• The Bulletin of The Korean Astronomical Society
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• v.38 no.1
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• pp.50.2-50.2
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• 2013

In the process of star formation, the density and temperature of associated material, which are the physical conditions for the molecular chemistry, vary dramatically. As a result, the connection between physical and chemical conditions has been used to trace the evolutionary stages in star formation. One chemical tracer for the physical conditions in star forming material is the [HCN]/[HNC] abundance ratio since the ratio strongly depends on the kinetic temperature in molecular clouds. Here we investigate the [HCN]/[HNC] abundance ratios in objects related to the massive star formation. For the investigation, we carried out $H^{13}CN$ and $HN^{13}C$ line observation toward objects in three different evolutionary stages of massive star formation: Infrared dark clouds (IRDCs), High-mass protostellar object (HMPOs), and Ultra-compact HII regions (UCHIIs). According to our observational results, both $H^{13}CN$ and $HN^{13}C$ lines have been detected toward 19 IRDCs, 25 HMPOs, and 31 UCHIIs. We will discuss about the [HCN]/[HNC] abundance ratios in different evolutionary stages of massive star formation and associate the results with the physical conditions of the targets.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.1
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• pp.28.2-28.2
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• 2017

Using numerical hydrodynamics code RAMSES, we perform idealized galaxy merger simulations and study the star formation of merging disk galaxies. In our simulations, we consider the active galactic nucleus (AGN) feedback effect. In order to investigate the star formation influenced by AGN, we run ~60 simulations with various initial conditions. We confirm that star formation is more efficiently suppressed in merging galaxies than in isolated galaxies. In the mergers, AGN effect is more significant when the masses of two galaxies are similar. Furthermore, we find that bulge fraction does not affect the star formation when the AGN effect is considered. We discuss the implications on semi-analytic galaxy formation models and the limitation of the current AGN prescriptions.

• Journal of The Korean Astronomical Society
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• v.54 no.1
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• pp.17-35
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• 2021

We study galaxies undergoing ram pressure stripping in the Virgo cluster to examine whether we can identify any discernible trend in their star formation activity. We first use 48 galaxies undergoing different stages of stripping based on H i morphology, H i deficiency, and relative extent to the stellar disk, from the VIVA survey. We then employ a new scheme for galaxy classification which combines H i mass fractions and locations in projected phase space, resulting in a new sample of 365 galaxies. We utilize a variety of star formation tracers, which include g - r, WISE [3.4]-[12] colors, and starburstiness that are defined by stellar mass and star formation rates to compare the star formation activity of galaxies at different stripping stages. We find no clear evidence for enhancement in the integrated star formation activity of galaxies undergoing early to active stripping. We are instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, in agreement with previous studies. Our results suggest that if there is any ram pressure stripping induced enhancement, it is at best locally modest, and galaxies undergoing enhancement make up a small fraction of the total sample. Our results also indicate that it is possible to trace galaxies at different stages of stripping with the combination of H i gas content and location in projected phase space, which can be extended to other galaxy clusters that lack high-resolution H i imaging.

• The Bulletin of The Korean Astronomical Society
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• v.39 no.1
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• pp.47.1-47.1
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• 2014

We examine the effect of environment on star formation activity of a sample of galaxy group catalogue given in Tempel et al.(2012) constructed from the Sloan Digital Sky Survey Data Release 8. In order to compare galaxies in different environment, we classify galaxies into two groups: galaxies in low density environment and galaxies in high density environment. After matching colors and apparent magnitudes of the galaxies, we are left with 5912 galaxies in each of the environment category. The fraction of star-forming galaxies in low-density environment is ~34%, higher than ~15% in high-density environment. Star-forming galaxies in low density environment have a higher average SFR value than those in high density environment. The bulge-to-disk ratio for galaxies in two different environment shows bimodal distribution. Regardless of the environment, we find galaxies with high star formation rate despite their red (g-r) color, for which the origin enhancing their star formation rate is investigated.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.2
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• pp.49.1-49.1
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• 2015

Star formation in galaxies that lie in dense environment tends to increase as the redshift of the cluster increases. At z~1.4, the situation turns to be complex; some clusters still harbor galaxies with vigorous star formation, and others are populated with relatively old, massive galaxies. We present the result from narrow-band photometric study of the fields around the radio galaxy 6CE 1100+3505 at z=1.44. Deep H- and H-narrow band data have been obtained using CFHT/WIRCAM which cover the corresponding wavelengths for redshifted $H{\alpha}$. While the number of IRAC 3.6, and $4.5{\mu}m$ selected sources show clear excess within the central ~1Mpc area from the radio galaxy, number of galaxies identified to show excess in H-narrow band is very small. We discuss the possible integrated star formation rate in this overdense structure, and the implication to the evolution of cosmic star formation rate as a function of environment.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.55.2-55.2
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• 2013

We examine the effect of environment on star formation activity of a sample of a galaxy group catalogue constructed from the Sloan Digital Sky Survey(SDSS DR8) given in Tempel et al.(2012). As an environmental parameter, we use the richness of the galaxy group. According to this parameter, we select 6846 galaxies in dense environment and 297335 galaxies in low environment. By comparing the two samples, we identify the different relationship between star formation rate and stellar mass. In order to compare galaxies in different environment, we fixed other parameters(color, apparent magnitude), which can affect star formation efficiency except for stellar mass. Also, based on HI mass from the ALFALFA survey, we study the environmental dependence of Kennicutt-Schmidt law which show the correlation between star formation rate and gas content.

• 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.

• Journal of The Korean Astronomical Society
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• v.40 no.4
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• pp.127-129
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• 2007

It is well known that water vapor maser emission at 22.2 GHz is associated with the earliest stages of both low- and high-mass star formation and it can be considered a reliable diagnostic of their evolutionary state. Bright Rimmed Clouds (BRCs) are clouds that have been compressed by an external ionization-shock front which focuses the neutral gas into compact globules. The boundary layer between the neutral gas and the gas ionized by the incident photons is often called "bright rim" but the clumps are sometimes classified also as speck globules or cometary globules depending on their appearance. Small globules with bright rims have been considered to be potential sites of star formation and have been studied in several individual regions. We present results from high resolution VLA observations searching for new candidates of recent star formation in bright-rimmed clouds/globules associated with IRAS point sources.

• The Bulletin of The Korean Astronomical Society
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• v.41 no.1
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• pp.39.2-39.2
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• 2016

The central regions of barred-spiral galaxies contain interesting gaseous structures such as dust lanes located at the leading side of the bar and nuclear rings that are sites of intense star formation. Our previous studies showed how gas structures form under the influence of a non-axisymmetric bar potential and temporal/spatial behavior of the star formation in nuclear rings. However, previous works were limited to 2-dimensional infinitesimally-thin, unmagnetized and isothermal disks. To study effects of cooling/heating, vertical motions of gas structures and magnetic field, we use Mesh-Free magneto-hydrodynamic simulation code GIZMO. We find that temporal variations of the star formation rates in the nuclear ring in the three-dimensional model are overall similar those in the previous two-dimensional results, although the former shows more violent small-scale fluctuations near the early primary peak. We will present our recent results about evolution of gaseous structures and star formation rate compare with results of previous studies.

• The Bulletin of The Korean Astronomical Society
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• v.45 no.1
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• pp.48.1-48.1
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• 2020

The cosmological constant is accountable for the accelerated expansion of our Universe. Observational data have provided a tight constraint on the cosmic star formation history from z = 8 to the present. What happens to the star formation rate beyond z=0? I will discuss the star formation rates, along with the properties of the intergalactic mediumfrom our suite of simulations into the future. Since Lambda becomes dominant in the future of our universe, I further simulate counter-factual universes to assign anthropic weights to each universe within the multiverse setting. I will argue that using the asymptotic star formation efficiency as weights, we almost double previous estimates of observers living in universes similar to ours. The expected value of the energy density of Lambda is also closer to the observed value. I will also discuss potential future works to improve the applicability of the anthropic reasoning of the cosmological constant.