Sungsoon Lim

MERGING GALAXY CLUSTER A2255 IN MID-INFRARED

We present the mid-infrared (MIR) observation of a nearby galaxy cluster, A2255, by the AKARI space telescope. Using AKARIs continuous wavelength coverage between 3 and 24 μm and the wide field of view, we investigate the properties of cluster member galaxies to see how the infall of the galaxies, the cluster substructures, and the cluster-cluster merger influence their evolution. We show that the excess of MIR (~11 μm) flux is a good indicator for discriminating galaxies at different evolutionary stages and for dividing galaxies into three classes accordingly: strong MIR-excess (N3 – S11 > 0.2) galaxies that include both unobscured and obscured star-forming galaxies; weak MIR-excess (–2.0 5 Gyr) galaxies where the MIR emission arises mainly from the circumstellar dust around AGB stars; and intermediate MIR-excess (–1.2 < N3 – S11 < 0.2) galaxies in between the two classes that are less than a few Gyr old past the prime star formation activity. With the MIR-excess diagnostics, we investigate how local and cluster-scale environments affect the individual galaxies. We derive the total star formation rate (SFR) and the specific SFR of A2255 using the strong MIR-excess galaxies. The dust-free, total SFR of A2255 is ~130 M_⊙ yr^(–1), which is consistent with the SFRs of other clusters of galaxies at similar redshifts and with similar masses. We find no strong evidence that supports enhanced star formation either inside the cluster or in the substructure region, suggesting that the infall or the cluster merging activities tend to suppress star formation. The intermediate MIR-excess galaxies, representing galaxies in transition from star-forming galaxies to quiescent galaxies, are located preferentially at the medium density region or cluster substructures with higher surface density of galaxies. Our findings suggest that galaxies are being transformed from star-forming galaxies into red, quiescent galaxies from the infall region through near the core which can be explained well by ram-pressure stripping as previous simulation results suggest. We conclude that the cluster merging and the group/galaxy infall suppress star formation and transform galaxies from star-forming galaxies into quiescent galaxies, most likely due to ram-pressure stripping.

DISCOVERY OF THE MOST ISOLATED GLOBULAR CLUSTER IN THE LOCAL UNIVERSE

We report the discovery of two new globular clusters in the remote halos of M81 and M82 in the M81 Group based on Hubble Space Telescope archive images. They are brighter than typical globular clusters (MV = ?9.34?mag for GC-1 and MV = ?10.51?mag for GC-2), and much larger than known globular clusters with similar luminosity in the Milky Way Galaxy and M81. Radial surface brightness profiles for GC-1 and GC-2 do not show any features of tidal truncation in the outer part. They are located much farther from both M81 and M82 in the sky, compared with previously known star clusters in these galaxies. Color-magnitude diagrams of resolved stars in each cluster show a well-defined red giant branch (RGB), indicating that they are metal-poor and old. We derive a low metallicity with [Fe/H] ?2.3 and an old age ~14?Gyr for GC-2 from the analysis of the absorption lines in its spectrum in the Sloan Digital Sky Survey in comparison with the simple stellar population models. The I-band?magnitude of the tip of the RGB for GC-2 is 0.26?mag fainter than that for the halo stars in the same field, showing that GC-2 is ~400?kpc behind the M81 halo along our line of sight. The deprojected distance to GC-2 from M81 is much larger than any other known globular clusters in the local universe. This shows that GC-2 is the most isolated globular cluster in the local universe.

Ks -BAND LUMINOSITY EVOLUTION OF THE ASYMPTOTIC GIANT BRANCH POPULATION BASED ON STAR CLUSTERS IN THE LARGE MAGELLANIC CLOUD

We present a study of Ks -band luminosity evolution of the asymptotic giant branch (AGB) population in simple stellar systems using star clusters in the Large Magellanic Cloud (LMC). We determine physical parameters of LMC star clusters including center coordinates, radii, and foreground reddenings. Ages of 83 star clusters are derived from isochrone fitting with the Padova models, and those of 19 star clusters are taken from the literature. The AGB stars in 102 star clusters with log(age) = 7.3-9.5 are selected using near-infrared color-magnitude diagrams based on Two Micron All Sky Survey photometry. Then we obtain the Ks -band luminosity fraction of AGB stars in these star clusters as a function of ages. The Ks -band luminosity fraction of AGB stars increases, on average, as age increases from log(age) ~ 8.0, reaching a maximum at log(age) ~ 8.5, and it decreases thereafter. There is a large scatter in the AGB luminosity fraction for given ages, which is mainly due to stochastic effects. We discuss this result in comparison with five simple stellar population models. The maximum Ks -band AGB luminosity fraction for bright clusters is reproduced by the models that expect the value of 0.7-0.8 at log(age) = 8.5-8.7. We discuss the implication of our results with regard to the study of size and mass evolution of galaxies.

A STUDY OF DWARF GALAXIES EMBEDDED IN A LARGE-SCALE Hɪ RING IN THE LEO I GROUP

A large-scale neutral hydrogen (H i) ring serendipitously found in the Leo I galaxy group is 200 kpc in diameter with MH i ∼ 1.67 × 10M , unique in size in the Local Universe. It is still under debate where this H i ring originated whether it has formed out of the gas remaining after the formation of a galaxy group (primordial origin) or been stripped during galaxy-galaxy interactions (tidal origin). We are investigating the optical and H i gas properties of the dwarf galaxies located within the gas ring in order to probe its formation mechanism. In this work, we present the photometric properties of the dwarfs inside the ring using the CFHT MegaCam u∗, g ′, r ′ and i ′-band data. We discuss the origin of the gas ring based on the stellar age and metal abundance of dwarf galaxies contained within it.

Star clusters in the starburst galaxy M82

We present a study of star clusters in the starburst galaxy M82 using the BV I mosaic images taken with the Hubble Space Telescope’s Advanced Camera for Surveys. We have selected about 850 clusters with V < 23 mag based on their morphological information. The brightest cluster is as bright as V ∼ 16.5 mag (MV ∼ −11.2 mag) and most clusters are fainter than V = 18 mag (MV = −9.7 mag). The V -band luminosity function is represented by a power law with a slope α ∼ −2.0 in the range of −9.5 < MV < −7 mag. The star clusters in M82 are mainly distributed in the galaxy’s disk. However, about a dozen clusters are found far from the disk and are considered to belong to the halo of M82. The color–magnitude diagrams of star clusters show that most star clusters are highly reddened. We have derived the age of the star clusters using a spectral-energy-distribution fitting method with Bruzual & Charlot simple stellar population models. We discuss the age distribution as well as the photometric properties of the star clusters in regard to the formation history of the M82 cluster population.