Curriculum Vitae

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Publications

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ApJS Special Issue: BASS DR2

Collection of our recent research papers from the BASS DR2, an all-sky survey of the brightest and most powerful nearby hard-X-ray-selected AGN is published (2022).

(Click the figure below to see more about the ApJS special issue)

BASS XXIV: The BASS DR2 Spectroscopic Line Measurements and AGN Demographics

Oh et al. (2022)

ADS . arXiv

We present the second catalog and data release of optical spectral line measurements and AGN demographics of the BAT AGN Spectroscopic Survey, which focuses on the Swift-BAT hard X-ray detected AGNs. We use spectra from dedicated campaigns and publicly available archives to investigate spectral properties of most of the AGNs listed in the 70-month Swift-BAT all-sky catalog; specifically, 743 of the 746 unbeamed and unlensed AGNs (99.6%). We find a good correspondence between the optical emission line widths and the hydrogen column density distributions using the X-ray spectra, with a clear dichotomy of AGN types for NH = 10^22 cm−2. Based on optical emission-line diagnostics, we show that 48%–75% of BAT AGNs are classified as Seyfert, depending on the choice of emission lines used in the diagnostics. The fraction of objects with upper limits on line emission varies from 6% to 20%. Roughly 4% of the BAT AGNs have lines too weak to be placed on the most commonly used diagnostic diagram, [OIII]λ5007/Hβ versus [NII]λ6584/Hα, despite the high signal-to-noise ratio (S/N) of their spectra. This value increases to 35% in the [OIII]λ5007/[OII]λ3727 diagram, owing to difficulties in line detection. Compared to optically-selected narrow-line AGNs in the Sloan Digital Sky Survey, the BAT narrow-line AGNs have a higher rate of reddening/extinction, with Hα/Hβ > 5 (∼ 36%), indicating that hard X-ray selection more effectively detects obscured AGNs from the underlying AGN population. Finally, we present a subpopulation of AGNs that feature complex broad-lines (34%, 250/743) or double-peaked narrow emission lines (2%, 17/743).

Compact elliptical galaxies hosting Active Galactic Nuclei in isolated environment

Rey, Oh, & Kim (2021)

ADS . arXiv

We present the discovery of rare active galactic nuclei (AGNs) in nearby (z < 0.05) compact elliptical galaxies (cEs) located in isolated environments. Using spectroscopic data from the Sloan Digital Sky Survey (SDSS) Data Release 12, four AGNs were identified based on the optical emission-line diagnostic diagram. SDSS optical spectra of AGNs show the presence of distinct narrow-line emissions. Utilizing the black hole (BH) mass-stellar velocity dispersion scaling relation and the correlation between the narrow L([OIII])/L(Hβ) line ratio and the width of the broad Hα emission line, we estimated the BH masses of the cEs to be in the range of 7 x 10^5 - 8 x 10^7 Msun. The observed surface brightness profiles of the cEs were fitted with a double Sersic function using the Dark Energy Camera Legacy Survey r-band imaging data. Assuming the inner component as the bulge, the K-band bulge luminosity was also estimated from the corresponding Two Micron All Sky Survey images. We found that our cEs follow the observed BH mass-stellar velocity dispersion and BH mass-bulge luminosity scaling relations, albeit there was a large uncertainty in the derived BH mass of one cE. In view of the observational properties of BHs and those of the stellar populations of cEs, we discuss the proposition that cEs in isolated environments are bona fide low-mass early-type galaxies (i.e., a nature origin).

An observational link between AGN Eddington ratio and [N II]λ6583/Hα at 0.6 < z < 1.7

Oh et al. (2019)

ADS . arXiv

We present an observed relationship between the Eddington ratio (λEdd) and the optical narrow-emission-line ratio([NII]λ6583/Hα) of X-ray-selected broad-line active galactic nuclei (AGNs) at 0.6<z<1.7. We use 27 near-infrared spectra from the Fiber Multi-Object Spectrograph along with 26 sources from the literature. We show that the λEdd and [NII]λ6583/Hα ratio at 0.6<z<1.7 exhibits a similar anti-correlation distribution of λEdd−[NII]λ6583/Hα, as has been found for local (<z> = 0.036), ultra-hard, X-ray-selected AGNs. The observed distribution implies that there is a consistent relationship from local to z∼1.7, which corresponds from the present time to 4 Gyr old. Further study of high-redshift, low-Eddington-ratio AGNs (log λEdd < −2) is necessary to determine fully whether the λEdd−[NII]λ6583/Hα anti-correlation still holds in high-redshift AGNs at low Eddington ratios.

The Swift-BAT 105 month all-sky hard X-ray survey

Oh et al. (2018)

ADS . arXiv

The 105 month Swift-BAT survey is a uniform hard X-ray all-sky survey with a sensitivity of 8.40×10 −12 ergs −1 cm −2 over 90% of the sky and 7.24 × 10 −12 erg s −1 cm −2 over 50% of the sky in the 14 − 195 keV band. The Swift -BAT 105 month catalog provides 1633 (423 new detections) hard X-ray sources in the 14 − 195 keV band above the 4.8σ significance level. Adding to the previously known hard X-ray sources, 32% (137/423) of the new detections are identified as Seyfert AGN in nearby galaxies (z < 0.2). The majority of the remaining identified sources are X-ray binaries (7%, 29) and blazars/BL Lac objects (9%, 36). As part of this new edition of the Swift-BAT catalog, we release eight-channel spectra and monthly sampled light curves for each object in the online journal and at the Swift -BAT 105 month Web site.

The observed relationship between MBH, Lbol, and λEdd with optical emission line ratios

Oh et al. (2017)

ADS . arXiv

Using nearby and bright hard X-ray selected AGN, I showed that [NII]λ6583/Hα ratio exhibits a significant correlation with λ Edd(RPear=-0.44, p-value=3x10-13, σ=0.28 dex), and correlation is not solely driven by MBH or Lbol.

The observed correlation between [NII]λ6583/Hα ratio and MBH is stronger than the correlation with L bol, but both are weaker than the λEdd correlation. This implies that the larg-scale narrow lines of AGN host galaxies carry information about the accretion state of the AGN central engine. We propose that the [NII]λ6583/Hα is a useful indicator of Eddington ratio with 0.6 dex of rms scatter, and that it can be used to measure λEdd and thus MBH from the measured Lbol, even for high redshift obscured AGN. Possible physical mechanisms behind this correlation are 1) mass-metallicity relation, 2) X-ray heating, and 3) radiatively driven outflows. 

A New Catalog of Type 1 AGNs and its Implications on the AGN Unified Model

Oh et al. (2015)

ADS . arXiV

I discovered a substantial number of unidentified and unexplored population of galaxies featuring weak broad-line region in the nearby Universe (z < 0.2) and investigated their implications on the AGN unified model. Through this research, I proved that the receding torus model is only partially valid and it requires more valid formulation to explain the current observational results. Furthermore, I showed that neither black hole mass nor bolometric luminosity solely determines the type 1 AGN fraction which is a measure of an average torus opening angle.

Improved and Quality-assessed Emission and Absorption Line Measurements in SDSS Galaxies

Oh et al. (2011)

ADS . arXiv

Spectroscopic data provides a fossil record of galaxies featuring stellar absorption and nebular emission lines. The prominent stellar absorption features give us the average surface gravities, effective temperatures, metal abundances, and kinematics of stellar components. Nebular emission lines, on the other hand, probe the physical state of the ionized gas in galaxies and thus can be used to trace the nuclear activity of central supermassive black holes, or their instantaneous star formation rate. From the detailed and improved analysis of more than half million SDSS galaxy spectra (z < 0.2, N=664,187), I released a publicly available galaxy line strength database (the OSSY database) to the community which provides the strength of nebular emission lines, stellar absorption lines, and stellar velocity dispersion. The OSSY database provides more accurate line strengths by decomposing nebular emission lines from stellar absorption features. The OSSY database also achieved better spectral line fitting by adopting empirical stellar templates compared to the SDSS pipeline. Moreover, the database provides a quality parameter assessing the goodness of spectral line fitting so that users can chose proper datasets based on their purposes. 

“(The OSSY database is reachable from this link: http://gem.yonsei.ac.kr/ossy/)”

Talks

Conference presentations

1. Korea Astronomical Society Meeting, Busan, Korea (Apr 2022)

2. BAT AGN Spectroscopic Survey team meeting, (virtual, July 2020)

3. Korea Astronomical Society Meeting, Busan, Korea (Apr 2019)

4. BAT AGN Spectroscopic Survey team meeting, University of Florida, Gainesville, USA (Feb 2019)

5. East Asia AGN Workshop 2019, Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan (Jan 2019)

6. Formation and evolution of SMBHs revealed by 'Wide field', 'Multi-wavelength', and 'Transient' surveys with HSC, Tohoku University, Japan (Nov 2018)

7. Korea Astronomical Society Meeting, Cheongsong, Korea (Oct 2018)

8. Korea Astronomical Society Meeting, Hongcheon, Korea (May 2018)

9. KOOLS-IFU workshop, Kyoto University, Japan (Feb 2018)

10. American Astronomical Society 231st Meeting, Washington DC, USA (Jan 2018): Special Session

11. Korea Astronomical Society Meeting, Daejeon, Korea (Oct 2016)

12. East Asia AGN Workshop 2016, Seoul National University, South Korea (Sep 2016)

13. The X-ray View of Black Hole Activity in the local Universe, ETH Zurich, Switzerland (Feb 2016)

14. Demographics and environments of AGN from multi-wavelength surveys, Crete, Greece (Sep 2015)

15. International Astronomical Union 284 Symposium, Preston, UK (Sep 2011)

16. American Astronomical Society 215th Meeting, Washington DC, USA (Jan 2010)

17. The 2nd Korea-Japan Science Seminar, Hiraizumi, Japan (Nov 200)

18. Korean Astronomical Society Meeting, Gwacheon, Korea (April 2009)

Departmental seminars

1. Department of Astronomy and Space Science, Chungnam National University, South Korea (Jan 2017)

2. Department of Astronomy, Kyoto University, Japan (Oct 2016)

3. National Astronomical Observatory of Japan (Mitaka) (Oct 2016)

4. Kavli Institute for the Physics and Mathematics of the Universe, Japan (Oct 2016)

5. Instituto de Astrofisica, Pontificia Universidad Cato ́lica de Chile, Chile(Mar 2016)

6. Physics and Astronomy department, Seoul National University, Korea (Aug 2015)

7. Korea Astronomy and Space Science Institute, South Korea (July 2015)

8. Department of Astronomy, University of Maryland, USA (May 2015)

9. Astrophysics division (code 600), NASA Goddard Space Flight Center, USA (May 2015)

10. Institute for Astronomy, Department of Physics, ETH Zurich, Switzerland (Apr 2014)

11. Institute for Astronomy, Department of Physics, ETH Zurich, Switzerland (Oct 2013)

12. School of Physics and Astronomy, The University of Nottingham, UK (Jan 2012)

Awards

1. USD 305k (PI): Young Researcher Program, National Research Foundation of Korea (Feb., 2020)

2. USD 640k (co-I): NASA Astrophysical Data Analysis Program (NNH18ZDA001N) (Sep., 2019)

3. USD 5.5k (PI): The CSR travel grant, Swiss Society for Astrophysics and Astronomy (July, 2017)

4. USD 5.5k (PI): MERAC funding and travel grant, Swiss Society for Astrophysics and Astronomy (Feb., 2016)

5. Outstanding paper award (the best thesis), Graduate School of Yonsei University (Oct., 2015)

6. USD 23k (PI): Swiss Government Excellence Scholarship (2013 - 2014)

7. Outstanding paper award (journal), Graduate School of Yonsei University (Oct., 2011)

8. USD 30k (PI): Lotte Scholarship, The Lotte Scholarship Foundation (2006 - 2007, 2010)

9. The Highest Honors, Yonsei University (May, 2007)

10. High Honors, Yonsei University (Oct., 2006)

11. Honors, Yonsei University (May, 2006)

12. Yonsei designated scholarship (Mar., 2003)

Accepted Telescope Time Proposals

1. (coI) ESO VLT 8.2m, period 112, XSHOOTER at Paranal, Chile: 32 hours (2023)

2. (coI) ESO VLT 8.2m, period 110, XSHOOTER at Paranal, Chile: 18.8 hours (2022)

3. (coI) ESO VLT 8.2m, period 109, XSHOOTER at Paranal, Chile: 30.7 hours (2022)

4. (delegated PI) ESO VLT 8.2m, period 108, XSHOOTER at Paranal, Chile: 36.1 hours (2021)

5. (delegated PI) ESO VLT 8.2m, period 106, XSHOOTER at Paranal, Chile: 39 hours (2020)

6. (delegated PI) ESO VLT 8.2m, period 105, XSHOOTER at Paranal, Chile: 40.75 hours (2019)

7. (delegated PI) ESO VLT 8.2m, period 104, XSHOOTER at Paranal, Chile: 55 hours (2019)

8. (delegated PI) ESO VLT 8.2m, period 103, XSHOOTER at Paranal, Chile: 62 hours (2019)

9. (delegated PI) ESO VLT 8.2m, period 102, XSHOOTER at Paranal, Chile: 61 hours (2018)

10. (coI) SOAR 4.1m, GOODMAN at CTIO, Chile: 8 nights (2018)

11. (delegated PI) ESO VLT 8.2m, period 101, XSHOOTER at Paranal, Chile: 29 hours (2018)

12. (coI) Magellan Clay 6.5m, LDSS3 at Las Campanas, Chile: 2 night (2018)

13. (PI) ESO VLT 8.2m, period 100, XSHOOTER at Paranal, Chile: 1 hour (2017)

14. (PI) ESO VLT 8.2m, period 99, FORS2 at Paranal, Chile: 31 hours (2017)

15. (PI) ESO VLT 8.2m, period 99, XSHOOTER at Paranal, Chile: 37 hours (2017)

16. (PI) ESO VLT 8.2m, period 98, XSHOOTER at Paranal, Chile: 13 hours (2016)

17. (coI) Magellan Clay 6.5m, LDSS3 at Las Campanas, Chile: 1 night (2016)

18. (coI) Ir ́en ́ee du Pont 2.5m, B&C at Las Campanas, Chile: 10 nights (2016)

19. (coI) Wisconsin-Indiana-Yale-NOAO (WIYN) 3.5m, MiniMo at Kitt-Peak, USA: 5 nights (2011 - 2012)