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"Traveler, there is no path; the path is made by walking"
"Traveler, there is no path; the path is made by walking"
Antonio Machado
1. Radio-(sub)millimeter observation of galaxies and active galactic nuclei in local and distant Universe
1. Radio-(sub)millimeter observation of galaxies and active galactic nuclei in local and distant Universe
Galaxy model SED from radio to FIR wavelength with a composite of four emission processes: synchrotron, free-free, thermal dust emission and anomalous microwave emission.
Galaxy spectral energy distribution in the radio-(sub)millimeter wavelength
Radio-submillimeter-FIR continuum SED has a wealth of information on star formation in galaxies, which is complementary to the UV-optical SED. Photons from star-forming regions (UV-optical SED) heat the cold gas and dust, which emits radiation at longer wavelengths (radio-submillimeter-FIR continuum). I am studying physical processes responsible for the emissions in the radio-submillimeter-FIR continuum SED.
Left: numerical simulation of cold gas accretion. Right: cold gas stream of neutral carbon (blue) in Anthill Galaxy (4C41.17) with extended Lyman alpha halo (purple). Image credit: Emonts et al. 2023
Star formation and inter/circum-galactic medium in galaxy proto-clusters
Galaxy protoclusters are building blocks of today's large galaxy clusters and are much more complicated and evolved structures than we think. High-resolution and high-sensitivity ALMA observations reveal surprisingly bright and extended emission from cold gas and dust in the CGM and IGM inside the protocluster. I study the gas and dust in protoclusters in distance universe.
HST image of J1015 hosting unobscured AGN, an offset nucleus spatially coincident with the location of X-ray emission (white circle) and obscured AGN visible in radio (white contour: VLA and blue contour: VLBA). Image credit: Kim et al., 2019
Radio imaging to search for dual/binary/recoiling supermassive black holes
Almost all galaxies have a supermassive black hole at their center. Therefore, in a hierarchical galaxy formation scenario, the existence of binary black holes are inevitable, but finding them is very hard because we need high spatial resolution to confirm two AGNs, or both AGNs are not always bright in the observing band. Radio and optical observations together often confirm the two AGNs (optically-bright and radio-bright AGN).
2. Study of the first galaxies in cosmic dawn using ALMA and JWST, and next-generation observing facilities
2. Study of the first galaxies in cosmic dawn using ALMA and JWST, and next-generation observing facilities
Color composite (RGB) image of the Hubble Frontier Field cluster Abell 2744, derived by combining the F090W+F115W+F150W as blue, F200W+F277W as green and F356W+F410M+F444W as red. Colored borders show the position of the three different JWST data sets used here: GLASS (green), UNCOVER (GO-2561, blue) and DD-2756 (red). The entire image (including the empty space) is approximately 12.7 × 5.9 arcmin on the sky. Image credit: STScIO
Early galaxies discovered by JWST
JWST opens up a new era of galaxy formation study with a surprisingly large number of galaxies in the cosmic dawn (z>10). ALMA observations following up on z>10 galaxy candidates discovered by JWST pose challenges: very weak or non-detection of thermal dust emission, implying low dust content. Rest-frame UV-optical observations using JWST reveal different emission line features: some galaxies are line-rich while other galaxies are not. To understand the ISM properties of z>10 galaxies correctly, we need both JWST and ALMA to cover multiphase ISM. I am studying gas and dust from these high-z galaxies using ALMA data.
Prospects of the next-generation radio-FIR facilities
The current FIR and radio facility will be upgraded or fully replaced by the next-generation facilities (ALMA WSU, SKA, ngVLA). To prepare for science in the new era, I revisited the galaxy SED in the radio-FIR along the redshift up to z=20 by incorporating the effect of CMB that heats dust but also decreases the contrast of the observed thermal continuum emission against the background.
3. Legacy Survey of Space and Time (LSST) by Vera C. Rubin Observatory
3. Legacy Survey of Space and Time (LSST) by Vera C. Rubin Observatory
Burke-Spolaor et al., 2019
Binary and recoiling supermassive black holes in the era of LSST
LSST will revolutionize our understanding of galaxies and AGNs. Especially, finding binary and recoiling supermassive black holes is one of the key science goals in the LSST AGN Science Collaboration. I am leading and contributing to several collaborative programs to discover close binary AGNs that eventually lead to a recoiling event and become a big siren for the gravitational wave experiments.
Graham et al., 2021
Galaxy evolution study using LSST
My PhD thesis is a statistical inference of galaxy structure using morphology analysis within a Bayesian framework. LSST data will be the most comprehensive observational dataset for this research. I am involved in the LSST Galaxies Science Collaboration to keep my early research interest and contribute to the collaborative effort.
4. Dynamical evolution and equilibrium of gravitational system
4. Dynamical evolution and equilibrium of gravitational system
Dynamical evolution of rotating star cluster
Significant fraction of globular clusters is rotating. This rotation accelerates the core-collapse via gravo-gyro catastrophe, which is conceptually similar to gravo-thermal catastrophe. One of our collaborator has extended Fokker-Planck(FP) model of rotating stellar system to study "after core-collapse", using suitable diffusion term in the FP equation. I have carried out direct nbody simulation of rotating star clusters using Nbody6 code and compared to FP results. Two results shows reasonably good agreements with small difference in detail due to intrinsically different approaches. More details.
Self gravitating system in the external potential well
I investigated the problem of equilibrium of isothermal sphere bounded by solid wall. Instead of solid wall, I posed a situtation where the self gravitating system is in the center of deep external potential well and calculated an equilirium condition of the system. This study showed that a potential well can be a heat bath increasing the velocity dispersion of the embedded system and proposed a new equilibrium model of the system. I also simulated the dynamical evolution of star cluster embedded in the external potential using the modified direct Nbody simulation code, Nbody6. This direct simulation shows an equilibrium density profile which is similar to the analytic model. See more detail.
5. Advanced statistical analysis of astronomical data
5. Advanced statistical analysis of astronomical data
Marginal likelihood computation
In Bayesian statistics, marginal likelihood or often called 'Bayesian evidence' is used for model comparison, which has a broad application to astronomical modeling. However it is challenging to compute a reliable marginal likelihood if the model has large dimensional parameter space, owing to the 'curse of dimensionality'. I tested a practical approach to improve the reliability of marginal likelihood computed from the simulated posterior samples. Combining samples from tempered MCMC simulation increases the samples in the posterior region which is sparsely sampled and makes the marginal likelihood computation quickly converged with smaller number of samples than running a single MCMC simulation.
Bayesian galaxy image decomposition software GALPHAT
We have developed a new image decomposition software GALPHAT (GALaxy PHotometric ATtributes) to model galaxy structure in a statistically robust way. GALPHAT is an application of Bayesian Inference Engine which has been primarily developed by Prof. Martin Weinberg and computer science people in UMASS.
Bayesian MCMC draws the random samples from the posterior distribution of model parameters and thus we can characterize the full probability distribution of model parameters. This is a big difference from previous approaches in galaxy image analysis. In many cases, people have estimated just best-fit parameters using simple chi^2 minimization type technique, but have not carefully taken care of all the correlations in the parameters, which is required to derive any robust conclusion about galaxy evolution and formation hypotheses.
We have extensively tested the software and convinced ourselves that it is working as what we expect. We will use this software for scientific analysis of galaxy image data and put statistically robust confidence on the theory/hypothesis of galaxy formation and evolution. This will be a part of my Ph.D. thesis. More detail...
AzTEC data reduction pipeline for extended sources
AzTEC is a bolometer array camera for detecting thermal continnum emission from dust in the universe and will be one of primary instruments for LMT, a 50m single dish in Mexico. As my second year project in UMASS, I was involved in the development of AzTEC data reduction pipeline focusing on diffuse emission from any extended astronomical sources. In contrast to the detection of point sources like high-z submm galaxy population, reducing signal(which is basically time series) from the extended source is very diffucult since the signal is dominated by large temporal and spatial variation of thermal emission from water vapor. Therefore standard technique (e.g. principal component analysis) used in reducing the data from point sources can not cleanly separate the signal from the background due to the degeneracy.
I have implemented a simple algorithm for doing it and written an IDL code in Object Oriented Programming style. Preliminary test result looked promising however, the significant improvement is required to apply to real data. I am still interested in it.
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