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Research Interest
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Probing The Origin of Supermassive Black Seed Formation Mechanisms with Low-Mass Nearby Galaxies
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Supermassive black holes are know to live at the centers of all massive galaxies with bulges. They power active galactic nuclei and play an import role in the evolution of the host galaxies. However, the birth and growth of the first supermassive black hole "seeds" in the early Universe is far from understood. The primary goal of my research is to improve our understanding on the origin of supermassive black holes by searching for and studying the small black holes in present-day, low-mass, and nearby galaxies. Dynamically measuring the masses of black holes in dwarf galaxies have significant science impacts. These measurements will help to anchor whether the galaxy-black hole scaling relations hold up at lower mass? Constraining the slopes and scatters of these relations in the low-mass regime and the "occupation fraction" of black holes in low-mass galaxies, which will tell us the formation paths of black hole seeds in the early Universe and how they affect the galaxy formations and evolutions throughout the cosmic history.
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Black Hole - Host Galaxy Relations
Scaling relations between central black hole mass and host galaxy properties, e.g., the bulge mass component and bulge velocity dispersion of stars, hint to a joint evolution of black holes and galaxies. Estimating the dynamical mass of black holes and their host galaxies at different redshifts is fundamental to establish their growth scenarios over the cosmic time. However, the presence of supermassive black holes and the importance of active galactic nuclei (AGN) feedback in lower mass galaxies is less clear. In particular it remains unclear if the scaling relations between supermassive black hole masses and galaxy properties that hold at higher mass break down for lower-mass galaxies (see left panel of Figure 1). Increased scatter around the relation has been seen for Milky Way like galaxies, while black hole masses in the lowest mass galaxies seem to fall below the bulge mass relation seen for higher mass galaxies. The cause of this change at low masses is still debated: perhaps it is tied to the formation history of the bulge or perhaps to the star formation history of the galaxy more generally.
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​My expertise emphasizes on UV-optical, Infrared spectroscopy both in IFU and ​Slit, and Radio interferometric data. Currently, I'm using both stellar and cold gas dynamics to estimate the masses of central black holes in galaxies, which live in the local volume (distances < 10 Mpc), with the aid of the stellar and gas dynamical models, respectively.
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Figure 1: Our four low-mass early-type galaxies (ETGs) in context of the Mbh-Mbulge (left) and Mbh-dispersion (right) scaling relations. The data of both ETGs (black dots with open circles) and late-type galaxies (LTGs, black open circles) are taken from the compilation of Saglia et al. 2016. The dotted-, dashed-, long dashed-, and solid-lines indicate their linear best-fit in log scale of the relations from Scott et al. 2013; McConnell et al. 2013; Kormendy et al. 2013, and Saglia et al. 2016 for LTGs, respectively. The BH masses of M32, NGC 5102, and NGC 5206 and 3 sigma upper limit mass of NGC 205 with the downward arrow are plotted in red dots + open circles as in the legend. We also add BHs with dynamical masses below 1 million solar mass, including the dwarf AGN LTG NGC 4395 (yellow open circle, den Brok et al. 2015) and ETG NGC 404 (Nguyen et al. 17).
Nuclear Star Cluster (NSC) at High Resolution
​NSC
Figure 2: HST WFPC2 F547M of NGC 5102 (left). The location of NSC appears as the bright and compact core that is consistent with both photometric and kinematic center. Dynamical mass vs. effective radius of nuclei star clusters (right).
Using high-resolution imaging and spectroscopy and the proximity of our low-mass early-type galaxy sample, I examine in detail their NSCs morphology, composition, kinematics, and dynamical masses to shed a light on their formation origins either gas in situ or stellar migration. In low-mass galaxies, the coexistence of both NSCs and black holes are expected and the relationship between NSCs and black holes is not well understood due to the sample limited. However, the formation of NSCs and black holes could be explicitly linked with NSCs creating the needed initial seed black holes during formation, or strong gas inflow creating the NSC and BH simultaneously.
Search for Molecular Tori in Nearby AGN
The dusty molecular torus has been considered a cornerstone of unified schemes of quasars and Seyfert galaxies. However, this picture has recently been challenged through high spatial resolution infrared observations by the discovery of polar elongated dust structures, rather than doughnut shaped distributions. Similarly, recent spatially unresolved X-ray observations have provided critical new insights into the toroidal obscurer and scatterer, potentially requiring a reinterpretation of long-established concepts.
Figure 3: CO(2-1) integrated intensity (moment zero) map of the NGC 3504's nucleus from our ALMA Cycle-5 observations (left) and that of the NGC 404's nucleus from our ALMA Cycle-3 observations (right). The torus' cavities' diameters are roughly estimated of <1 pc and ~0.1 pc for NGC 3504 and NGC 404, respectively.
I'm interested in using dense gas molecular tracers to obtain detail physical, morphological and dynamical information of the dusty molecular tori of the nearby face-on AGNs from a ~10 pc down to <1 pc scale. Adding the very high resolution with ALMA will enable us to investigate the molecular torus structure and its cavity surrounding the AGN engine, which is powered by an accreting supermassive black holes at the center, and verify/reform the AGN unification model observationally in details. We also want to probe for the first time how do the accretion disk and molecular torus physics depend on accretion rate, luminosity, and supermassive black hole mass.
@2018 Dieu D. Nguyen
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