Dr. Mo and his team will use the Sloan Digital Sky Survey (SDSS), a large survey of nearby galaxies, to reconstruct the distribution of dark matter in the local Universe at the present time. With the aid of large gravitational N-body simulations, they will deduce what that matter distribution must have been earlier in cosmic history, before the galaxies began to form their stars. At those early times, normal matter in the form of gas would have been evenly mixed with the dark matter. The team will then use smooth-particle hydrodynamic simulations to follow the combined development of the dark matter and the gas, under various assumptions about how the gas is turned into stars, and how those stars feed energy back into the intergalactic gas. Their aims include testing a currently-popular model, according to which gas is shocked and heated as it falls into massive systems, but flows quietly into smaller galaxies, remaining cold so that it can readily form stars. They will estimate how much gas is associated with various cosmic environments, such as filaments and sheets of galaxies, and examine how metals produced by the nuclear reactions inside stars are fed into the intergalactic gas.
A graduate student will be trained by participating in the research, and the team also expects to offer small-scale research projects for undergraduates. The density fields derived from the SDSS catalog, and various catalogs from the simulations, will be made available to the astronomical community. The team will produce three-dimensional maps of the reconstructed distribution of dark matter, luminous galaxies and intergalactic gas, for use in teaching and public outreach.
The observed total mass in baryonic matter (like protons and neutrons) in stars and cold gas today is only about 10 percent of the total baryonic matter in early times when the Universe was only one tenth of its present age. Where is the missing baryonic material done? Where is it hidden in the local Universe? A key step in answering these questions is to study the distribution, kinematics, state and chemical composition of the gas that has not been incorporated into galaxies, and how such gas is related to the galaxy population we can observe today. One promising way to study the bulk of the diffuse IGM is through its absorption of bright background sources, such as quasars and galaxies. However, absorption studies at low-redshift have suffered from a lack of suitable background sources and from the difficulty in obtaining high-quality UV spectroscopy needed to detect the atomic transitions of atomic gas. It is imperative to have as much theoretical and empirical input as possible both to design an optimal observational strategy and to help interpret the limited amount of observational data. One important advantage one has in the study of the IGM in the local universe is the availability of detailed information about the environment in which the galaxy population resides and from which gas absorption arises. The goal of the project was to conduct a systematic theoretical investigation of the structure of mass distribution in the local universe and its relation to galaxies. The results have provided new avenues for interpreting observations from major observational programs, such as SDSS, FUSS, and HST/COS. Our proposed projected also provide multidisciplinary training opportunities for graduate students and postdoctoral researchers. During the grant period, we have (i) constructed galaxy systems from large observational surveys; (ii) established the relationship between galaxy systems and dark matter halos (dark matter clumps within which galaxies reside); (iii) established the relationship between dark matter halos and the underlying density field; (iv) developed methods for reconstructing the initial conditions that are responsible to the formation and evolution of structures seen today; (v) tested the methods using large and high-resolution $N$-body simulations; (vi) applied to the methods to the Sloan Digital Sky Survey to investigate the structures observed in the local Universe; (vii) studied how dark matter and gas are associated with the galaxies we observe directly. The grant has generated a total of 18 referred journal articles. The results have been presented as invited talks at 7 international conferences and 6 colloquia. The grant has supported two graduate students for parts of their thesis works, one postdoctoral research associate, and two undergraduate students for their independent study. All the group/halo catalogs, and the current and initial density fields reconstructed from the SDSS sample are released on a website constructed for the project, so that the whole community can make use of them for their own research. We have also releaseed three-dimensional maps of the reconstructed dark-matter and gas distributions, along with the galaxy distribution, in the local universe, which will be useful for classroom teaching and for public education.
