Galaxy thick disk

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 in Other Galaxies and How To Form Them... 

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. 

It is also interesting to note that solar metallicities are reported for z=2, e.g. [22], and that disks of old stars have been found at redshifts as high as z=2.5, see e.g. [23]. These types of findings indicate that indeed the formation of the thick disk in our galaxy might have happened well in the past. 

Based on currently available elemental abundance data and age determinations, the thick disk could have formed either through a violent, heating merger or through accretion of (substantial) satellites in a hierarchical galaxy formation scenario. The fast monolithic-like collapse is getting more and more problematic as data are gathered. It would be especially crucial to establish if there is an age-metallicity relation in the thick disk or not as in that case the thick disk could not have formed in that way (since the models indicate that the formation time-scale for the stars in the thick disk would be very short, see [7]). 

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

Abundances and Ages of the Deconvolved Thin/Thick Disks of the Galaxy... 

Abstract. We have investigated the abundance of several chemical elements in two large stellar samples kinematically representative of the thin and the thick disks of the Galaxy. Chemical, kinematical and age data have been collected from high quality sources in the literature. Velocities (U,V,W) have been computed and used to select stars with the highest probability to belong to the thin disk and the thick disk respectively. Our results show that the two disks are chemically well separated. Both exhibit a decline of [a/Fe] with increasing [Fe/H]. A transition between the thin/thick disks stars is observed at 10 Gyr... 

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. 

Fig. 5. Quantitative tests of numerical galaxy models are now becoming feasible. It is interesting to note how little apparent substructure is evident in the best conserved quantity, angular momentum, and how complex is the evolutionary history of a stellar population , such as the thick disk, in this model. This figure is from Abadi et al. 2003.

Fig. 8.17. Age-metallicity relation for disk stars using data from Edvardsson et al. (1993). Open circles, filled circles and crosses represent respectively stars with mean Galactocentric distances 7 to 9 kpc (like the Sun), stars from the inner Galaxy (under 7 kpc) and from the outer Galaxy (over 9 kpc). Model curves assume linear star-formation laws with

What happened to the gas expelled from the halo A traditional answer based on monolithic models of a Galaxy collapsing through successive stages of halo, thick disk and thin disk would be that the expelled gas formed the raw material of the disk. This hypothesis faces some severe difficulties. For one thing, the remaining mass in the halo should then be about 10 per cent of the mass of the disk, whereas it is probably a factor of 5 or so less than this (Carney, Latham Laird 1990). Another difficulty is the specific angular momentum, as is apparent qualitatively from Fig. 8.15 and is illustrated quantitatively in Fig. 8.21. 

Accumulation either rapid or slow of metal enriched gas from the halo or thick disk in the Galaxy center. 

When the metallicity distribution of damped Lya systems is compared with those of different stellar populations of the Milky Way, we find that is broader and peaks at lower metallicities than those of either thin or thick disk stars (Figure 8). At the time when our Galaxy s metal enrichment was at levels typical of DLAs, its kinematics were closer to those of the halo and bulge than a rotationally supported disk. This finding is at odds with the proposal that most DLAs are large disks with rotation velocities in excess of 200 km s 1, put forward by Prochaska Wolfe (1998). 

In this picture, 1 Gyr is therefore the time over which the halo of our Galaxy became enriched to a metallicity [Fe/H] = — 1, ultimately reflecting the rate at which star formation proceeded in this stellar component of the Milky Way. Clearly, the situation could be different in other environments (Gilmore Wyse 1991 Matteucci Recchi 2001). The thick disk, for example, evidently reached solar abundances of the a-elements in less than 1 Gyr, since the a. overabundance—or more correctly the Fe deficiency—seems to persist to this high level of metallicity (Fuhrmann 1998). 

Interstellar strontium and barium, both being heavier than iron, are synthesized in supernovae via the rapid capture by iron nuclei of a succession of neutrons—the r-process. The barium and strontium in old stars in the outer portions of galaxies (sometimes called thick-disk stars) probably synthesized via the r-process, as the proportion of heavy elements in that more primitive interstellar medium owed its existence almost solely to Type II supernovae. 

Abstract. In an effort to determine accurate stellar parameters and abundances for a large sample of nearby stars, we have performed the detailed analysis of 350 high-resolution spectra of FGK dwarfs and giants. This sample will be used to investigate behavior of chemical elements and kinematics in the thick and thin disks, in order to better constrain models of chemical and dynamical evolution of the Galaxy. 

Most of the gas in the Galaxy is contained within the disk and in particular in the spiral arms, hence in a layer only a few light-years thick. Although we cannot claim that the space between the stars is empty since the interstellar medium is actually observable, it is not far from being so. It contains on average about 1 atom cm, far less than the best laboratory vacuum. 

The thickness of the disk of the galaxy is of order 300 pc = 1000 light years, which is much shorter than the characteristic propagation time of 10 million years. The explanation is that the charged particles are trapped in the turbulent magnetized plasma of the interstellar medium and only diffuse slowly away from the disk, which is assumed to be where the sources are located. Measurements of the ratio of unstable to stable secondary nuclei (especially 10Be/9 lie) are used to determine resc independently of the product np resc and hence to constrain further the models of cosmic-ray propagation. 

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