Galaxies groups

The phenomenon of cooling flows , described for luminous clusters, also applies to other systems with gaseous atmospheres. In particular, galaxy groups, the poorer cousins of rich clusters, (e.g., Helsdon et al. 2001) and individual early type galaxies with their luminous, X-ray coronae (Forman, Jones Tucker 1985) also frequently have short gas cooling times at their centers (Thomas et al. 1986). 

Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

Studies in chemistry or any realm of science commonly consist of a series of directed examinations of parts of nature s realm called systems. A system is an identifiable fragment of the world that is recognizable and that has attributes that one can identify in terms of form and/or function. We can give examples at any level of size and complexity and in essentially any context. Indeed, a dog is a system at a pet show whereas the human heart is a system to the cardiologist a tumor cell is a system to the cancer specialist a star or planet or galaxy is a system to an astronomer a molecule or a collection of molecules is a system to a chemist and an atom or group of atoms is a system to a physicist. A system is, then, whatever we focus our attention upon for study and examination. 

The group identified a circular belt between 7 and 9 kpc1 from the centre of the galaxy. This zone (Fig. 11.6) consists of a population of stars which developed between 8 and 4 billion years ago it contains about 10% of all the stars in the galaxy, and around 57% of the stars in the habitable zone are older than our sun (Lineweaver et al., 2004). 

In which development phase of the universe could there have been the greatest chance of panspermia processes taking place A research group from the Potsdam Institute for Climate Research has tried to provide answers to this difficult question on the basis of research results from astronomy and astrophysics. Using mathematical models, they concluded that the maximum number of habitable planets in our galaxy must have been present at the time when our solar system and the young Earth were evolving (von Bloh et al., 2003). 

In the Galaxy, we know 93 (3 Cephei (Stankov Handler 2004) and about 100 SPB-type stars (De Cat et al. 2004). They fall within the instability strips predicted by the theory. The K-mechanism driving pulsations in (3 Cephei and SPB stars strongly depends on the abundance of the iron-group ions in the driving zone at temperatures around 2 x 105 K (Dziembowski Pamyatnykh 1993, Dziembowski et al. 1993). Theoretical models predict that pulsations of (3 Cephei and SPB-type vanish for Z = 0.01 and Z = 0.006, respectively (Pamyatnykh 1999). 

The Metallicity-Luminosity Relation for Local Group Dwarf Galaxies... 

The accretion history of a parent galaxy is constructed using a semi-analytical code. The full phase-space evolution during each accretion event is then followed separately with numerical simulations [2]. Star-formation and chemical evolution models are implemented within each satellite. The star formation prescription matches the number and luminosity of present-day galaxies in the Local Group, whereas the chemical evolution model takes into account the metal enrichment of successive stellar populations as well as feedback processes. Below we present results of a sample of four such simulated galaxy halos, denoted as Halos HI, H2, H3 and H4. 

NGC 6822 is an isolated irregular galaxy of the Local Group located at 495 kpc from our Galaxy and at 880 kpc from M31, there are no small galaxies in its neighbourhood, and it does not show tidal effects. 

Nowadays, the star formation history (SFH), initial mass function (IMF) and detailed chemical properties have been determined for many dwarfs, both in the Local Group and outside it (e.g. Grebel, Shetrone, Tolstoy, these proceedings). This in principle allows us to base theories of late-type galaxy formation and evolution on firmer grounds, by reducing the free parameter space. 

The Milky Way belongs to a cluster of galaxies called the Local Group that occupies about 3 Gly in space and contains over 30 galaxies, although the precise number is hard to determine. Placing the Milky Way at the centre (Figure 2.14),... 

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... 

Local group The collection of galaxies, including the Milky Way and Andromeda, that form part of the Virgo cluster as part of the local supercluster of galaxies. 

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