An X-ray hubble diagram for quasi-stellar objects

To form the Hubble diagram for quasi-stellar objects (QSOs),we have made use of the recently published data on X-ray fluxes of 159 QSOs observed from the Einstein Observatory. The scatter in the Hubble diagram and the lack of an obvious redshift-flux density correlation for these QSOs have been attributed to the observational selection effect that the intrinsically less luminous QSOs can be detected only in the nearby region of space. When the optical, radio and X-ray selection effects are removed, keeping only the intrinsically brighter sources, we obtain a sample of 16 QSOs having a small dispersion in X-ray luminosities (〈 logL _x〉) = 46.12 ± 0.28), a statistically significant linear correlation between (logf _x, logcz) pairs and a slopeA =-1.906 ± 0.061 of the linear regression oflog f _x on logcz. This slope is consistent, at a confidence level of 95 per cent or greater, with the slope of-2.0 expected theoretically based on the assumption that the redshifts of QSOs are cosmological in nature.     

Galactic evolution along the hubble sequence

A generalization of the multiphase chemical evolution model applied to a wide set of theoretical galaxies is shown. This set of models has been computed by using the so-called Universal Rotation Curve from (Persic, Salucci and Steel, 1996) to calculate the radial mass distributions of each theoretical galaxy. By assuming that the molecular cloud and star formation efficiencies depend on the morphological type of each galaxy, we construct a bi-parametric grid of models whose results are valid in principle for any spiral galaxy, of given maximum rotation velocity or total mass, and morphological type. This revised version was published online in August 2006 with corrections to the Cover Date.     

Some statistical properties of spiral galaxies along the Hubble sequence

A statistical study has been made for the variations along the Hubble sequence, os such parameters as the degree of tightness of winding of spiral arm λ, the pitch angle μ, the flatness of the disk H/D₂₅ and the thickness H along the Hubble sequence for 365 spiral galaxies published in A&Ap Supplement Series. The mean values of these quantities for the various Hubble types have been obtained for the first time. The results of the statistics show clearly 1) that the Hubble classification of spiral galaxies is one which has only a qualitative and statistical significance, and 2) that the dispersion relation in the density wave theory is valid for most spiral galaxies, i.e., the arms of most spiral galaxies satisfy the requirements of being tightly wound.     

New estimates of scale heights and spiral structures for non-edge-on spiral galaxies

On the basis of Poisson＇s equation for the logarithmic perturbation of matter density, we provide improved estimates of scale heights and spiral structures for non-edge-on spiral galaxies by subtracting the surface brightness distributions from observed images. As examples, the non-edge-on spiral galaxies PGC 24996, which is face-on, and M31, which is inclined, are studied. The scale height, pitch angle and inclination angle of M31, our nearest neighbor, that are presented in this work, agree well with previous research.     

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.     

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.     

Sufficiency conditions for the development of spiral arms in galaxies

From previously published Markov chain derivations of the coefficients (in terms of transition probabilities) in the collisionless Boltzmann equation it proves possible, with plausible assumptions about the transition probabilities, to reduce to a partial differential equation with spiral characteristics. This is applied to the case of a rotating galaxy which has already become disk-shaped. The contangent of the angle between radial and tangential directions is then given by the ratio of radial velocity to circumferential velocity.Which already takes account of the deterministic effects of gravitation and angular momentum.     

Rotation of internal regions of spiral galaxies. II

Special Astrophysical Observatory, USSR Academy of Sciences: P. K. Shternberg State Astronomical Institute. Translated from Astrofizika, Vol. 28, No. 2, pp. 245–254, March–April, 1988.     

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.