Milky Way

The regions of space where molecules have been detected are the nebulae which are found not only in our own galaxy but also in other galaxies. In our galaxy the nebulae are found in the Milky Way, which appears as a hazy band of light as a result of its containing millions of stars. Associated with the luminous clouds composing the nebulae are dark clouds of... 

Figure 3. Sky coverage of an adaptive optics at an 8m telescope. Pupil sampling by the actuators of the deformable mirror 0,5m, 0 = 0,17m, From top to bottom in a direction of galactic latitude 6 = Of (the Milky Way), b = 20f and b = 90f (the galactic pole). From left to right K, 1 and V bands.

Atoms are extremely small. Measurements show that the diameter of a single carbon atom is approximately 0.000 000 0003 meters (about 0.000 000 001 feet). To give you some idea of just how small that is, a sample of carbon the size of the period at the end of this sentence contains more atoms than the number of stars in the Milky Way. Any sample of matter large enough for us to see or feel contains an unfathomable number of atoms. 

According to present-day concepts, our solar system was formed from a huge gas-dust cloud several light years across in a side arm of the Milky Way. The particle density of this interstellar material was very low, perhaps 108-1010 particles or molecules per cubic metre, i.e., it formed a vacuum so extreme that it can still not be achieved in the laboratory. The material consisted mainly of hydrogen and helium with traces of other elements. The temperature of the system has been estimated as 15 K. 

Fig. 3.10 Distribution of matter in the Milky Way. From Feitzinger, J. V., Die MilchstraBe - Innenansichten unserer Galaxie, Spektrum Akademischer Verlag, Heidelberg Berlin 2002...

The mean particle density of ISM is 106 particles per cubic meter there are, however, great variations from this mean value. Between the spiral arms of the Milky Way, there are between 104 and 105 hydrogen atoms per cubic metre in the dark clouds and the HII regions, there are 10s—1010. Up to 1012—1014 hydrogen atoms per cubic metre are present in regions with OH sources and in certain infrared objects. 

It is assumed that the greatest part of our solar system, and indeed of the Milky Way, is hostile to life. The term habitable zone (Franck et al., 2002) takes into account... 

Can we assume that there is a habitable zone somewhere in our galaxy This seems feasible astronomers divide the Milky Way into four regions, which cannot always be exactly separated ... 

Australian astronomers have attempted to characterise the habitable zone of the Milky Way more exactly. The models which they used were based on the following assumptions for complex life in our galaxy ... 

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...

Open clusters (OCs) are important tools both for stellar and for galactic astrophysics, as tests of stellar evolution theory for low and intermediate mass stars and as tracers of the Galactic disk properties. Since old OCs allow us to probe the lifetime of the Milky Way disk, up to about 10 Gyr ago, they can be used to study the disk evolution with time, and in particular its chemical history. 

Abstract. The Milky Way harbours two disks that appear distinct concerning scale-heights, kinematics, and elemental abundance patterns. Recent years have seen a surge of studies of the elemental abundance trends in the disks using high resolution spectroscopy. Here I will review and discuss the currently available data. Special focus will also be put on how we define stars to be members of either disk, and how current models of galaxy formation favour that thick disks are formed from several accreted bodies. The ability for the stellar abundance trends to test such predictions are discussed. 

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. 

I ll review here three different scenarios that still appear viable (based on the currently available data for the Milky Way see summary in Fig. 3). For each example I have chosen one or two references that have done detailed models as illustrations to compare the observational data to. This is not an exhaustive account for all the possibilities within each scenario but it gives a flavour of the types of comparisons we ought to make and, hopefully it also illustrates the shortcomings both of the observed as well as simulated data. 

Fig. 3. Summary of current observational knowledge about the thin and the thick disks in the Milky Way. The two trends for [O/Fe] vs [Fe/H] are depicted in blue, dashed line (thin disk) and red (thick disk). The most debated issues are marked in blue (i.e. thick disk all the way to [Fe/H]=0, hiatus in star formation, SF, and AMR in thick disk) and topics of some debate in purple ( knee and <5age between various sub-populations, such as Sage between the youngest thin disk and the oldest thick disk). The one issue all agree upon, the a-enhancement, is indicated in green (light grey)...

Thick disks are common in other galaxies, especially in merger environments, hence perhaps we should prefer the merger scenario for the Milky Way ... 

Fig. 1. The Galactic fluorine abundances obtained to date. Three samples are represented the disk of the Milky Way (crosses), including the two young Orion pre-main-sequence stars (open circles), and u> Centauri giants (filled circles).

Conclusion. Thanks to our large sample, the statistic is improved and the separation between the two disks is quantified. It is now clear that the thin and the thick disks are chemically well separated. We found a transition in the age distribution of the thin disk and the thick disk stars at 10 Gyr but no clear vertical gradient in the thick disk. These results constrain the formation scenarii of the Milky Way s disks. 

Cayrel R., Spite M., et al. 2005, ESO Astrophysics Symposia Chemical Abundances and Mixing in Stars in the Milky Way and its Satellites , S. Randich and L. Pasquini eds. (this book). 

Scl is a close companion of the Milky Way, at a distance of 72 5 kpc [7], with a low total (dynamical) mass, (1.4 0.6) x 107Mq [8], and modest luminosity, My = —10.7 0.5, and central surface brightness, Soy = 23.5 0.5 mag/arcsec2 [9] with no HI gas [10]. CMD analysis, including the oldest Main Sequence turnoffs, has determined that this galaxy is predominantly old and that the entire star formation history can have lasted only a few Gyr [11]. 

The most metal-rich stars in dwarf spheroidals (dSph) have been shown to have significantly lower even-Z abundance ratios than stars of similar metallicity in the Milky Way (MW). In addition, the most metal-rich dSph stars are dominated by an s-process abundance pattern in comparison to stars of similar metallicity in the MW. This has been interpreted as excessive contamination by Type la super-novae (SN) and asymptotic giant branch (AGB) stars ( Bonifacio et al. 2000, Shetrone et al. 2001, Smecker-Hane McWilliam 2002). By comparing these results to MW chemical evolution, Lanfranchi Matteucci (2003) conclude that the dSph galaxies have had a slower star formation rate than the MW (Lanfranchi Matteucci 2003). This slow star formation, when combined with an efficient galactic wind, allows the contribution of Type la SN and AGB stars to be incorporated into the ISM before the Type II SN can bring the metallicity up to MW thick disk metallicities. 

The halo globular cluster system also provides valuable information, since accurate distances, and hence reliable ages, can be derived. Mackey Gilmore (2004) recently acquired and compiled a new, nearly complete, internally consistent set of photometric studies of the globular cluster population in both the Milky Way and its satellite galaxies they deduce, from analysis of HB morphol-ogy, age, abundance and structural information, somewhat more relaxed limits... 

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