Research Interests of Qingjuan Yu
Galactic Dynamics and Planetary Dynamics
In astrophysics, there are rich dynamical structures associated with various systems on scales from the solar system, star clusters, galaxies, to clusters of galaxies and beyond. (See some example pictures of these systems: the Kuiper belt in our solar system, our solar system, solar nebula in which planets formed, "grand design" spiral galaxy M74, colliding galaxies antenna and cluster of galaxies Abell 2218.) With advancements in techniques, some tremendous observational progresses have been made in the past decade. (1) Discovery of many (binary) objects in the Kuiper belt in the outer edge of the solar system. These objects are believed to be the remnants of the initial planetesimal disk, in which the eight planets were formed, and record its information. (2) Discovery of more than 200 exoplanets, with diverse masses, semimajor axes and eccentricities, surrounding nearby stars. (3) Ubiquitous existence of supermassive black holes in galactic nuclei. These supermassive black holes may be dynamically important in forming galaxies through AGN feedback or shaping the structure of galaxies, e.g., through heating or slingshots of stars during the orbital evolution of supermassive binary black holes. (4) Observations of galaxies at high redshifts, which are analogous to the progenitors of nearby galaxies (including our own Milky Way). Interactions and mergers of these progenitors could produce the structures of galaxies seen in the nearby universe. With these breakthroughs in observations, addressing questions on how these dynamical systems work and evolve becomes vital and is essential, which would provide deep insights into their origin and further into the origin of ourselves and the universe.
Supermassive Black Holes and Quasars/Active Galactic Nuclei
Quasars/Active Galactic Nuclei are the brightest objects in the universe, which are believed to be powered by accretion of materials onto supermassive black holes. Observations have shown that supermassive black holes exist in most, if not all, nearby normal galactic centers, as the remnants of Quasars/Active Galactic Nuclei. Some supermassive black holes with masses larger than billions of solar masses were formed even at the time when the age of the universe is smaller than 10 percent of its present age. Furthermore, if each galaxy has a central supermassive black hole, mergers of galaxies may form binary black holes. Collisions of supermassive black holes, which emits enormous gravitational waves, are one of the main objective sources of Laser Interferometer Space Antenna (LISA). How did quasars and black holes form and evolve? What determine their spatial, mass and spin distributions in the universe? Did supermassive black holes obtain their mass through gas accretion or mergers of small black holes? What is the merger rate of binary black holes? One may be able to address those questions in the framework of co-evolution of galaxies and supermassive black holes in the &LambdaCDM cosmogony, together with investigations on detailed physics of disk accretion and dynamical evolution of binary black holes. New data from optical and X-ray surveys should give further censuses on quasars and supermassive black holes and help to address the above questions.
Galaxy Formation and Evolution, and the Reionization of the Universe
How do galaxies form and evolve? In the &LambdaCDM cosmogony, semi-analytical models (SAM; or SAM painted in N-body simulations) based on hierarchical mergers of dark matter halos, as a framework of galaxy formation and evolution, have been developed. In short, dark matter halos were formed from tiny quantum fluctuations on the wake of the big bang and the inflation. Cooling of baryonic gas led to the formation of first stars, galaxies and quasars in the centers of dark matter halos. The universe was reionized by UV photons emitted from the first emerged galaxies and quasars. Hierarchical mergers of small halos and small galaxies and further cooling of baryonic gas and star formation then led to the formation of present-day massive galaxies. (See a picture for this structure formation.) However, both the formation and evolution of galaxies and the reionization of the universe remain puzzles since observations on primeval galaxies are elusive and many factors contribute to the chemical, dynamical and morphological developments of a galaxy. With surveys at a large span in both space and redshifts, some significant progresses have been made in observations in the past decade. (1) More accurate statistics on the properties of galaxies are obtained from surveys such as SDSS and 2dF; (2) Meaningful statistics is accumulated from observations on early-type (or red) galaxies at redshifts z=1-3; (3) A large number of Lyman-break galaxies, which could be the progenitors of early-type galaxies, are detected at redshifts z=3-5; (4) A number of Lyman-alpha emitters are found at redshifts z>6; (5) Discovery of quasars at redshifts z>6 and detection of the Gunn-Peterson trough; (6) The CMB polarization data from the three-year WMAP observations suggests that the universe was reionized at redshift around 10. With the flooding of these data, formation and evolution of galaxies is still one of the most active areas in contemporary astrophysics. Strong constraints would be obtained by modeling various statistical properties of galaxies and the reionization process, which would help to reveal the general mechanisms underlying the formation of and evolution of galaxies and how the reionization process proceeded.

This file is maintained by Qingjuan Yu and last updated on Oct. 3, 2007.