Matter and Galaxy Formation

Astrophysics, Denys Wilkinson Building, Keble Road, Oxford 0X1 3RH, UK silk astro.ox.ac.uk 

Abstract I describe recent challenges in dark matter. I review the budgets for baryonic and nonbaryonic dark matter. Problems with cold dark matter in the context of galaxy formation are summarized, and possible solutions are presented. I conclude with a description of the prospects for observing cold dark matter. 

We are confronted by a paradox. Baryonic dark matter (BDM) exists and contributes to fib- There are examples of baryonic dark matter, but we cannot reliably calculate the BDM mass fraction. On the other hand, cold dark matter (CDM) is motivated by theory and explains much of the large-scale structure of the universe. CDM dominates Qm. We can calculate the relic CDM mass fraction, but no CDM candidate particles are known to exist. 

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The distribution of elements in the cosmos is the result of many processes, and it provides a powerful tool to study the Big Bang, the density of baryonic matter, nucleosynthesis and the formation and evolution of stars and galaxies. This textbook, by a pioneer of the field, provides a lucid and wide-ranging introduction to the interdisciplinary subject of galactic chemical evolution for advanced undergraduates and graduate students. It is also an authoritative overview for researchers and professional scientists. 

In summary, the extinct radioactivities which have a limited time of existence in the solar system, constrain the time interval between the late stages of stellar nucleosynthesis and the formation of the solar system. Some production may also occur within the solar system during active periods of the young Sun. There have been numerous studies about how this matter was added into the solar system as a late spike of about 10 solar masses of freshly stellar processed material or from constant production in the galaxy (Wasserburg et al. 1996 Goswami and Vanhala 2000 Russell et al. 2001). These models are refined constantly with the input of new data and will probably continue to evolve in the future. 

As we see, the emphasis of the conference was on the discussion of the physical nature of dark matter and its role in the formation of galaxies. These preliminary studies demonstrated that both suggested models for coronas had difficulties. It is very difficult to explain the physical properties of the stellar corona, also no fast-moving stars as possible candidates for stellar coronas were found. 

Our analysis gave strong support to the Zeldovich pancake scenario. This model was based essentially on the neutrino dominated dark matter model. However, some important differences between the model and observations were detected. First of all, there exists a rarefied population of test particles in voids absent in real data. This was the first indication for the presence of biasing in galaxy formation - there is primordial gas and dark matter in voids,... 

With the initial conditions specified, it became possible to simulate galaxy formation. Three distinct approaches have emerged numerical, semi-analytical and hybrid. The fully numerical approach canot yet cope with the complexities of star formation, but has been instrumental in guiding us towards an understanding of the dark matter distribution. The semi-analytical approach has had most success, because it can cope with a wide dynamic range via the extended Press-Schechter formalism, to which is added a prescription for star formation... 

The upper limit on Qvh2 forbids currently known neutrinos from being the major constituents of dark matter. Moreover, since they are light and relativistic at the time of galaxy formation, the three neutrinos known to exist are hot, not cold, dark matter. 

Dark matter neutrinos with a mass around 1 eV would be relativistic at the time of galaxy formation ( keV), and would thus be part of hot dark matter. From the bounds on hot dark matter in the preceding Section, however, they cannot be a major component of the dark matter in the Universe. 

The visible matter of galaxies is concentrated in mainly three components stars, interstellar matter, and stellar remnants. Since the early days of galaxy formation there is a vivid exchange of matter between the stellar component and the interstellar matter. Stars are formed in local concentrations of the ISM, the molecular clouds they live for a certain period of time while burning their nuclear fuels and they die... 

After about 3 10 y the temperature was about 3 10 K and by combination of nuclei and electrons the first atoms of hydrogen and helium were formed. At further stages of expansion and cooling the first H2 molecules became stable. Due to gravitation, the matter began to cluster and the formation of galaxies and stars began after about 10 y. The further individual development of the stars depended and still depends mainly on their mass. 

Within the year, all the best people (48, 49) were including cold dark matter (CDM) in their models for galaxy formation. From then to now, speakers and writers of the CDM words generally mean WIMPs and their ilk, though axions and a few other candidates behave in much the same way during structure formation. "Bias," the idea that luminous baryons will be more tightly clustered than the CDM (presumably because they are dissipative) dates from the same period. 

The intensity of this ripple effect is very small indeed, amounting to only a few parts per 100,000. While this anisotropy (differences in quantity depending on direction) is exceedingly small, it is not at all insignificant. Indeed, astronomers believe that these tiny variations in CMB may be related to differences of distribution of matter in the early universe and are, therefore, a possible key to the later formation of stars and galaxies. 

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