Research

"Oh Traveler, there are no paths; paths are made by walking"

Antonio Machado

I have research interests in broad range of topics.

1. Molecular gas in galaxies

2. Atomic hydrogen gas in galaxies

3. Supermassive Black Hole

4. Dynamical evolution and equilibrium of gravitational system

5. Astronomical data reduction and analysis technique

Molecular gas and dust surrounding QSO

Star formation rate and dust properties in QSO is important information to understand the connection between QSO and ULIRG. Using ALMA, I studied the molecular gas and dust distribution in optically bright QSO selected from Palomar-Green sample. High resolution ALMA observation reveals a diverse morphological and kinematical structure of molecular gas in the vicinity of AGN, showing rotating disk and outflowing molecular gas.

Collaborators:

HI contents in groups and clusters

Using ALFALFA survey, we selected galaxies in groups and clusters and measure their HI masses. We then investigate the statistical distribution of the HI mass and stellar mass ratio and the detection rate compared with the ones for the comparison samples with similar stellar mass and redshift, as function of group centric distance.

Collaborators: Jessica L. Rosenberg

Relation between HI mass and halo mass in local universe

HI masses in dark matter halos is important observational constraint to connect the dark matter halo distribution robustly predicted from theory and the observed distribution of HI masses in the universe. It is an essential ingredient in the HI intensity mapping which is a new observational technique to study the large scale matter distribution and the galaxy formation physics. We measured the correlation function of HI sources and the HI masses in dark matter halos in local universe, by combining ALFALFA and SDSS group catalog in order to constrain model parameters of the relation between HI mass and halo mass.

Collaborators:

Weighting supermassive black hole mass using molecular gas kinematics

I investigated an optimal spatial and velocity resolution for ALMA to detect an impact of supermassive black hole (SMBH) to the galaxy rotation curve. Depending on the galaxy stellar mass profile shape, the required resolution for reliable measurement of SMBH mass can be much larger than the black hole sphere of influence (SOI).

Collaborators:

Search for recoiling supermassive black hole

When two galaxies merge, two progenitor supermassive black holes (SMBH) will also merge and the merge SMHB can recoiled if emitting gravitational wave is anisotropic. This recoiling SMBH has important implications to the coevolution of SMBH and host galaxy and the "changing-look AGN", however has not been studied well due to small number of candidate detected so far. We are carrying out observational study for the selected recoiling SMBH candidate to confirm if they are real recoiling SMBH or binary SMBH. In the LSST era, the systematic search for recoiling SMBHs and the follow up investigations of their physical properties will shed light on the black hole and galaxy co-evolution.

Collaborators:

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.

Collaborators:

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.

Collaborators: Hyung Mok Lee

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.

Collaborators: Martin Weinberg, Neal Katz

New 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...

Collaborators: Martin Weinberg, Neal Katz

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.

Supervisor: Grant Wilson, Min Yun