Research

Quasar Clustering

SDSS quasars across cosmic time

Just as galaxies, quasars and AGNs as a cosmological luminous population can be used to trace the underlying large-scale structure (LSS) mapped by the invisible dark matter. An important statistical description of the LSS is the two-point correlation function (or clustering for short). I have been working on the measurement of quasar clustering using large, homogeneous quasar samples from the Sloan Digital Sky Survey (SDSS). The strength of quasar clustering tells us about what dark matter halos quasars live in, and quasar clustering has become an important ingredient in cosmological quasar models.

Phenomenology and Physical Properties of Quasars

Quasars are much more than just points in the sky. They have quite diverse properties inferred from their multi-wavelength spectral energy distribution (SED). These spectral properties contain rich information about the physics and astrophysics of the BH, the accretion disk, and other emission regions such as the broad line region. I am working on various aspects of quasar phenomenology, in order to understand quasars and their innermost structures.

Here is my BASI review on the mass of quasars.

Cosmological SMBH Models

With increasingly larger data sets, it has become feasible and important to fit the SMBH population in the cosmological framework, namely, how did the SMBH population evolve across cosmic time. My particular interest on this topic is to investigate how different observations of quasar statistics can be integrated to constrain certain evolutionary models, and how these models make predictions for future surveys.

Binary SMBHs

Binary SMBHs

Most (if not all) astronomers believe that binary SMBHs do exist. Indeed, there have been many reports of spatially-resolved binary SMBHs above pc scales. Under certain circumstances, a SMBH binary will merge to form a single BH. This is an exciting field, not only because numerical GR has been able to predict many interesting phenomena during/after BH coalescence, but also because the data flood in astronomical surveys has enabled systematic searches for these systems/events. If the BBH eventually coalesces, it will produce a gravitational siren that could be detected with ongoing or future low-frequency gravitational wave experiments. In addition, the dynamics of binary SMBHs has always been an interesting topic.

Quasar Absorption Lines

Quasar as a backlight

Many quasars show absorption features imprinted on their spectra. These include broad absorption lines (BALs), high-velocity narrow absorption lines (NALs), and associated absorption lines (AALs). Many of the NALs are not physically connected to the quasar, and are simply absorption from cosmologically intervening systems. Intervening quasar absorbers have been a useful tool to study the intergalactic medium (IGM) and high-redshift foreground galaxies, using background quasars. On the other hand, the intrinsic absorber population is a great tool to probe the physical conditions and dynamical processes within the quasar hosts, and carries a key piece of information about the formation and evolution of quasars and massive galaxies. They are as useful as emission lines (even better than emission lines under many circumstances) in the studies of quasar evolution and feedback.