It is argued that spiral patterns in galaxies generally are transient features which will decay away in a few dynamical times, only to be replaced by other similarly short-lived waves. The evidence for this point of view is presented and the current state of the theory is discussed. A major consequence of such behaviour is that disc stars are scattered into gradually more eccentric orbits by successive spiral waves. This has two further implications: (a) the velocity dispersion of stellar populations will increase with their ages and (b) persistent spiral structure demands a gas component in the disc that can dissipate some of the random moiton energy. That the theory offers such a matural explanation for these two well established aspects of galactic structure is cited as indirect evidence in its favour.
Quasi Stationary Spiral Structure is the natural Interpretation for grand deqlgn spiral galaxies. The best theoretical tool to descrlbe QSSS morphologles is the study of spiral modes. Modal studies are characterlzed by two distinct stages. At the dynamical level, one wishes to compute spiral modes efflcently on a variety of equilibrium models and to master the relation between the properties of the basic states and the propertles of the relevant spiral modes. At the astrophysical level, realistic basic states are identified by dlscussing the processes of evolutlon and
self-regulatlon; in partlcular, we Identify some basic states conslstent wlth QSSS grand designs and others where more flocculent spiral structure is expected to occur. For thls second stage It is cruclal to have a proper interpretation of the results obtained from one-component models In vlew of the multiple component astrophysical systems to be described (which include stars, Population 1 objects, and cold gas). Followlng this outllne, under the guidance of a slmple analytlc (asymptotic) theory, a very large set of galaxy models has been studled numerically. Thls is essentlally a three dimensional survey wlthln a flexlble class of models, where one parameter measures the amount of partlclpatlng disk mass, a second parameter determlnes the
hotness of the disk and the third is a scale length which combines propertles of the slze of the nuclear bulge and the dlstrlbutlon of the cold gas component. All the essentlal morphologlcal types found in the Hubble diagram have been reproduced; in partlcular, parameter reglmes are identified for SA and SB galaxies and speclfically for SBO, SB-r and SB-s objects. Dlstinction is made between systems where gas is expected to play an actlve or a passlve role.
Improving precious studies based on galaxies belonging to various aggregations, our examination of the radio and optical properties of Virgo spirals indicates that the ratio of the radio continuum-to-optical luminosiy of spirals is linked to their HI content. Specifically, HI deficient spirals tend to be weaker radio continuum emitters.
This paper deals with the determination of global star formation rates (SPRs) from radlo free-free and submm/FIR dust emission. Masslve, hot and luminous stars interact with the surrounding
interstellar matter (ISM) lonlzing the gas and heating the dust. O star formation rates (OSFR) in the galactic disk are estimated with observed Lyman continuum photon production rates. Extrapolation to lower mass stars with a constant inltial mass function (IMF) yields, however, too high total SFRs. Furthermore, the lock-up rate, i.e. the rate at whlch gas transformed into stars is permanently locked up in low mass and dead stars, can not reproduce the present-day mass distribution of the galactic disk. Agreement between Lyc photon production rate and time integrated lockup rate can be reached by introducing bimodal star formation in the galactic disk. Thls means that induced star formation in main spiral arms produces only masslve stars ≥3mq. while spontaneous star formation in the interarm region produces stars in the total mass range ≥0.1 mq.
Estimates of SFR based on Lyc photon production rates can not easily be applied to external galaxies because of the difficulty to separate radio synchroton and free-free emission. It is found that slmilar problems are encountered in separating the emission from warm dust (heated by OB stars) and cold dust (heated by the general Interstellar radiation field). The relation between IR luminoslťy and star formation actlvity of galaxies is much more complex than previously assumed.
The problem of interpretation of the so-called Central Void /CV/ in the Jagiellonian Field /JF/ is consldered. The galaxies in JF are inclined to group around CV into a pallium-like configuration. The popular suggestion that CV may reflects a really existlng and genuinely empty volume of the inter-multicluster space has been verified. So far any interstellar cloud was nover regarded as a possiblo explanation because of: 1. the cosecant-type laws descrlbing interstellar extlnctlon in the Milky Way, whlch made the galactlc extlnction at b=72” improbable, 2. the claims of the Zwicky´s School that JF was free of the extlnction, 3. the traditlonal view that multlclustering was not caused by absorptive dusty clouds, i.e. that the extended galaxy structures were not only apparent. The results of our work concem the conceptual foundatlons of the researoh programme oontained in early works of Splawa-Neyman /superclustors of galaxies called "clouds"/ and recent searches of Batuski and Burns on a poasible 300 Mpc filament of clusters of galaxies in Perseus-Pegasus. In the paper It ia proved that a local dusty cloud Is projected just onto the region of the Central Void.