Intae Jung

JWST Postdoc (Sponsor: CUA)
@ NASA's Goddard Space Flight Center (Astrophysics Science Division, Code 665)

Lyman-alpha Emitter as a Probe of Cosmic Reionization -- High-z Galaxy (Star-formation Quenching) -- Galaxy SED Fitting Using a MCMC Algorithm

A Spectroscopic Study of Lyman Alpha Emission at z = 6 - 8 as a Probe of Reionization

    I am currently working on spectroscopic search for Lyman alpha emission at the epoch of reionization by performing deep spectroscopic observations with the DEIMOS optical spectrograph (Oct 2014, PI: Dr. Rachael Livermore) and the MOSFIRE near-infrared spectrograph (Nov 2015, PI: Dr. Steve Finkelstein) on the Keck 10-meter telescope to constrain the end of cosmic reionization.
    Measuring the equivalent width (EW) distribution of Lyman-alpha emission from continuum-selected galaxies with follow-up spectroscopy is an immediately accessible method for studying the intergalactic medium in the reionization era. Neutral hydrogen in the intergalactic medium affect the observed features of Lyman-alpha emission, for example, the equivialent width distribution (Figure 1 in the summary slide). The models predict a steeper slope in the Lyman-alpha EW distribution with the higher fraction of neutral hydrogen in the intergalactic medium (Figure 2).
    Utilizing both optical (DEIMOS) and near-IR (MOSFIRE) spectrographs, we are searching for Lyman-alpha emission with a comprehensive wavelength coverage of Lyman-alpha emission from z ~ 5-8 galaxies (the survey design in Figure 3 & five Lyman-alpha detections from DEIMOS in Figure 5). Moreover, our MOSFIRE observions provide the deep spectroscopic dataset for 72 galaxies z > 7, which includes the deepest (>15hr) NIR spectroscopic data for six candidate galaxies, discovering a new Lyman-alpha at z=7.6 (Figure 5)
    By simulating mock emission lines, considering observational conditions (e.g., observational depth, wavelength coverage, and sky emission) and the probability distribution function of galaxy photometric redshifts, we predict the expected number of detections of Lyman-alpha emitters from our spectroscopic dataset with varying Lyman-alpha EW distribution, particularly changing the characteristic scale length, W_{0}, in the exponential function (Figure 6).
    In Figure 7, our measure of the equivalent width distribution of Lyman-alpha emission at z ~ 6.5 shows a weak evidence that the e-folding scale begins to drop at z > 6 (~36Å in 1-sigma upper limit, red arrow) from our DEIMOS dataset, and this study will be extended toward higher redshift ranges, incorporating the MOSFIRE data in the fitting process.

A Spatially-Resolved Stellar Population Study Using the HST Imaging:
reduced star-formation rates in the centers of massive galaxies at z = 4


In the PhD program at the University of Texas at Austin, I have dealt with the Hubble Space Telescope imaging data for studying spatially-resolved star formation histories of distant galaxies, which was published, Evidence for Reduced Specific Star Formation Rates in the Centers of Massive Galaxies at z = 4 (Jung et al. 2017, ApJ, 834, 81). In this work, I developed my own galaxy spectral energy distribution fitting IDL procedure based on a Markov Chain Monte Carlo (MCMC) algorithm, and performed a spatially-resolved stellar population study of z = 4 galaxies. A key discovery of the study is that the galaxies with the highest central mass densities have reduced star formation in their centers, possibly observing the earliest phases of bulge formation.

Building Dark Matter Halo Merger Trees (ySAMtm)

[Dark Matter Halo Merger Trees]
(Left) schematic diagram showing the processes of a dark matter halo merger (Jung et al. 2014, ApJ, 794, 74).

During my master's program, I studied the theoretical understanding of galaxy evolution using N-body simulations and a semi-analytic model with Prof. Sukyoung K. Yi at Yonsei University in Seoul, South Korea. Particularly, I developed a dark matter (DM) halo merger tree builder, called ySAMtm, which constructs DM halo merger trees from cosmological N-body simulations and provides a DM halo backbone for the semi-analytic model of Prof. Yi's group (ySAM; Lee & Yi, 2013). Jung et al. (2014) also study large-scale environmental effects on galaxy assembly by analyzing the semi-analytic model galaxies.