• 천문학회보
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• 제38권1호
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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.

• 천문학논총
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• 제30권2호
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• pp.487-488
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• 2015

In type Ia supernovae (SNe Ia) cosmology, a well-established correlation exists between the mass of host galaxies and the Hubble residual (HR) of SNe Ia. In order to investigate the origin of this correlation, we used low-resolution spectroscopic data of early-type host galaxies obtained from our YOnsei Nearby Supernovae Evolution Investigation (YONSEI) project. We measured velocity dispersions and Lick/IDS absorption line indices from these fully calibrated spectra. These indices were used to estimate the luminosity-weighted mean age, metallicity and mass of host galaxies. We found a tight correlation between host mass and population age, which is consistent with the "downsizing" trend in early-type galaxies. This suggests that the well-established correlation between HR and host mass is most likely due to the difference in population age. More observations, which are in progress, are required to understand the impact of luminosity evolution on SNe Ia cosmology.

• 천문학회보
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• 제35권2호
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• pp.39.2-39.2
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• 2010

We have investigated the evolution of the galaxy morphology in the hierarchical universe taking advantage of Semi-Analytic Model (SAM). It is well known that the galaxy morphology is related to the dynamical and the chemical evolution. This implies that we need to understand overall physical processes in the galaxy to reproduce its morphology. Thus we implemented gradual hot gas stripping of satellite galaxies in a galaxy cluster and recycling of stellar mass losses into our model in order to describe star formation rate of galaxies accurately. To morphologically classify galaxies, the evolution of disc and bulge components is traced carefully. We compute our models based on the dark matter halo merger trees generated by N-body simulations as well as the Extended Press-Schechter (EPS) formalism. We present morphological differences caused by the use of different merger trees.

• 천문학회보
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• 제37권2호
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• pp.76.1-76.1
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• 2012

We have employed SNANA supernova analysis package to make YONSEI Supernova Catalogue 1, which contains distance modulus, light-curve shape parameters, and color or extinction values of each supernova. This database is used to study the dependence of Type Ia supernovae (SNe Ia) luminosities on the host-galaxy morphologies. The redshift range of this catalogue is 0.010 < z < 1.555, and we use three light-curve fitters: SALT2, MLCS2k2 (Rv = 3.1), and MLCS2k2 (Rv = 1.7). We find a systematic difference in the Hubble residual (HR) of $0.1{\pm}0.031$ mag between E-S0 and Scd/Sd/Irr host-galaxies, and of $0.16{\pm}0.044$ mag between passive and star-burst host-galaxies. This difference is significant over the $3{\sigma}$ level. Considering the significant difference in the mean age of stellar population between these morphological types, the difference in the HR reported here suggests that the evolution effect of SNe Ia luminosity should be considered in the cosmological application of SNe Ia data.

• 천문학회보
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• 제39권2호
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• pp.63.1-63.1
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• 2014

The galaxy disk warp is a common phenomenon, yet their properties and formation mechanism(s) are still unclear. Here, we introduce a new automatic measurement method for the warp properties of stellar disks in nearby edge-on galaxies, including warp's angle, shape, and asymmetry. We obtain isophotal maps of edge-on galaxies and express each of isophote contours in polar coordinates (${\Phi}$, R) centered on the galaxy centers. Two peaks in the ${\Phi}$-R diagram correspond to the outermost tips of each isophote. The locations of peaks, in turn, inform us of the misalignment between their inner and outer galactic planes, i.e., the warp. We apply this method to SDSS Stripe 82 co-added data and discuss its reliability and validity. Based on the measurement of warp properties, we also investigate their correlations with both intrinsic and environmental properties of warped galaxies.

• 천문학회보
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• 제38권2호
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• pp.62.2-62.2
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• 2013

Recent spectroscopic observations have provided evidence for complex chemical evolution by supernovae and/or asymptotic giant branch (AGB) stars in the two metal-poor globular clusters (GCs) NGC2419 and M15. In particular, the horizontal branches (HBs) of these metal-poor GCs are very extended in the Hertzsprung-Russell diagram. The origin of these peculiar features, as well as that for the Sandage period-shift effect observed in these clusters, are yet to be understood. Here we show, by constructing population models including the Nitrogen enhanced subpopulation, that the second generation populations in these clusters would be enhanced not only in Helium, but also in Nitrogen. This working hypothesis can simultaneously explain the observed extended feature on the HB and the period-shift of RR Lyrae variables.

• 천문학논총
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• 제30권2호
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• pp.485-486
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• 2015

We use light-curve fitting models (MLCS2k2, SALT2, and SNooPy) as implemented in SNANA to make the YOnsei Nearby Supernova Evolution Investigation (YONSEI) Supernova Catalogue. The catalogue consists of several hundred Type Ia supernovae (SNe Ia) in the redshift range from 0.01 to 1.35, and provides distance moduli, light-curve shape parameters, and color or extinction values for each supernova. This data set will be used to study the dependence of SNe Ia luminosities on the host galaxy morphologies. In this paper, we present the YONSEI Supernova Catalogue and preliminary systematic tests for the catalogue.

• 천문학논총
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• 제30권2호
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• pp.473-474
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• 2015

Halo merger trees are the essential backbone of semi-analytic models for galaxy formation and evolution. Srisawat et al. (2013) show that different tree building algorithms can build different halo merger histories from a numerical simulation for structure formation. In order to understand the differences induced by various tree building algorithms, we investigate the impact of halo merger trees on a semi-analytic model. We find that galaxy properties in our models show differences between trees when using a common parameter set. The models independently calibrated for each tree can reduce the discrepancies between global galaxy properties at z=0. Conversely, with regard to the evolutionary features of galaxies, the calibration slightly increases the differences between trees. Therefore, halo merger trees extracted from a common numerical simulation using different, but reliable, algorithms can result in different galaxy properties in the semi-analytic model. Considering the uncertainties in baryonic physics governing galaxy formation and evolution, however, these differences may not necessarily be significant.

• 천문학회보
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• 제38권2호
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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.

• 천문학회보
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• 제38권2호
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• pp.59.2-59.2
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• 2013

The luminosity evolution of Type Ia supernova (SN Ia) and dust extinction play major roles in the systematic uncertainties in the SN cosmology. In order to overcome these obstacles, here we propose to use GMT-GMACS to take spectra for early-type host-galaxies of SNe Ia in the redshift range between 0.2 and 1.0. This high-redshift sample will be taken from Dark Energy Survey (DES), which expects more than 200 early-type hosts at this redshift range. They will be compared with nearby early-type hosts, for which we are now obtaining low-resolution spectra. We will select host-galaxies of same population age range for both nearby and high-redshift samples to reduce the possible evolution effect. Since we are dealing with early-type galaxies, our test is also less affected by dust extinction. We expect that our evolution-free and dust-free dark energy test will provide more robust results on the nature of dark energy.