The super massive black holes at the center of galaxies gravitationally attract the surrounding gas and dust. They heat up and release energy in the form of electromagnetic radiation. Objects in this state are called “quasars”. Quasars are considered important targets for studying galaxy formation and evolution. Exposure to the strong UV radiation results in the ionization of neutral hydrogen gas around the quasars. If the quasar’s UV radiation is isotropic, the ionization level should be almost constant regardless of the direction seen from the quasars.
However, previous studies have shown that this is not actually the case. Hydrogen gas in the region transverse to quasars (i.e., sideways to the direction of observation by telescopes) are less ionized than those along the line of sight (i.e., the direction of observation from Earth). The reason for this remains to be elucidated, although previous studies have suggested that a doughnut-shaped structure of gas and dust called a “dust torus” may block UV radiation toward the tangential direction.

In previous studies targeting normal quasars, the inclination angle of dust torus seen from Earth cannot be determined. Professor Misawa and his team were able to narrow down the inclination angle of the dust torus by observing “BAL quasars” (*2) that are characterized by broad absorption lines in their spectra. They revealed that the existence of the dust torus is likely to cause the anisotropy of the ionization level around the quasars.
Their findings suggest that quasars are observed as BAL quasars when the dust torus is observed from the side. The past studies have already suggested that normal quasars (i.e., non-BAL quasars) would be observed when we observe the dust torus from the face-on direction; however, this was only speculation. Professor Misawa and his team observed the degree of ionization of hydrogen gas in the tangential direction of BAL quasars and revealed that the ionization level is higher than those in the line-of-sight direction, which is an opposite trend seen around normal quasars.
BAL quasars had been rarely observed for studying the ionization condition around quasars owing to the difficulty of analysis. However, this study succeeded in making a new discovery by daring to observe BAL quasars.

The star formation efficiency depends on the ionization level of hydrogen gas. Thus, the results of this study will provide important information regarding the star formation activity around quasars. In addition, the indirect confirmation of the existence of a dust torus marks an important step toward understanding the internal structure of quasars.
The Subaru Telescope (*3) has begun operation of a new instrument, the Prime Focus Spectrograph (PFS), in 2025. The PFS will enable the simultaneous observation of up to 2,400 targets. Professor Misawa aims to continue to further unravel the mysteries of quasars and galaxies using this spectrograph.