• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.40.1-40.1
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• 2017

Massive early-type galaxies (ETG) have been spectroscopically confirmed up to z>3 which, together with their ages and abundances at z>1.5, implies that their progenitors must have converted gas into stars on short timescales. The termination of star formation in these galaxies can occur through several channels, but they remain largely conjectural, in part due to the current lack of direct measurements of the amount of residual gas in high redshift ETGs. Here I will present constraints on the star formation rate and dust/gas content of z=1.4-2.5 ETGs. These galaxies, close to their epoch of quenching, contained more than 2 orders of magnitude more dust than their local counterparts, which suggests the presence of substantial amounts of gas and a low star formation efficiency.

• Publications of The Korean Astronomical Society
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• v.34 no.3
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• pp.67-79
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• 2019

We present a semi-analytical method to calculate the global evolution of the ionized state of the intergalactic medium, on the basis of physically motivated star formation histories in the early universe. This method incorporates not only the conventional scenarios in which the star formation rate is proportional to the growth rate of the halo collapse fraction, but also the more sophisticated scenarios in which the star formation is self-regulated. We show that this variance in the star-formation model strongly impacts the resulting reionization history, which bears a prospect for observational discrimination of these models. We discuss how observations of the anisotropic polarization of the cosmic microwave background and the global 21cm signal from the high-redshift universe, most notably by Planck and EDGES, may probe the history of reionization.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.29.2-29.2
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• 2013

The presence of dark energy, suggested from Type Ia supernovae (SNe) cosmology, is the most pronounced astronomical discovery made during the past decade. The basic assumption of this discovery is that the look-back time evolution of SNe luminosity would be negligible after light-curve correction. Several recent works, however, show that there are some differences in Hubble residual among host galaxies having different morphology and mass, indicating that SNe luminosity might be affected by population age. In order to investigate this more directly, we are continuing the YONSEI (YOnsei Nearby Supernovae Evolution Investigation) project, where we are obtaining low-resolution spectra of some 60 nearby early-type host galaxies. The early-type galaxies are preferred because the mean population ages and metallicities can be estimated from the absorption lines, and they are less affected by dust extinction. In this talk, we will report our progress in determining the ages and metallicities of host galaxies to investigate their correlation with the Hubble residual.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.56.1-56.1
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• 2013

Supernovae type Ia (SNe Ia) cosmology is providing the only direct evidence for the presence of dark energy. This result is based on the assumption that the look-back time evolution of SNe Ia luminosity, after light-curve shape correction, would be negligible. However, the most recent compilation of SNe Ia data shows systematic difference in the Hubble residual (HR) between the E and Sd/Irr galaxies, indicating that the light-curve fitters used by the SNe Ia community cannot quite correct for a large portion of the population age effect. In order to investigate this possibility more directly, we have obtained low-resolution spectra for 30 nearby early-type host galaxies. This data set is used to estimate the luminosity-weighted mean ages and metallicities of host galaxies by employing the population synthesis models. We found an interesting trend between the host galaxy age and HR, in the sense that younger galaxies have positive residuals (i.e., light-curve corrected SNe Ia luminosity is fainter). This result is rather independent of the choice of the population synthesis models employed. Taken at face value, this age (evolution) effect can mimic a large fraction of the HR used in the discovery of the dark energy. This result is significant at 1.4 - 3 sigma levels, depending on the light curve fitters adopted, and further observations and analyses are certainly required to confirm the trend reported here.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.59.1-59.1
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• 2013

Type Ia supernovae (SNe) are providing the most conclusive evidence for accelerating universe with dark energy in observational cosmology. In these investigations, look-back time evolution of SNe luminosity is regarded as negligible on the basic assumption. However, several recent works present some systematic differences among hosts which have different characteristics of stellar population. For more direct investigation, we are proceeding with our YONSEI (YOnsei Nearby Supernovae Evolution Investigation) project. Only early-type hosts in our catalogue were chosen in order to estimate the luminosity-weighted mean age and metallicity directly using Single Stellar Population (SSP) models and ignore the effect from the dust extinction. Observations using low-resolution spectrographs are still in progress at Las Campanas Observatory with 2.5m telescope and at McDonald Observatory with 2.7m telescope. We have thus far obtained spectra for 30 early-type hosts. After weak emission line correction, Lick/IDS absorption-line indices are measured and YEPS spectroscopic evolution model was applied to determine mean population ages and metallicities. Our preliminary results show that SNe Ia hosted in older galaxies seem to be brighter at 1.4 - 3 sigma levels, however, more observations and analyses are still needed to confirm this correlation.