Main Research Interests

While my astrophysical research has covered a broad range of topics, my primary interest is in understanding the structure and evolution of disk galaxies that are similar to our own Milky Way Galaxy. I have concentrated on four closely-related aspects of this research topic: 1) examining various heating sources of the interstellar medium (ISM); 2) characterizing the global structure and physical state of the diffuse hot ISM; 3) investigating the interplay of the hot ISM with other galactic components; and 4) exploring the interaction of galaxies with their environment, particularly the hot intergalactic medium (IGM). In short, my research is focused on the high-energy ISM and IGM.

Current and planned Projects:

A new era for the study of the high-energy ISM and IGM has just started. Rapid progress is expected in this field over the next several years, as unprecedented observing capabilities are being offered by such major space-based observatories as Chandra, XMM-Newton, FUSE, and HST as well as an array of powerful ground-based telescopes. With members in  the UMass high-energy astrophysics group, we will capitalize on this excellent opportunity. The following is a brief description of our major ongoing/planned research activities in the field:

Hyper-energetic Interstellar Structures

An outstanding issue in the ISM study is the origin of various hyper-energetic shell-like gaseous structures, which have been detected in our Galaxy and many nearby galaxies. The mechanical energy contained in such a structure 10^52 - 10^54 ergs/s is substantially greater than that from an ordinary supernova explosion. The overall structure size ranges from a few tens to a few thousands of pc, and the dynamic age from about 10^4 to several 10^7 yrs. It has been demonstrated that many of these structures cannot be due to multiple supernova explosions and energetic stellar winds from massive stars or to the impact of high-velocity clouds.

I have proposed two alternative scenarios for the creation of such hyper-energetic structures: 1) hypernovae which might be related to the formation of black holes and 2) energetic outflows from luminous X-ray binaries. Recent optical imaging and spectroscopic observations have indeed shown that many of the so-called ultra-luminous X-ray sources are surrounded by very-energetic shell-like nebulae in nearby disk galaxies. Most interestingly, the ionization state of the nebulae provides tight constraints on the possible X-ray beaming effect. Some of these sources are apparently X-ray binaries containing intermediate-mass black holes. Our Chandra Galactic center survey has further revealed a diffuse thermal X-ray-emitting region around a Galactic micro-quasar. We are conducting a systematic multiwavelength observing campaign to determine the true nature of such associations. In addition to X-ray data we have been obtaining, we have access to optical data from both space- and ground-based optical observations and are planning for follow-up radio and far-UV observations. Theoretical studies are also being coordinated. This investigation will have strong implications for understanding the physics of the ultra-luminous X-ray sources as well as the hyper-energetic ISM structures.

The Global Interstellar Medium

The manifestation of high-energy activities in the ISM is the creation of a pervasive hot gas component. The presence of this rarefied interstellar component can profoundly affect the geometry and dynamics of the cooler phases of the ISM, the propagation of cosmic rays and UV/soft X-ray photons, the strength and topology of magnetic field, the galactic disk-halo interaction, the distribution of metal abundances, and so forth. However, there is little agreement yet on such basic issues as how much hot gas there is, which thermal state the gas is typically in, how the gas is distributed relative to cooler phases, what fraction of supernova mechanical energy is deposited into the hot gas, and where the energy is transferred or dissipated.

Aim to address the above issues, We have a multiwavelength program to characterize the global hot gas component. We will continue to use our Galactic center region as a unique laboratory to study detailed physical processes involved in heating, mass loading, and outflows of hot gas. We have obtained data or observing time from HST, FUSE, Chandra, and XMM-Newton to map out hot gas and to study its interplay with other galactic components in nearby galaxies. We will further examine the relationship of global hot gas properties with the star formation rate, morphological type, and clustering environment of galaxies.

Interplay with the Large-scale Structure Formation

High-energy activities in galaxies are intimately related to the structure formation on large scales. The morphology and color of galaxies are known to depend on their environment, although the physical mechanism responsible for this dependence remains greatly uncertain. For example, whereas galaxy-galaxy interaction can naturally trigger massive starburst and active galactic nuclear activities, the ram-pressure stripping of the ISM may have the opposite effect. Conversely, the thermal and chemical feedback in form of hot gas outflows from galaxies is fundamentally important to the evolution of the intergalactic medium, therefore to the structure formation in general.

We have ongoing programs to identify and study X-ray-emitting superstructures on scales greater than individual clusters of galaxies. The superstructure associated with Abell 2125 at z=0.25, for example, contains an unusually large population of active galaxies. Our recent deep Chandra observation of the superstructure shows that most of the 100+ X-ray sources detected in the field have optical and radio IDs. While the brightest X-ray galaxies appear to be active galactic nuclei, starburst galaxies with optical emission-lines appear to lie in or near local concentrations of the hot intergalactic medium. Complemented by the extensive radio, millimeter, and optical surveys of the field, the X-ray data will enable us to determine the dynamic state of the superstructure, the thermal and chemical properties of the IGM, the environmental reason for the large population of active galaxies, and the relationship of high-energy activities in individual galaxies with their intergalactic environments.

We have also obtained Chandra observations to further our extragalactic X-ray shadowing experiment. We will map out the X-ray absorption produced by galaxy disks to measure the intensity and spectral shape of the extragalactic background in the 0.1-1 keV range. The measurements will hopefully provide fundamental constraints on the thermal and chemical states as well as the overall content of the hot intergalactic medium.