Magnetic spiral arms in galaxies

The Doppler parameter distribution of Lyα absorption lines is calculated for a set of numerical simulations with different re-ionization histories. The differences in temperature between different re-ionization histories are as large as a factor of 3–4 depending on the spectrum of the ionizing sources and the redshift of helium re-ionization. These temperature differences result in observable differences in the Doppler parameter distribution. The best agreement with the observed Doppler parameter distribution between redshifts of 2 and 4 is found if hydrogen and helium are re-ionized simultaneously at or before a redshift of 5 with a quasar-like spectrum.     

On estimation of the pitch angle for spiral arms of galaxies

The shape of the arms of spiral galaxies is usually described by logarithmic spiralswith constant pitch angles, however, some galaxies display variations of the pitch angle along their spiral arms. We propose a method of determining the pitch angle of the arms in spiral galaxies based on the window Fourier analysis of their images. This method allows us not only to infer the average pitch angle, but also to obtain its value as a function of galactocentric radius. The method is implemented in the form of freely available programs.     

Properties of star formation in the spiral arms of barred galaxies

It is assumed that the two-fold disc-wide symmetry of spirals is caused by density waves, but also the potential of a bar component may have a significant influence on structural properties. The strength of the bar component appears to be anti-correlated with the degree of symmetry of star-forming regions in the spiral arms (Rozas et al., 1998). We present new results of R and Hα surface photometry of a sample of bright barred spirals. A photometric decompositon of the galaxy components is carried out in order to make a more accurate measurement of the strength of the bar and its interrelation to gas and stars in the disc. This revised version was published online in August 2006 with corrections to the Cover Date.     

Vorontsov-Velyaminov rows: Straight segments in the spiral arms of galaxies

The phenomenon of rows—straight features in the spiral patterns of galaxies, which was discovered by Vorontsov-Velyaminov, is investigated. The rows are not artifacts; in several cases, they outline regular spiral arms almost over their entire lengths. The galaxies M 101, M 51, and a number of more distant spirals are used as examples to demonstrate major geometrical and physical properties of these structures. It is shown that the row lengths increase nearly linearly with distance from the disk center, and that the angle between adjacent rows is almost always close to 2π/3. The galaxies with rows generally belong to moderate-luminosity Sbc-Sc systems with low rotational velocities, regular spiral patterns (Grand Design), and an H I content normal for these types of galaxies. Two types of rows are shown to exist, which differ in thickness and appear to be evolutionarily related. The formation mechanism of the rows should probably be sought in the peculiar behavior of the gas-compression wave in spiral density waves.     

The relation between magnetic and gas arms in spiral galaxies

The intriguing question of the origin of the arm-like magnetic structures seen between the optical spiral arms of certain spiral galaxies is addressed. Using a two-dimensional approximation to the non-linear disc dynamo equation, it is shown that gas streaming along the arms may produce such a field configuration. Another possibility is a spiral modulation of the turbulent diffusivity, associated with an enhancement of turbulence in the interstellar medium within the arms. The effects of a similar modulation of the alpha-effect are also examined. Finally, the consequences of a non-linear feedback of the large-scale field on the turbulent diffusivity ('η-quenching') are briefly discussed.     

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.     

Model of outgrowths in the spiral galaxies NGC 4921 and NGC 7049 and the origin of spiral arms

NGC 4921 and 7049 are two spiral galaxies presenting narrow, distinct dust features. A detailed study of the morphology of those features has been carried out using Hubble Space Telescope archival images. NGC 4921 shows a few but well-defined dust arms midway to its centre while NGC 7049 displays many more dusty features, mainly collected within a ring-shaped formation. Numerous dark and filamentary structures, called outgrowths, are found to protrude from the dusty arms in both galaxies. The outgrowths point both outwards and inwards in the galaxies. Mostly they are found to be V-shaped or Y-shaped with the branches connected to dark arm filaments. Often the stem of the Y appears to consist of intertwined filaments. Remarkably, the outgrowths show considerable similarities to elephant trunks in H ii regions. A model of the outgrowths, based on magnetized filaments, is proposed. The model provides explanations of both the shapes and orientations of the outgrowths. Most important, it can also give an account for their intertwined structures. It is found that the longest outgrowths are confusingly similar to dusty spiral arms. This suggests that some of the outgrowths can develop into such arms. The time-scale of the development is estimated to be on the order of the rotation period of the arms or shorter. Similar processes may also take place in other spiral galaxies. If so, the model of the outgrowths can offer a new approach to the old winding problem of spiral arms.     

The phase shift between the stellar and gaseous arms in the spiral galaxies M81 and M51

In this paper, the phase shift between the stellar and gaseous arms in the spiral galaxies M81 and M51 has been computed. It is found that the calculated pattern of the spiral arms agree in general with the observational results of 21 cm radio emission.     

The shape of spiral arms

The response by a gaseous disc to perturbations is investigated. Spectral analysis of the resulting pattern indicates that information about the nature of the perturber is maintained only if the perturbation is strong.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984.     

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.     

Radio emission spectra of isolated spiral galaxies and spiral members of pairs of galaxies

It is shown that objects with steep radio emission spectra occur significantly more often among isolated spiral galaxies than among spiral members of double galaxies. This, along with some other facts, indicates that members of pairs of galaxies are at a more active phase of their evolution than isolated galaxies. Translated fromAstrofizika, Vol. 40, No. 1, pp. 39–43, February, 1997.     

A dynamical model for the extraplanar gas in spiral galaxies

Recent H  i observations reveal that the discs of spiral galaxies are surrounded by extended gaseous haloes. This extraplanar gas reaches large distances (several kiloparsecs) from the disc and shows peculiar kinematics (low rotation and inflow). We have modelled the extraplanar gas as a continuous flow of material from the disc of a spiral galaxy into its halo region. The output of our models is pseudo data cubes that can be directly compared to the H  i data. We have applied these models to two spiral galaxies (NGC 891 and NGC 2403) known to have a substantial amount of extraplanar gas. Our models are able to reproduce accurately the vertical distribution of extraplanar gas for an energy input corresponding to a small fraction (<4 per cent) of the energy released by supernovae. However, they fail in two important aspects: (1) they do not reproduce the right gradient in rotation velocity; (2) they predict a general outflow of the extraplanar gas, contrary to what is observed. We show that neither of these difficulties can be removed if clouds are ionized and invisible at 21 cm as they leave the disc but become visible at some point on their orbits. We speculate that these failures indicate the need for accreted material from the intergalactic medium that could provide the low angular momentum and inflow required.     

A dispersion relation for open spiral galaxies

The Lin-Shu dispersion relation is applicable in the (asymptotic) case of tight spirals (large wave numberk _R). Here we reconsider the various steps leading to the Lin-Shu dispersion relation in higher approximation, under the assumption that the wave numberk _R is not large [(k _Rr) =O(1)], and derive a new dispersion relation. This is valid for open spiral waves and bars. We prove that this dispersion relation is the appropriate limit of the nonlinear self-consistency condition in the case where the linear theory is applicable.     

A new model for barred spiral galaxies

Computer simulations are used to demonstrate that normal spiral galaxies with symmetrically warped arms can appear to have bar and bar-within-ring structures when viewed from appropriate position angles.     

Unusual properties of bar-within-ring galaxies and the projection effect model for barred spiral galaxies

Five anomalous properties of bar-within-ring galaxies, which comprise the majority of barred spiral galaxies, are reported. These properties appear to be in conflict with conventional ideas about galactic morphology, but are in keeping with the projection effect model for barred spiral galaxies.     

Formation of the spiral structure in SB galaxies

We investigate regions near the ends of the principal spiral arms in SB galaxies, where the non-axially symmetric part of the gravitational potential rapidly transforms to its asymptotic quadrupole form. The galactic disk responds to this transformation of the potential by forming nearly circular spirals with an angular extent of the order of π/2 (quarter-turn spirals). We consider the resonance mechanism for the formation of principal spirals. Expressions are derived for the resonance responses of disks with circular and nearly circular stellar orbits.