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Research Interests

Supermassive black holes (SMBH) existing at the centers of massive galaxies can outshine their host galaxies when they accrete a large amount of matters to feed themselves. Such phenomena, known as quasi-stellar objects, or quasars, are used as key probes to investigate the SMBH activity over the cosmic time. Since my PhD years at NAOJ, I have worked on the observational frontiers of high-redshift quasar studies, i.e., active SMBHs within the first billion years of the universe (z=6-7.5), capitalizing on rich datasets from wide-field surveys.

My primary research interests are 

  • the SMBH activity and chemical enrichment of the highest-redshift quasars

  • the full census of SMBHs and their host galaxies at redshift over 6

  • large-scale environments of the high-redshift quasars

More detailed description of my research works can be found in the following:

Hunting Quasars in the Distant Universe

People have searched for quasars within the first billion years of the universe (z>6) since the first discovery by the SDSS (Fan et al. 2001). Thanks to the advents of wide-field surveys, there are now more than 200 in this redshift range with the current record holder at z=7.54 (ULAS J1342+0928; Bañados et al. 2018).
I have been deeply involved in this exciting and competi-tive business since my PhD years at NAOJ using the 8.2m Subaru telescope.
The figure on the right shows that the 1000 deg2-class wide-and-deep survey of Hyper Supreme Cam Subaru Strategic Program (HSC-SSP) has succeeded in discovering ~100 quasars that are mostly a dex fainter than the SDSS-class quasars (e.g., Matsuoka, MO et al. 2016).

スクリーンショット 2020-12-05 6.53.05.png

Deep NIR Spectroscopy of the Most Distant Quasar at z=7.54 (MO+20, Simcoe, MO+20)

One of the beauties of quasars is that they are so bright that we can observe many metal emission lines and underlying continua even from the high-redshift universe; thus, quasars can be used to study the chemical enrichment history of the host galaxies. With ultradeep NIR follow-up spectroscopy of ULAS J1342+0928, the most distant quasar at that time (now second), with VLT/XShooter (23hr) and Gemini/GNIRS (9hr), I studied the properties of the ionized gas clouds orbiting around the central SMBH (i.e., broad-line-region). Thanks to the high quality of the spectrum, I found that the broad-line-region is chemically matured despite its young age (0.7Gyr from the Big Bang). This result is especially surprising for iron, because supernovae nucleosynthesis models predict that iron enrichment should delay from alpha elements (e.g., magnesium) by ~1Gyr.

Moreover, the high-resolution XShooter spectrum enabled us to search for unseen galaxies that appear in the NIR spectrum as absorption lines. Combining with another deep spectrum taken by Magellan/FIRE, we investigated the chemical abundance and ionizing condition of a system at z=6.84, which is at the highest-redshift as a metal absorber. Based on various metal absorption lines identified in the deep spectrum, this system is reminiscent of metal-poor Damped Lyman Alpha system known in the lower-redshift universe. Such metal-poor galaxy would be common in the reionization epoch. 

スクリーンショット 2020-12-17 12.59.49.png

Characterization of SMBH activity at z>6


Large-scale Environments of Quasars


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