Galaxy star formation rate

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. 

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. 

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. 

Assuming the star formation rate for the Galaxy given in Table 7.9 and that all stars between 10 and 100 M explode as Type II supemovae, estimate the corresponding supernova rates for the IMFs in Table 7.8. How much difference does it make if the upper mass limit for SN is 50 M (The observed rate for SN II in galaxies like our own is of the order of 2 to 3 per century.)... 

Thus the mass of stars and that of the whole system steadily increase while z soon approaches 1 and the stellar metallicity distribution is very narrow (see Fig. 8.24). The accretion rate is constant in time if the star formation rate is any fixed function of the mass of gas. Other models in which the accretion rate is constant, but less than in the extreme model, have been quite often considered in the older literature (e.g. Twarog 1980), but are less popular now because they are not well motivated from a dynamical point of view, there is an upper limit to the present inflow rate into the whole Galaxy of about 1 M0yr 1 from X-ray data (Cox Smith 1976) and they do not provide a very good fit to the observed metallicity distribution function. 

Universe model specifically 2m = 0.37, Qt> = 0.037, 2a = 0.63, h = 0.7. The key lies in the dependence of star formation rate on ambient density and temperature, roughly parameterized by the relative overdensity 8 = p/(p) — 1, the change in physical density from expansion being partly compensated by the drop in ambient temperature. Galaxies and clusters of stars are deemed to be formed in a cell in the computation when three criteria are satisfied (Cen Ostriker 2000) ... 

The third parameter in chemical evolution models of the galaxy, the star formation rate, serves to define the rate at which evolution proceeds. Unfortunately, there is a great deal of uncertainty in its estimation. There is no theory of star formation worthy of the name. In decline as time goes by, the star formation rate is often assumed to be proportional to the gaseous fraction of the Galaxy, which is itself in permanent decline, or to some power of it, but less than 2. 

This parameter is nevertheless constrained by the relationship between the age and iron content of stars, the observed rate of supernova events (of the order of three per century), which is related to the current star formation rate, and the present gaseous fraction. After 10 billion years of evolution, the region of the Galaxy accessible to us, that is, the solar neighbourhood, still retains about 10% gas. 

Recent modeling based on the lifetimes of stars, their IMF, the star formation rate as a function of time, and nucleosynthesis processes have succeeded in matching reasonably well the abundances of the elements in the solar system and in the galaxy as a whole (e.g. Timmes et al., 1995). These models are still very primitive and do not include nucleosynthesis in low and intermediate-mass stars. But the general agreement between model predictions and observations indicates that we understand the basic principles of galactic chemical evolution. 

When were the atoms that became our solar system synthesized The star formation rate in the galaxy was highest early in galactic history and a lot of heavy elements were produced. However, most of this material was subsequently incorporated into stars, and much of that has been permanently sequestered. On the other hand, much of the recently synthesized material has not yet been incorporated into a new generation of stars. This balance between synthesis and sequestration means that the birth dates of the elements that became the solar system are roughly evenly distributed over the 7.5 Gyr of galactic history prior to the solar system s birth (Clayton, 1988). 

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... 

Figure 29. Histograms of assembled stellar mass and star formation rates from Shapley et al. (2001). By redshift 3 a significant fraction of LBGs seem to be approaching the stellar mass of today s L galaxies, Wstar — 4 x 1010M .

Virtually all models for the ultimate energy source of GRBs involve an endpoint of stellar evolution, particularly of the most massive stars. Thus it has been proposed that the burst rate must be proportional to the overall cosmic star formation rate. This view is supported by the fact that the typical redshifts (z 1) associated with GRB host galaxies correspond to an epoch of early active star formation in the Universe. Burst counterparts also tend to be... 

Studies of galactic evolution have focused on the comparison between the atomic (HI) and molecular (H2) gas properties and star formation rates as a function of environment, luminosity, and galaxy type. The general conclusions from these studies are as follows ... 

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