My research is motivated by one big question: “How do supermassive black holes (SMBHs) grow and finish their lives?”. To tackle this problem, my research in observational/archive data astronomy falls into a few different approaches, which are listed below. Plewase click the links when you want to know more details.

1. Can we see AGN dying (=final phase of AGN activity)?

スクリーンショット 2020-04-22 14.46.37AGN lifetime is observationally known to be very long with >10^5 yrs. Thus we know very little  on the ignition/shut-down mechanism of AGN: how silent SMBH becomes AGN and how AGN again returns to a dormant SMBH again. Considering that there is maximum mass limit for SMBH (~100 billion times the mass of the Sun), AGN must shut-down their activity at some point, but finding such shut-down AGN has been very difficult because of the SMBH becomes dormant very rapidly after the quenching of AGN.
We have serendipitously found a dying AGN in a galaxy called Arp 187 by carefully checking the surrounding environment of SMBH with different physical scale from less than 10 light year to 1 million light year.

References: Ichikawa et al. (2016), PASJ, 68, 9; Ichikawa & Tazaki (2017), ApJ, 844, 21; Ichikawa et al. (2019a), ApJ, 870, 65; Ichikawa et al. (2019b), ApJL, 883, 13
Talk slides: IPMU lunch talk in 2019

2. quenching of SMBH growth at around 109.5 Msun ?

スクリーンショット 2017-04-20 10.24.44

There is a long-standing problem “why is there a maximum mass limit (of a few 1010 Msun) of supermassive black holes (SMBHs) in the Universe?” Our work reports, in light of a large number of SDSS selected quasars, the first observational evidence that the number fraction of radio-loud quasars (hereafter, the RL fraction) increases with masses of SMBHs from above a critical BH mass of 2×109 Msun, and the feature is independent of redshifts in a range of 0<z<2. These observational trends can be explained by a scenario proposed by Inayoshi & Haiman (2016) of suppressing BH growth above the apparent maximum mass. Our finding also supports that small-scale physics of nuclear accretion disks around SMBHs determine the BH mass limit, rather than large-scale environments around the host galaxies.

Reference: Ichikawa & Inayoshi (2017), ApJL in press.

3. How does dusty torus and the AGN central engine connect?


The unified model for active galactic nuclei (AGN) propose the ubiquitous presence of a dusty, obscuring, and geometrically thick “torus” around the central engine (e.g., Antonucci & Miller 1985). Considering this torus is the most prominent mass provider of the central SMBH, understanding the dusty torus structures is crucial.  Since those dusty tori are bright in mid-IR (MIR) band, we acquired the almost complete MIR and statistically significant FIR photometric data of AGN obtained from the Swift/BAT hard X-ray all-sky survey. We find a good correlation between their hard X-ray and MIR luminosities over nearly 5 orders of magnitude (41 < log Lx < 46). Both type-1 (blue points) and type-2 (red) AGN follow the same correlation, implying isotropic MIR emission, as strongly expected in clumpy dusty torus models (e.g., Nenkova et al. 2008) or dusty outflow models (e.g., Hoenig et al. 2013).

Reference: Ichikawa et al. (2012), ApJ, 754, 45 , Ichikawa et al. (2017), ApJ, 835, 74, Ichikawa et al. (2019), ApJ, 870, 31
Talk slides: Elusive AGN workshop in 2017 (for 2017 paper) and the talk is available here. or X-ray AGN conference in Corfu, Greece in 2019 (for 2019 paper).
All IR data points are available from here.

4. How much do mergers enhance AGN activity?


Many studies imply that the cosmic evolution of obscured AGN are somehow couple to the star formation activity, which peaks at z~2. Therefore, complete AGN survey covering heavily obscured (=buried) AGN including Compton-thick populations are crucial to understand the cosmic BH growth history hidden by dust/gas obscuration in which even the current hard X-ray surveys might be missing (e.g., Comastri et al. 2015).  Candidates of galaxies that host such buried AGN are infrared galaxies, defined as those having infrared luminosity with L_IR > 10^10 Lsun. We conducted AKARI 2.5-5.0 um spectroscopy of such infrared galaxies without any signs of AGN in the optical and X-ray energy bands. Applying our AGN diagnostics to the AKARI spectra by decomposing the spectra into stellar and AGN dust-torus components, we found that both the fraction and the energy contribution of buried AGN increase with infrared luminosity from 10^10 to 10^13 Lsun. However, the energy contribution of buried AGN to the total infrared luminosity is only ~7% in LIRGs and ~20% in ULIRGs, suggesting that the majority of the infrared luminosity originates from starburst activity, not AGN in the local universe.

Reference: Ichikawa et al. (2014), ApJ, 754, 45 

5. Application of clumpy torus models to near-/mid-IR SEDs of AGN


The above research (Ichikawa et al. 2012, 2017) suggests that the clumpy torus model is one of the most preferable dusty torus model. The next step is to apply this clumpy torus model to the observed dust-torus SEDs, where high spatial resolution infrared observations of AGN are crucial to disentangle the emission from the host galaxy components. Thanks to the help of “Los Piratas AGN team”, which lead the project to pursue the understanding the properties of the AGN torus and its connection with the host galaxy by using 8m class telescopes such as Gemini, VLT, and Subaru. The team also provides the clumpy torus model called Bayesclumpy (Asensio Ramos & Ramos Almeida 2009), therefore we applied this model to the torus SEDs and compared the torus geometry such as the torus size, scale height, and the covering factor. We found that each AGN type (type-1, type-2 with signs of hidden BLRs, type-2 with no-signs of hidden BLRs) have different covering factor and the torus geometry could produce such an type differences.

Reference: Ichikawa et al. (2015), ApJ, 803, 57

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