arXiv subjects

Narae Hwang

Publications and source records attributed to Narae Hwang.

At least 55 records · Page 3Linked to original sources

arXiv2014

Spectroscopic Study of Extended Star Clusters in Dwarf Galaxy NGC 6822

We present a spectroscopic study of the four extended star clusters (ESCs) in NGC 6822 based on the data obtained with Gemini Multi-Object Spectrograph (GMOS) on the Gemini-South 8.1m telescope. Their radial velocities derived from the spectra range from $-61.2 \pm 20.4$ km s$^{-1}$ (for C1) to $-115.34 \pm 57.9$ km s$^{-1}$ (for C4) and, unlike the intermediate age carbon stars, they do not display any sign of systematic rotation around NGC 6822. The ages and metallicities derived using the Lick indices show that the ESCs are old ($\geq 8$ Gyr) and metal poor ([Fe/H]$\lesssim -1.5$). NGC 6822 is found to have both metal poor ([Fe/H]$\approx -2.0$) and metal rich ([Fe/H]$\approx -0.9$) star clusters within 15 arcmin (2 kpc) from the center, while only metal poor clusters are observed in the outer halo with $r \geq 20$ arcmin (2.6 kpc). The kinematics, old ages, and low metallicities of ESCs suggest that ESCs may have accreted into the halo of NGC 6822. Based on the velocity distribution of ESCs, we have determined the total mass and the mass-to-light ratio of NGC 6822: $M_{N6822} = 7.5^{+4.5}_{-0.1} \times 10^{9} M_{\odot}$ and $(M/L)_{N6822} = 75^{+45}_{-1} (M/L)_{\odot}$. It shows that NGC 6822 is one of the most dark matter dominated dwarf galaxies in the Local Group.

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arXiv2013

A new HST/Herschel deep field at the North Ecliptic Pole: preparing the way for JWST, SPICA and Euclid

We propose a co-ordinated multi-observatory survey at the North Ecliptic Pole. This field is the natural extragalactic deep field location for most space observatories (e.g. containing the deepest Planck, WISE and eROSITA data), is in the continuous viewing zones for e.g. Herschel, HST, JWST, and is a natural high-visibility field for the L2 halo orbit of SPICA with deep and wide-field legacy surveys already planned. The field is also a likely deep survey location for the forthcoming Euclid mission. It is already a multi-wavelength legacy field in its own right (e.g. AKARI, LOFAR, SCUBA-2): the outstanding and unparalleled continuous mid-IR photometric coverage in this field and nowhere else enables a wide range of galaxy evolution diagnostics unachievable in any other survey field, by spanning the wavelengths of redshifted PAH and silicate features and the peak energy output of AGN hot dust. We argue from the science needs of Euclid and JWST, and from the comparative multiwavelength depths, that the logical approach is (1) a deep (H-UDF) UV/optical tile in the NEP over ~10 square arcminutes, and (2) an overlapping wide-field UV/optical HST survey tier covering >100 square arcminutes, with co-ordinated submm SPIRE mapping up to or beyond the submm point source confusion limit over a wider area and PACS data over the shallower HST tier.

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arXiv2013

The Star Cluster System in the Nearby Starburst Galaxy M82

We present a photometric study of star clusters in the nearby starburst galaxy M82 based on the UBVI, YJ and H band Hubble Space Telescope images. We find 1105 star clusters with V<23 mag. 1070 of them are located in the disk region, while 35 star clusters are in the halo region. The star clusters in the disk are composed of a dominant blue population with a color peak at (B-V)0 ~ 0.45, and a weaker red population. The luminosity function of the disk clusters shows a power-law distribution with a power law index alpha=-2.04+-0.03, and the scale height of their distribution is h_z=9".64+-0".40 (164 +- 7 pc), similar to that of the stellar thin disk of M82. We have derived the ages of ~630 star clusters using the spectral energy distribution fit method by comparing UBVI(YJ)H band photometric data with the simple stellar population models. The age distribution of the disk clusters shows that the most dominant cluster population has ages ranging from 100 Myr to 1 Gyr, with a peak at about 500 Myr. This suggests that M82 has undergone a disk-wide star formation about 500 Myr ago, probably through the interaction with M81. The brightest star clusters in the nuclear region are much brighter than those in other regions, indicating that more massive star clusters are formed in the denser environments. On the other hand, the colors of the halo clusters are similar to those of globular clusters in the Milky Way Galaxy, and their ages are estimated to be older than 1 Gyr. These are probably genuine old globular clusters in M82.

astro-ph.CO↗

arXiv2012

GMC Evolutions in the Nearby Spiral Galaxy M33

We present a Giant Molecular Cloud (GMC) catalog toward M33, containing 71 GMCs in total, based on wide field and high sensitivity CO(J=3-2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and HII regions enable us to classify GMCs into four categories: Type A showing no sign of massive star formation (SF), Type B being associated only with HII regions, Type C with both HII regions and <10 Myr-old YSGs and Type-D with both HII regions and 10--30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >10^5 Mo is estimated to be 20--40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J=3-2)/CO(J=1-0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.

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arXiv2012

Origin of Extended Star Clusters

We have discovered new extended star clusters (ESCs) in a nearby dIrr galaxy NGC 6822. These clusters are the nearest sample of ESCs available to date. The key characteristic of ESCs is their large size compared to typical globular clusters even though the two cluster populations are rather similar in terms of other parameters, i.e., color and luminosity. Several scenarios have been suggested to explain the formation of ESCs. However, the currently known ESCs may be a mixture of populations with heterogeneous formation histories. Future observational and theoretical studies are expected to better constrain the origins of ESCs as well as to increase their sample size.

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arXiv2011

Extended Star Clusters in the Remote Halo of the Intriguing Dwarf Galaxy NGC 6822

We present a study on four new star clusters discovered in the halo of the intriguing dwarf irregular galaxy NGC 6822 from a wide field survey covering 3 deg x 3 deg area carried out with MegaCam at Canada-France-Hawaii Telescope (CFHT). The star clusters have extended structures with half-light radii R_h ~ 7.5 -- 14.0 pc, larger than typical Galactic globular clusters and other known globular clusters in NGC 6822. The integrated colors and color magnitude diagrams (CMD) of resolved stars suggest that the new star clusters are 2 -- 10 Gyr old and relatively metal poor with Z=0.0001--0.004 based on the comparison with theoretical models. The projected distance of each star cluster from the galaxy center ranges from 10.7 arcmin (~ 1.5 kpc) to 77 arcmin (~ 11 kpc), far beyond the optical body of the galaxy. Interestingly, the new star clusters are aligned along the elongated old stellar halo of NGC 6822, which is almost perpendicular to the HI gas distribution where young stellar populations exist. We also find that the colors and half-light radii of the new clusters are correlated with the galactocentric distance: clusters farther from the galaxy center are larger and bluer than those closer to the galaxy center. We discuss the stellar structure and evolution of NGC 6822 implied by these new extended star clusters in the halo. We also discuss the current status of observational and theoretical understandings regarding the origin of extended star clusters in NGC 6822 and other galaxies.

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arXiv2011

HII Region Luminosity Function of the Interacting Galaxy M51

We present a study of HII regions in M51 using the Hubble Space Telescope ACS images taken as part of the Hubble Heritage Program. We have catalogued about 19,600 HII regions in M51 with Ha luminosity in the range of L = 10^{35.5} erg/s to 10^{39.0} erg/s. The Ha luminosity function of HII regions (HII LF) in M51 is well represented by a double power law with its index alpha=-2.25\pm0.02 for the bright part and alpha=-1.42\pm0.01 for the faint part, separated at a break point L= 10^{37.1} erg/s. This break was not found in previous studies of M51 HII regions. Comparison with simulated HII LFs suggests that this break is caused by the transition of HII region ionizing sources, from low mass clusters (with ~ 10^3 M_sun, including several OB stars) to more massive clusters (including several tens of OB stars). The HII LFs with L < 10^{37.1} erg/s are found to have different slopes for different parts in M51: the HII LF for the interarm region is steeper than those for the arm and the nuclear regions. This observed difference in HII LFs can be explained by evolutionary effects that HII regions in the interarm region are relatively older than those in the other parts of M51.

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arXiv2010

Merging Galaxy Cluster Abell 2255 in Mid-Infrared

We present the mid-infrared (MIR) observation of a nearby galaxy cluster, Abell 2255 by the AKARI space telescope. Using the AKARI's continuous wavelength coverage between 3-24 micron 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 micron) flux is a good indicator to discriminate galaxies at different evolutionary stages, and divide 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 Gyrs 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 of ~130 Msun/yr for A2255 using the strong MIR-excess galaxies, which is consistent with other clusters of galaxies at similar redshifts and with similar masses. We find no strong evidence that supports enhanced star formation neither inside the cluster nor in the substructure region. 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. 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 well-explained by the ram-pressure stripping as previous simulation suggests.

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arXiv2010

Wandering globular clusters: the first dwarf galaxies in the universe?

In the last decade we witness an advent of new types of dwarf stellar systems in cluding ultra-compact dwarfs, ultra-faint dwarf spheroidals, and exotic globular clusters, breaking the old simple paradigm for dwarf galaxies and globular clusters. These objects become more intriguing, and understanding of these new findings be comes more challenging. Recently we discovered a new type of large scale structure in the Virgo cluster of galaxies: it is composed of globular clusters. Globular clusters in Virgo are found wandering between galaxies (intracluster globular clusters) as well as in galaxies. These intracluster globular clusters fill a significant fraction in the area of the Virgo cluster and they are dominated by blue globular clusters. These intracluster globular clusters may be closely related with the first dwarf galaxies in the universe.

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arXiv2010

A Wide Field Survey of Satellite Galaxies around the Spiral Galaxy M106

We present a wide field survey of satellite galaxies in M106 (NGC 4258) covering a $1.7\degr \times 2\degr$ field around M106 using Canada-France-Hawaii Telescope/MegaCam. We find 16 satellite galaxy candidates of M106. Eight of these galaxies are found to be dwarf galaxies that are much smaller and fainter than the remaining galaxies. Four of these galaxies are new findings. Surface brightness profiles of 15 out of 16 satellite galaxies can be represented well by an exponential disk profile with varying scale length. We derive the surface number density distribution of these satellite galaxies. The central number density profile (d $<100$ kpc) is well fitted by a power-law with a power index of $-2.1\pm0.5$, similar to the expected power index of isothermal distribution. The luminosity function of these satellites is represented well by the Schechter function with a faint end slope of $-1.19^{+0.03}_{-0.06}$. Integrated photometric properties (total luminosity, total colour, and disk scale length) and the spatial distribution of these satellite galaxies are found to be roughly similar to those of the Milky Way and M31.

astro-ph.CO↗

arXiv2010

A deep radio survey of the AKARI North Ecliptic Pole Field - WSRT 20 cm Radio survey description, observations and data reduction

The Westerbork Radio Synthesis Telescope, WSRT, has been used to make a deep radio survey of an ~ 1.7 sq degree field coinciding with the AKARI North Ecliptic Pole Deep Field. The observations, data reduction and source count analysis are presented, along with a description of the overall scientific objectives. The survey consisted of 10 pointings, mosaiced with enough overlap to maintain a similar sensitivity across the central region that reached as low as 21 microJy per beam at 1.4 GHz. A catalogue containing 462 sources detected with a resolution of 17"x15" is presented. The differential source counts calculated from the WSRT data have been compared with those from the shallow VLA-NEP survey of Kollgaard et al 1994, and show a pronounced excess for sources fainter than ~ 1 mJy, consistent with the presence of a population of star forming galaxies at sub-mJy flux levels. The AKARI North Ecliptic Pole Deep field is the focus of a major observing campaign conducted across the entire spectral region. The combination of these data sets, along with the deep nature of the radio observations will allow unique studies of a large range of topics including the redshift evolution of the luminosity function of radio sources, the clustering environment of radio galaxies, the nature of obscured radio-loud active galactic nuclei, and the radio/far-infrared correlation for distant galaxies. This catalogue provides the basic data set for a future series of paper dealing with source identifications, morphologies, and the associated properties of the identified radio sources.

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arXiv2009

Vestige of the Star Cluster Burst in M51

We present a study of the star cluster formation in M51 based on the image data taken with Advanced Camera for Surveys (ACS) and Wide Field Planetary Camera 2 (WFPC2) of Hubble Space Telescope (HST). We have derived the star cluster formation rate using the ages and masses of about 2,000 star clusters estimated by comparing photometric data (F336W, F435W, F555W, and F814W) with theoretical population synthesis models. The star cluster formation rate increased significantly during the period of 100 ~ 250 Myr ago. This period roughly coincides with the epoch of dynamical encounters of two galaxies, NGC 5194 and NGC 5195, expected by theoretical models. The age distribution of the star clusters also shows two peaks at about 100 Myr and 250 Myr ago. The star cluster mass ranges from 10^3 to 10^6 M_{\odot} and the mass function can be represented by a power law with an index ranging from α= -2.23 \pm 0.34 for t<10 Myr to α= -1.37 \pm 0.11 for t>100 Myr. The mass function of star clusters older than 10 Myr also appears to display the steepest distribution with α\approx -1.50 at around 200 Myr ago, near the expected epoch of the galaxy interaction. We also confirm the correlations of cluster size increasing with cluster mass (with a best fit slope of 0.16 \pm 0.02), and with cluster age (0.14 \pm 0.03).

astro-ph.CO↗

arXiv2009

Reddening and Distance of the Local Group Starburst Galaxy IC 10

We estimate the reddening and distance of the nearest starburst galaxy IC 10 using deep near infrared $JHK_{S}$ photometry obtained with the Multi-Object InfraRed Camera and Spectrograph (MOIRCS) on the Subaru telescope. We estimate the foreground reddening toward IC 10 using $UBV$ photometry of IC 10 from the Local Group Survey, obtaining $E(B-V)=0.52\pm 0.04$ mag. We derive the total reddening including the internal reddening, $E(B-V)=0.98\pm 0.06$ mag, using $UBV$ photometry of early-type stars in IC 10 and comparing $JHK_{S}$ photometry of red giant branch stars in IC 10 and the SMC. Using the 2MASS point source catalog of 20 Galactic globular clusters, we derive a relation between the metallicity [Fe/H]$_{CG97}$ and the slope of the red giant branch in the $K_{S}- (J-K_{S})$ color-magnitude diagram. The mean metallicity of the red giant branch stars in IC 10 is estimated to be [Fe/H]$_{CG97}=-1.08\pm0.28$. The magnitude of the tip of the red giant branch (TRGB) of IC 10 in the $K_{S}$ band is measured to be $K_{S,TRGB}=18.28\pm0.01$. Based on the TRGB method, we estimate the distance modulus of IC 10 to be $(m-M)_{0}=24.27\pm0.03{\rm (random)}\pm0.18{\rm (systematic)}$, corresponding to the distance of $d=715\pm10\pm60$ kpc. This confirms that IC 10 is a member of the Local Group.

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arXiv2009

The Mid-infrared View of Red Sequence Galaxies in Abell 2218 with AKARI

We present the {\it AKARI} InfraRed Camera (IRC) imaging observation of early-type galaxies in A2218 at z $\simeq$ 0.175. Mid-infrared (MIR) emission from early-type galaxies traces circumstellar dust emission from AGB stars or/and residual star formation. Including the unique imaging capability at 11 and 15 $μ$m, our {\it AKARI} data provide an effective way to investigate MIR properties of early-type galaxies in the cluster environment. Among our flux-limited sample of 22 red sequence early-type galaxies with precise dynamical and line strength measurements ($<$ 18 mag at 3 $μm$), we find that at least 41% have MIR-excess emission. The $N3-S11$ versus $N3$ (3 and 11 $μ$m) color-magnitude relation shows the expected blue sequence, but the MIR-excess galaxies add a red wing to the relation especially at the fainter end. A SED analysis reveals that the dust emission from AGB stars is the most likely cause for the MIR-excess, with low level of star formation being the next possible explanation. The MIR-excess galaxies show a wide spread of $N3-S11$ colors, implying a significant spread (2--11 Gyr) in the estimated mean ages of stellar populations. We study the environmental dependence of MIR-excess early-type galaxies over an area out to a half virial radius ($\sim$1 Mpc). We find that the MIR-excess early-type galaxies are preferentially located in the outer region. From these evidences, we suggest that the fainter, MIR-excess early-type galaxies have just joined the red sequence, possibly due to the infall and subsequent morphological/spectral transformation induced by the cluster environment.

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arXiv2009

North Ecliptic Pole Wide Field Survey of AKARI: Survey Strategy and Data Characteristics

We present the survey strategy and the data characteristics of the North Ecliptic Pole (NEP) Wide Survey of AKARI. The survey was carried out for about one year starting from May 2006 with 9 passbands from 2.5 to 24 micron and the areal coverage of about 5.8 sq. degrees centered on NEP. The survey depth reaches to 21.8 AB magnitude near infrared (NIR) bands, and ~ 18.6 AB maggnitude at the mid infrared (MIR) bands such as 15 and 18 micron. The total number of sources detected in this survey is about 104,000, with more sources in NIR than in the MIR. We have cross matched infrared sources with optically identified sources in CFHT imaging survey which covered about 2 sq. degrees within NEP-Wide survey region in order to characterize the nature of infrared sources. The majority of the mid infrared sources at 15 and 18 micron band are found to be star forming disk galaxies, with smaller fraction of early type galaxies and AGNs. We found that a large fraction (60~80 %) of bright sources in 9 and 11 micron stars while stellar fraction decreases toward fainter sources. We present the histograms of the sources at mid infrared bands at 9, 11, 15 and 18 micron. The number of sources per magnitude thus varies as m^0.6 for longer wavelength sources while shorter wavelength sources show steeper variation with m, where m is the AB magnitude.

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arXiv2007

A Catalog of Bright Star Clusters in the Interacting Galaxy M51

We present a catalog of star clusters with V_{F555W}<23 mag detected in the interacting spiral galaxy M51 system based on the mosaic images taken with HST ACS by the Hubble Heritage Team. We have selected about 3,600 clusters based on their morphological information through the visual inspection. The final star cluster catalog includes 2,224 clusters that are relatively well isolated and have a circular shape. The star clusters in M51 are mostly distributed around the spiral arms of NGC 5194. The color-magnitude diagrams show that most of the star clusters in M51 are bluer than (B_{F435W}-V_{F555W}) = 0.5 and (V_{F555W}-I_{F814W}) = 0.8. There are also some red star clusters with (B_{F435W}-V_{F555W}) > 0.7, uniformly distributed over the M51 field. Some of these red clusters are suspected to be a part of the halo or old disk population based on their old ages (t >= 10^9 yrs) and their spatial distribution. The luminosity function of the star clusters is fit well by a single power law with α= -2.59 \pm 0.03 for the range -10.0 <M_V < -8.0 mag. We find that the size distribution of the star clusters can be fit with three Gaussian components with peaks at effective radii of 2.27, 4.80 and 7.51 pc. Some large star clusters with red color are faint fuzzy clusters, and they are distributed not only around NGC 5195 but also around NGC 5194. These faint fuzzies are found to display an elongated spatial distribution, while the normal compact red clusters show a relatively uniform distribution around NGC 5194.

astro-ph↗

arXiv2007

Optical Identification of 15 Micron Sources in the AKARI Performance Verification Field toward the North Ecliptic Pole

We present the results of optical identifications for 257 mid-infrared sources detected with a deep 15um survey over approximately 80 arcmin^2 area in the AKARI performance verification field near the North Ecliptic Pole. The 15um fluxes of the sources range from 1 mJy down to 40 uJy, approximately a half of which are below 100 uJy. Optical counterparts were searched for within a 2-3 arcsec radius in both the BVRi'z' catalog generated by using the deep Subaru/Suprime-cam field which covers one-third of the performance verification field, and the g'r'i'z' catalog based on observations made with MegaCam at CFHT. We found B-R and R-z' colours of sources with successful optical identifications are systematically redder than that of the entire optical sample in the same field. Moreover, approximately 40% of the 15um sources show colours R-L15>5, which cannot be explained by the spectral energy distribution (SED) of normal quiescent spiral galaxies, but are consistent with SEDs of redshifted (z>1) starburst or ultraluminous infrared galaxies. This result indicates that the fraction of the ultraluminous infrared galaxies in our faint 15um sample is much larger than that in our brighter 15um sources, which is consistent with the evolving mid-infrared luminosity function derived by recent studies based on the Spitzer 24um deep surveys. Based on an SED fitting technique, the nature of the faint 15um sources is further discussed for a selected number of sources with available K_s-band data.

astro-ph↗

arXiv2007

Nature of Infrared Sources in 11 micron Selected Sample from Early Data of the AKARI North Ecliptic Pole Deep Survey

We present the properties of 11 $μ$m selected sources detected in the early data of the North Ecliptic Pole Deep (NEP-Deep) Survey of AKARI. The data set covers 6 wavelength bands from 2.5 to 11 $μ$m, with the exposure time of 10 ~ 20 minutes. This field lies within the CFHT survey with four filter bands ($g', r', i',z'), enabling us to establish nearly continuous spectral energy distributions (SEDs) for wavelengths ranging from 0.4 to 11 $μ$m. The main sample studied here consists of 71 sources whose 11 $μ$m AB magnitudes are equal to or brighter than 18.5 (251 $μ$Jy), which is complete to more than 90%. The 11 $μ$m band has an advantage of sampling star forming galaxies with low to medium redshifts since the prominent PAH feature shifts into this band. As expected, we find that the majority (~68%) of 11 $μ$m bright sources are star forming galaxies at 0.2 < z < 0.7 with $L_{IR} ~ 10^{10}$ -- $10^{12} L_{\odot}$ based on the detailed modelling of SEDs. We also find four AGNs lying at various redshifts in the main sample. In addition, we discuss a few sources which have non-typical SEDs of the main sample, including a brown dwarf candidate, a steep power-law source, flat spectrum object, and an early-type galaxy at moderate redshift.

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