Spiral galaxy

The astrophysical problem of justifying on theoretical grounds the morphology of galaxies (spiral and eUiptical, with their different content in stars and gas), their chemical evolution (initial rapid enrichment of metals, i.e., any element heavier than hydrogen and helium), and, finally, the attempt to trace a classification based on different physical aspects of the evolution, has been tackled by employing the approach of cooperative systems. In these models a scenario is proposed where the large-scale dynamics are related to the local microscopic interactions. At the same time a macroscopic description (e.g., the interplay of various phases, the metallicity) is derived by means of few (stochastic) variables. 

Galaxies are huge aggregations of stars, the mass of a galaxy may exceed 10 solar masses. There exist different types of galaxies spiral galaxies (Fig. 7.12) like... 

Key wordsi Photometry - Galaxies structure of - Galaxies spiral... 

Key words Spiral Galaxies - Spiral Arms - Imaging... 

P-xv artist s impression of galaxies being formed in the aftermath of the Big Bang. The spiral clouds of gas have already started condensing into the shapes of future galaxies. (Photo Researchers Inc.)... 

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

Fig. 11.6 Schematic representation of the Milky Way with the galactic life zone , in which life should be possible. The centre of the galaxy is kept practically sterile by extreme radiation, while areas in which stars are formed are localized in the spiral arms...

Thick disks are not unique to the Milky Way. Thick disks are seen in many spiral and lenticular galaxies, see e.g. [17], and in galaxies in merging environments, see e.g. [21]. Some, [9], even suggest that all spiral galaxies have thick disks. It is an important observational task to verify and extend these findings. 

Planetary nebulae (PNe) offer the opportunities 1) to study stellar nucleosynthesis in the advanced phases of stellar evolution of stars in the wide mass range - -O. S to Mq and 2) to probe radial and as well horizontal/vertical chemical gradients in spiral galaxies by the time of formation of their progenitors. 

As a consequence chemical abundances in PNe are of primary importance for the chemical evolution of spiral galaxies, including our own and related topics. 

The existence of abundance gradients of N/O in spiral galaxies imposes limits on the efficiency of HBB since for large efficiencies they would vanish [3,1],... 

The three broad classifications (elliptical, spiral and irregular) of star clusters that also cluster together to form the Local Group that contains the Milky Way and the Andromeda Galaxy, along with the Small and Large Magellanic Clouds... 

Galaxy A collection of stars located close in space, such as our spiral galaxy, the Milky Way. 

Fig. 3.25. Trends of nebular line strengths in H n regions with oxygen abundance. This figure shows oxygen abundance in H n regions of the Milky Way and spiral and irregular galaxies (determined using measured electron temperatures) vs. log R23, after Pilyugin (2003) the p parameter is the line ratio [O iii]/([0 11] + [O hi]).

LINERs are weaker emission-line regions in certain elliptical and early-type spiral galaxies (e.g. M51 and M81) showing relatively strong lines of [O I], [N ii] and [S n], similar to SNR. It is not clear whether they are excited by shocks like SNR or by a very dilute (i.e. low u) non-thermal spectrum. 

R. B. C. Henry and Guy Worthey, The Distribution of Heavy Elements in Spiral and Elliptical Galaxies , Publ. Astr Soc. Pacific, 111, 919, 1999. 

Typical timescales 1/v range from as little as 108 years (roughly the dynamical timescale) for a starburst galaxy to maybe 2 x 109 years in an early-type spiral to... 

Fig. 8.1. Ne/O ratio vs. O/H from (mainly optical) observations of H n regions in spiral and irregular galaxies and the Sun. Filled and open symbols represent results from different authors. After Garnett (2004).

Fig. 8.11. N/O ratio vs. O/H in H 11 regions of irregular (circles) and spiral galaxies (plus signs), adapted from Pilyugin, Thuan and Vilchez (2003). Solar values are indicated by the axes.

Fig. 8.12. Relation between oxygen abundance of H II regions in irregular (open squares) and spiral galaxies (filled circles, taking abundances at 0.4 of the de Vaucouleurs isophotal radius R25) plotted against the gas fraction, after Pilyugin, Vilchez and Contini (2004). The heavy curve shows expectation from the Simple model with an oxygen yield of 0.0027 (or about 0.5 Z ) and the broken curves show the same with the yield 1.5 x higher or lower, whereas the dotted curve shows a yield 4 x lower. The effective yield, defined as Zo/(— ln/z), increases systematically with luminosity, and the gas fraction decreases.

Fig. 8.13. H ii region oxygen abundance against rotational velocity for irregular and spiral galaxies (at a radius of 0.4/ s), after Pilyugin, Vflchez and Contini (2004).

Parameters of dynamically hot galaxies , i.e. various classes of ellipticals and the bulges of spirals, generally lie close to a Fundamental Plane in the 3-dimensional space of central velocity dispersion, effective surface brightness and effective radius or equivalent parameter combinations (Fig. 11.10). This is explained by a combination of three factors the Virial Theorem, some approximation to... 

---