiral structure of the Milky Way are long-standing problems in astrophysics.In this review we firstly summarize various methods with different tracers used to solve this puzzle.The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties, as we can currently obtain accurate parallaxes(a few μas) and proper motions(≈1 km s~(-1)) by using Very Long Baseline Interferometry(VLBI).On the other hand, the Gaia mission is providing the largest, uniform sample of parallaxes for O-type stars in the entire Milky Way.Based upon the VLBI maser and Gaia O-star parallax measurements, nearby spiral structures of the Perseus, Local, Sagittarius and Scutum Arms are determined in unprecedented detail.Meanwhile, we estimate fundamental Galactic parameters of the distance to the Galactic center,R_0, to be 8.35 ± 0.18 kpc, and circular rotation speed at the Sun, Θ_0, to be 240±10 km s~(-1).We found kinematic differences between O stars and interstellar masers: the O stars, on average, rotate faster,8 km s~(-1) than maser-traced high-mass star forming regions.     

Correlation between the densities of X-ray sources and interstellar gas

The distribution of X-ray sources in our galaxy is obtained, assuming that the absolute X-ray luminosities of these sources are the same. The distribution is found to be in good correlation with the distribution of interstellar gas. The density of X-ray sources is nearly proportional to the square density of gas. This indicates that X-ray sources are comparatively young. The relation between the densities of X-ray sources and gas allows us to estimate the X-ray intensities of various objects such as Magellanic Clouds and Andromeda nebula, and also to obtain the average X-ray luminosity of spiral galaxies. The latter should increase as the age of a galaxy decreases, since the amount of gas decreases as the galaxy evolves. Under the assumptions that the gas density is inversely proportional to the age and that galaxies older thant ₀/30 are visible in X-rays, wheret ₀ is the present age of the universe, the contribution of X-ray sources in distant galaxies to the background component is calculated. The intensity and the spectrum of the background component of X-rays thus obtained are in fair agreement with observed ones in the energy range between 1 and 4 keV but significantly deviate from the latter at high energies.     

Shock waves and the chemical structure of interstellar clouds

In this paper we study the effect of shock waves on the chemical structure of the interstellar clouds. A model of molecular cloud has been assumed. The chemistry is investigated in a time dependent model. Our chemical network contains 56 species in 251 reactions to including molecules of the elements H, O, C, N, S, and Si.The results indicate that the calculated fractional abundance of the molecules NS, H₂O, CN, NH, CO, and SO agrees well with the observations. The molecules OH, H₂S, CS, H₂CS, HS, NO, SiO, CH, CH₂, CH₃, HCO, C₂, and HCN reach high post-shock abundances.