One of the main goals of X-ray survey astronomy is to construct the luminosity function of active galactic nuclei (AGNs), i.e., the number of AGNs per unit volume of space as a function of luminosity. The luminosity function provides the most fundamental interface between observations and theories.

“...we have developed an extensive analysis procedure in calculating the absorption-corrected X-ray luminosity function by utilizing maximum likelihood method involving luminosities as well as the amount of absorption in each object.”

The paper in question made an extensive analysis of available hard X-ray survey data to construct the luminosity function of AGNs, including both unobscured (type 1) and obscured (type 2) AGNs for the first time. Many researchers used our luminosity function to compare with their theories on the evolution of AGNs and growth of supermassive black holes at galaxy centers.

The paper went as far as constructing an AGN population synthesis model, describing the global spectrum of the X-ray background. This gave a solution to the 40-year-long mystery in the history of X-ray astronomy, namely the origin of the X-ray background.

We discovered that the number density of more powerful AGNs (i.e., quasars) peaked earlier in the history of the universe than lower luminosity ones. This behavior, called anti-hierarchical evolution or down-sizing of AGN activities, apparently contradicts an expectation from the standard theory of structure formation.

On the methodology side, we have developed an extensive analysis procedure in calculating the absorption-corrected X-ray luminosity function by utilizing maximum likelihood method involving luminosities as well as the amount of absorption in each object.

We know that there is a supermassive black hole of four million solar masses at the center of our Galaxy (Milky Way). And almost all galaxies in the present universe also have a supermassive black hole. How they formed and how they gained their masses are among the most important questions in modern astronomy.

When material falls into a black hole (accretion), the black hole gains mass and the infalling material emits strong radiation in all wavelengths. This phenomenon is observed as an AGN. Hard X-ray surveys are the most complete and efficient way of finding AGNs, including those hidden by surrounding dust and gas, because of the strong penetrating power of hard X-rays and negligible contamination from stars in the host galaxy.

Utilizing a large set of X-ray survey results over the wide redshift range, we revealed the cosmological evolution of these AGNs, including hidden populations, for the first time. These results give a basis for observational constraints on the accretion history of the universe.

We have been involved in the X-ray survey projects using the fourth Japanese X-ray satellite, Advanced Satellite for Cosmology and Astrophysics (ASCA), which was launched in February, 1993. ASCA is the first imaging X-ray satellite covering the energy band above 2 keV, and achieved three orders of magnitude better sensitivity than previous observatories.

These projects provided us with a large sample of hard X-ray selected AGNs. We spent eight years to produce the X-ray catalogs and five years to complete their optical identifications utilizing various ground-based telescopes.

As stated above, constructing a luminosity function is one of the main goals of such X-ray survey efforts, which have been made possible through a tight collaboration between X-ray and optical astronomers.