• The Bulletin of The Korean Astronomical Society
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• v.39 no.2
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• pp.51.2-51.2
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• 2014

Ultra faint dwarf galaxies (UFDs) are larger but fainter than globular clusters, being the faintest galaxies in the universe. They have been found only in the Local Group. We report the discovery of an UFD in the intracluster field of the Virgo cluster (Virgo UFD1). It is located near the core of Virgo cluster, and far from any massive galaxies. The color magnitude diagram of resolved stars in Virgo UFD1 shows narrow, metal poor red giant branch (RGB), which is very similar to the UFDs in the Local Group. by comparing RGB in this galaxy with 12 Gyr stellar isochrones, we estimate its distance, $d=16.4{\pm}0.4$ Mpc and mean metallicity, $[Fe/H]=-2.4{\pm}0.4$. We derive its integrated photometric properties and structural parameters : V-band absolute magnitude of $MV=-6.3{\pm}0.2$, effective radius of $84{\pm}7pc$, and central surface brightness of ${\mu}V,0=26.49{\pm}0.09$ mag arcsec-2. These properties are similar to these of Local Group UFDs. Virgo UFD1 is the first UFD discovered beyond the Local Group. These results indicate that it may be a fossil remnant of the first galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.37 no.1
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• pp.49.1-49.1
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• 2012

Recently it is found that the globular clusters are not only bound in their host galaxies, but also are wandering between galaxies in Virgo and Coma clusters. The cluster-wide distribution of these intracluster globular clusters (IGCs) suggests that IGCs are an important probe to understand hierarchical structure formation. We present a survey of IGCs in Virgo cluster using HST archive images for four HST/ACS fields located from about 9 arcmin to 40 acrmin from the cluster center. We find ten new IGCs and confirm four previously known IGCs. The number density of IGCs decreases as the distance from the cluster center increases. We derive integrated photometry of IGCs. We also obtain photometry of resolved stars in the outer region of each cluster. These IGCs are fainter than $M_V{\approx}-9.5$ and mostly blue in (V-I) color. showing that they are mostly metal poor. The locations of red giant branch stars of IGCs in color-magnitude diagrams also show that they are meal-poor. We discuss the implications of these results.

• The Bulletin of The Korean Astronomical Society
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• v.42 no.2
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• pp.75.3-75.3
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• 2017

We Investigate dust stripping of Virgo cluster galaxies that are known to suffer HI gas stripping. The gas stripping phenomena of these galaxies may result from either ram pressure induced by the hot intracluster medium or gravitational tidal interactions between galaxies. While much efforts have been made to directly detect gas removed from cluster galaxies, the spatial distributions of dust, which should also be affected, are hardly known. Several previous studies have tried to directly detect the morphology of gas or dust using radio or infrared observations, but such approaches are hard to widely apply because of the limit of observational resolution and sensitivity. In this study, we try a different approach using optical data: measuring the background galaxy reddening by the dust stripped from the Virgo cluster members. Based on optical color excess maps of the background galaxies, we compare the ambient dust distribution with the HI morphology of the Virgo galaxies. We discuss how efficiently dust stripping can be detected with this method and how the stripped dust is associated with the removed gas according to HI gas stripping stage over the sample.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.46.2-46.2
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• 2013

We utilize Sloan Digital Sky Survey DR7 spectroscopic data of ~380 star forming galaxies in the Virgo cluster to investigate their chemical properties depending on the environments. The chemical evolution of galaxies is linked to their star formation histories as well as to the gas interchange in different environments. We derived star formation rate (SFR) and gaseous metallicity (e.g., oxygen abundance) of star forming galaxies. Combining with GALEX ultraviolet photometry and ALFALFA HI 21 cm data, we examine the relations between SFRs, metallicity, and HI deficiency of galaxies in various regions of the Virgo cluster. We also quantify the degree of ram pressure around galaxy using the ROSAT X-ray surface brightness map. We discuss environmental effects on the chemical properties and evolution of star forming galaxies.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.75.3-75.3
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• 2019

Understanding how ram pressure stripping (RPS) affects the star formation activity of cluster galaxies is one of the important issues in astrophysics. To examine whether we can identify any discernible trend in the star formation activity of galaxies undergoing ram pressure stripping, we study the star formation properties of galaxies in the Virgo cluster for which high-resolution HI images are available. We first classify galaxies in the Extended Virgo Cluster Catalog into different stages of RPS based on their HI morphology, HI deficiency, and location in phase space. We then examine various star formation activity indicators of these galaxies, which include starburstiness, g - r color, and WISE [3.4]-[12] color. No noticeable enhancement in star formation was identified for galaxies undergoing early or active stripping. Our results suggest that star formation activity at best seems to be enhanced locally in such galaxies, making it challenging to detect with integrated photometry. With the combination of HI deficiencies and locations in phase space, we were instead able to capture the overall quenching of star formation activity with increasing degree of ram pressure stripping, which agree with previous studies.

• The Bulletin of The Korean Astronomical Society
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• v.36 no.2
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• pp.78.2-78.2
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• 2011

We utilize Sloan Digital Sky Survey DR7 spectroscopy of ~600 emission line galaxies (ELGs) in the Virgo and Ursa Major clusters to investigate their chemical properties depending on the environments. We derived chemical abundances of galaxies using either a direct estimation of the electron temperature or empirical calibrations. We also estimated star formation rates (SFRs) using H alpha and GALEX ultraviolet (UV) luminosities. We see no significant difference of UV colors and SFRs of ELGs between the Virgo and Ursa Major, indicating weak dependence of their star formation activity on global cluster environment. We also discuss the segregation of gas-phase element abundances in cluster environment.

• The Bulletin of The Korean Astronomical Society
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• v.38 no.2
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• pp.48.1-48.1
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• 2013

We present galaxy luminosity functions (LFs) of subgroups in the Ursa Major cluster. The membership of galaxies is determined by radial velocities which are compiled from the SDSS and NED. We found that the faint end slope (${\alpha}$) of the LF of all sample galaxies in Ursa Major is ${\alpha}=-1.13$. This is consistent with previous result, but shallower than that of the Virgo cluster. Interestingly, the subgroups in Ursa Major show different slopes in their LFs. The NGC 3992 and NGC 4111, the massive subgroups in the Ursa Major cluster, exhibit steep slopes of their LFs comparable to that of the Virgo cluster. On the other hand, less massive group (NGC 3877) shows a very shallow slope of -0.84. Based on the results, we discuss the evolution of galaxies and the assembly history of the Ursa Major cluster.

• The Bulletin of The Korean Astronomical Society
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• v.35 no.2
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• pp.42.1-42.1
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• 2010

We present ultraviolet (UV) photometric properties of dwarf galaxies in Fornax cluster and Ursa Major group in comparison with Virgo cluster using GALEX data. We construct UV color-magnitude relations (CMRs) of dwarf galaxies matching with available optical photometry and SDSS data. Majority of dwarf galaxies in Fornax cluster show sequence in UV CMRs consistent with that of dwarf elliptical (dEs) in Virgo cluster indicating similar age and metallicity properties of dEs in two clusters. The dS0 sequence in Fornax cluster is not distinct as much as that in Virgo cluster. Dwarf galaxies in outer region of the Fornax cluster show more bluer UV colors with a wide scatter in CMRs, which indicates recent star formation activity. We show that the UV colors of dwarf galaxies are related with the distribution and strength of the X-ray emission in the cluster. In contrast to the Fornax cluster, most dwarf galaxies in Ursa Major group are located in the blue cloud showing recent or on-going star formation, and few galaxies show characteristics of dEs. We discuss relationship between UV properties of dwarf galaxies and different environment of cluster.

• Journal of The Korean Astronomical Society
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• v.29 no.spc1
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• pp.57-58
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• 1996

There is an excess gamma flux from the general direction of the Galactic North Pole compared with that from the south when allowance is made for the contribution from CR interactions with the HI gas (Osborne et al., 1994). The extent to which it is in accord with the predictions of Wdowczyk and Wolfendale (1990 a,b) for gamma rays secondary to very high energy CR escaping from the VIRGO cluster is examined and it is claimed that the observations may well be of the order of those expected.

• The Bulletin of The Korean Astronomical Society
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• v.40 no.1
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• pp.76.2-76.2
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• 2015

We present the results of the Westerbork Synthesis Radio Telescope (WSRT) HI imaging study of seven late-type galaxies. They are located in the outskirts of the Virgo cluster, possibly along a filament connected to Virgo from the north-west. Most galaxies in this region are found to be HI-rich, containing more HI gas compared to field galaxies with similar size and optical luminosity. The positions of the sample with respect to the cluster and their high HI mass-to-light ratios suggest that the selected galaxies might be accreting more gas from their surroundings while falling into the cluster. By high-resolution HI imaging, we aim to find evidence that galaxies are pre-processed by gas accretion from the intergalactic medium and/or gas-rich neighbors. We probe the detailed HI morphology/kinematics and the star formation properties of the sample. All of these galaxies are found with a large HI disk which is quite extended compared to their stellar disk. Together with kinematical peculiarities, this strongly suggests that cold gas accretion is responsible for active star formation in these galaxies.