Galaxy star formation history

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 Sagittarius dwarf Spheroidal galaxy (Sgr dSph) is currently disrupting under the strain of the Milky Way (MW) tidal field. The study of the Sgr chemical composition allows us to study at the same time the star formation history of a dwarf galaxy and the relevance of the hierarchical merging process for the formation of large galaxies such as the MW. 

We present chemical evolution models for NGC 6822 computed with five fixed parameters, all constrained by observations, and only a free parameter, related with galactic winds. The fixed parameters are i) the infall history that has produced NGC 6822 is derived from its rotation curve and a cosmological model ii) the star formation history of the whole galaxy based on star formation histories for 8 zones inferred from H-R diagrams iii) the IMF, the stellar yields, and the percentage of Type la SNe progenitors are the same than those that reproduce the chemical history of the Solar Vicinity and the Galactic disk. 

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. 

For metallicity distributions, one can examine lower spectral resolution diagnostics. The most useful of these has been the Ca II triplet indicator (Armandroff Da Costa 1991), which uses the combined equivalent width of the Ca II triplet near 8500 A, calibrated with metallicities of globular clusters, to infer the metallicity [Fe/H] (where the brackets denote the logarithmic abundance relative to that in the Sun). The main uncertainty of this method is that the Ca/Fe abundance ratio can vary depending on the star formation history, so the globular clusters may not provide the correct metallicity calibration for galaxies with a variety of star formation histories. Work needs to be done to calibrate the Ca II triplet with [Ca/H] rather than [Fe/H] to remove this ambiguity. 

These star formation histories are of vital interest to understanding the evolution of the dEs, and they have raised some puzzles. Among these are the question of how, on the one hand, the dEs lost their gas, and how, on the other hand, they retained gas to experience multiple episodes of star formation Combining the star formation history with the element enrichment history can, in principle, yield the information needed to understand the evolution of dwarf galaxies and their contribution to enrichment of the IGM. 

The abundance ratios of heavy elements are sensitive to the initial mass function (IMF), the star formation history, and variations in stellar nucleosynthesis with, e.g., metallicity. In particular, comparison of abundances of elements produced in stars with relatively long lifetimes (such as C, N, Fe, and the s-process elements) with those produced in short-lived stars (such as O) probe the star formation history. Below, I review the accumulated data on C, N, S, and Ar abundances (relative to O) in spiral and irregular galaxies, covering two orders of magnitude in metallicity (as measured by O/H). The data are taken from a variety of sources on abundances for H II regions in the literature. 

Figure 7. Predicted [a/Fe] vs. velocity dispersion for elliptical galaxies. Data are compared with predictions obtained by adopting the star formation history assumed in hierarchical clustering models for galaxy formation.The models and the figure are from Thomas et al. (2002).

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. 

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. 

In a closed box, the star formation rate can only decrease with time. Thus the timing of a collision is irrelevant for the metallicity history of such a galaxy the final abundance of metals will always increase relative to the moment of stripping. In contrast, in an open system, the global star formation rate can recover due to infall of externally or internally supplied diffuse gas and clouds. The metallicity history then becomes sensitive to the timing and frequency of such events. The final star formation rate may resemble an anemic systems. 

Present-day mass function (PDMF) of stars in the galaxy compared to the initial mass function (IMF). PDMF is the number of stars of a given mass in the galaxy today, whereas IMF is the number of stars of a given mass produced in a single episode of star formation. The difference in the two curves at high stellar mass reflects absence of the stars that have exhausted their nuclear fuel and died over galactic history from the PDMF. After Basu and Rana (1992). 

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