.: About Hai
I am a Postdoctoral Research Associate at the Department of Physics and Astronomy, University of California, Irvine. Before September 2011, I was a postdoc at Caltech Astronomy, a graduate student at the Institute for Astronomy, Hawaii, and an undergraduate at Nanjing University.
For my research, I am mostly intrigued by collisions of objects in the Universe, whether they are galaxies, black holes, or asteroids.
When I am far from a computer, I enjoy all kinds of outdoor activities, expecially, hiking, mountaineering, rock climbing, and bicycling.
I can be reached via email.
A central question in extragalactic research is the mass assembly history of galaxies and their black holes. Understanding this process requires measurements of the rates of star formation and black hole accretion throughout the cosmic ages. A major obstacle in this field is to separate star formation and SMBH accretion.
With NASA's Spitzer space telescope, we obtained mid-IR spectra of luminous galaxies at redshift 0.7, when the Universe was only half of its current age. The spectra allow us to separate the emission from star formation and black hole accretion and, subsequently, to study the co-growth of galaxies and their black holes.
We found that intense black hole accretion accompanies about a quarter of the galaxies when they are rapidly forming stars, and the growth in black hole mass is ~0.1% of the mass of newly formed stars. These observations show that the correlation between black hole mass and galaxy mass is maintained in major episodes of star formation.
During the collision of two galaxies, gas from the galaxy outskirts are driven to the centers of both galaxies because of strong gravitational torques. The gas flow can feed the central black holes in the merging galaxies. Like their host galaxies, the black holes will eventually merge and form a larger black hole. By observing this kind of objects we can learn more about the complicated physical processed in galaxy mergers and how the merger frequency evolves over cosmic time.
To catch such black hole binaries before they merge, I use the Keck 10-meter telescope to take sharp images of accreting black holes and look for companions in their close vicinity. The figure on the left illustrates an example.
Virtually every galaxy contains a massive black hole at its heart. Although neglibible in mass and size when compared to the host galaxies, these black holes appear to be able to control the growth of the galaxies, because the mass of the galaxies is tightly related to that of the black hole in nearby galaxies including our own Milky Way. To explain this observation, theorists have hypothesized a feedback mechanism from quasars. The feedback can expel most of the gaseous material to large distances impulsively, thus regulating both black hole growth and star formation in the galaxy. But did this actually happen?
To catch black hole feedback in action, we used the Gemini 8-meter telescope and the Hubble Space Telescope to study the fascinating filamentary nebulae around quasars. Our observations favor a scenario where the driving force producing the nebulae is a galactic-scale quasar superwind, in the form of a roughly spherical blast wave. Since such a mechanism is capable of ejecting a mass comparable to that of the total gaseous medium in our Milky Way, quasar extended nebulae provide local examples of the purported black hole feedback that may have regulated star formation and black hole growth in the early universe.
I am fortunate to have part of this research covered in SPACE.com.
Asteroids occassionally run into one another and create families of smaller asteroids with similar orbits. Although many asteroid families are known in the main belt, no such families have been identified among the hazardous near-Earth asteroids. These families would create spikes in the collision probability with our Earth (imagine being hit by a few asteroids at the same time!).
With numerical simulations I found that these hazardous families are only identifiable within ~300,000 years of formation, as they would quickly become well-mixed with background objects because of the disturbance from the planets. Random alignments of objects in the orbital space occur much more frequently than true families. So I developed a novel technique to distinguish between generic asteroid families and random orbital alignments. This technique can potentially discover the very first near-Earth asteroid families, as more and more near-Earth asteroids are identified by surveys like Pan-STARRS.
"Life is like riding a bicycle. To keep your balance you must keep moving." - Albert Einstein