Galaxies stellar population

How will we identify the extra astrophysics required to reconcile the properties of CDM dark haloes with those of luminous galaxies We can start by developing knowledge of the evolutionary history of at least one place in at least one galaxy. We would be unlucky if that place were far from the norm alternatively, any theory that predicts such a history to be very unusual might be suspect -the galaxian Copernican principle. Kinematics and current spatial location are of course critical parameters, so that traditional stellar populations analyses are... 

That is, the straightforward interpretation of abundance data for Galactic field stars in terms of stellar populations is feasible only because the Galaxy apparently acquired its gas early, or at a rate which was well-matched to the star formation rate across the whole volume now sampled by local halo stars, and kept this gas well-mixed and because the stellar IMF is (close to) invariant over time and metallicity. Neither deduction was obvious, nor is the underlying physics understood. However, these two deductions apply so well they have become assumed authors use any violation to rule out some possible Galaxy merger histories, as in the Venn et al. analysis from which Figure 1 is taken. 

Dwarf Spheroidal galaxies are the smallest and faintest galaxies known. They are typically dominated by old stellar populations (e.g. Sculptor and Sextans), but some of them (e.g. Fornax) exhibit more recent star formation episodes (2-8 Gyr ago). Analysis of the horizontal branch morphology shows that Red HB stars are more centrally concentrated than Blue HB stars which could be interpreted either as an age or a metallicity gradient or both ([1]). Only spectroscopic observations can unambiguously separate metallicity gradients and make a link with the kinematics. 

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. 

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.

Figure 3.38 shows a schematic cross-sectional view of the Galaxy, indicating the main stellar population groups (disk, bulge, halo and solar cylinder) that will figure in subsequent discussions and Table 7.9 gives some relevant statistics. 

The next problem was to find internally constitent values of physical parameters of stellar populations of different age and composition. For this purpose I developed a model of physical evolution of stellar populations (Einasto 1971). When I started the modelling of physical evolution of galaxies I was not aware of similar work by Beatrice Tinsley (1968). When my work was almost finished I had the opportunity to read the PhD thesis by Beatrice. Both studies were rather similar, in some aspects my model was a bit more accurate (evolution was calculated as a continuous function of time whereas Beatrice found it for steps of 1 Gyr, also some initial parameters were different). Both models used the evolutionary tracks of stars of various composition (metallicity) and age, and the star formation rate by Salpeter (1955). I accepted a low-mass limit of star formation, Mo 0.03 Msun, whereas Beatrice used a much lower mass limit to get higher mass-to-luminosity ratio for elliptical galaxies. My model... 

The second problem encountered in the modelling of M31 was the rotation and density distribution on the periphery. If the rotation data were taken at face value, then it was impossible to represent the rotational velocity with the sum of gravitational attractions by known stellar populations. The local value of M/L increases toward the periphery of M31 very rapidly if the mass distribution is calculated directly from the rotation velocity. All known old metal-poor halo-type stellar populations have a low M/L tv 1 in contrast, on the basis of rotation data we got M/L > 1000 on the periphery of the galaxy, near the last point with a measured rotational velocity. 

The extension of these models to two dimensions, a prerequisite for realistic models of spiral galaxies, can be accomplished by using stochastic methods for justification. A diffusion equation for the stellar population including birth and death terms was first asserted by Shore ° and also recently employed by Nozakura and Ikeuchi. It is possible, however, to derive this equation from first principles provided the spatial distribution for the stellar velocities has a random component as well as that due to the differential rotation of the galaxy. 

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