Narae Hwang

Evolution of infrared luminosity functions of galaxies in the AKARI NEP-deep field - Revealing the cosmic star formation history hidden by dust

Aims. Dust-obscured star-formation increases with increasing intensity and increasing redshift. We aim to reveal the cosmic starformation nhistory obscured by dust using deep infrared observation with AKARI. nMethods. We constructed restframe 8 μm, 12 μm, and total infrared (TIR) luminosity functions (LFs) at 0.15 < z < 2.2 using n4128 infrared sources in the AKARI NEP-deep field. A continuous filter coverage in the mid-IR wavelength (2.4, 3.2, 4.1, 7, 9, 11, n15, 18, and 24 μm) by the AKARI satellite allowed us to estimate restframe 8 μm and 12 μm luminosities without using a large nextrapolation based on an SED fit, which was the largest uncertainty in previous work. nResults. We find that all 8 μm (0.38 < z < 2.2), 12 μm (0.15 < z < 1.16), and TIR LFs (0.2 < z < 1.6) show continuous and strong nevolution toward higher redshift. Our direct estimate of 8 μm LFs is useful since previous work often had to use a large extrapolation nfrom the Spitzer 24 μm to 8 μm, where SED modeling is more difficult because of the PAH emissions. In terms of cosmic infrared nluminosity density (Ω_(IR)), which was obtained by integrating analytic fits to the LFs, we find good agreement with previous work at nz < 1.2. We find the ΩIR evolves as ∝(1 + z)^(4.4±1.0). When we separate contributions to Ω_(IR) by LIRGs and ULIRGs, we found more nIR luminous sources are increasingly more important at higher redshift. We find that the ULIRG (LIRG) contribution increases by a nfactor of 10 (1.8) from z = 0.35 to z = 1.4.

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{sup 0} x 3{sup 0} area carried out with MegaCam at the Canada-France-Hawaii Telescope. The star clusters have extended structures with half-light radii R{sub h} {approx} 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 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 ({approx}1.5 kpc) to 77 ({approx}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 H I 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 themorexa0» 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.«xa0less

A Catalog of Bright Star Clusters in the Interacting Galaxy M51

We present a catalog of star clusters with 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 ? 109 yrs) and their spatial distribution. The luminosity function of the star clusters is well fitted by a single power law with ? = ?2.59 ? 0.03 for the range ?10.0 < MV < ?8.0 mag. We find that the size distribution of the star clusters can be fitted 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.

An Optical Source Catalog of the North Ecliptic Pole Region

We present a five-band (u(*), g, r, i, z) optical photometry catalog of the sources in the north ecliptic pole (NEP) region based on deep observations made with MegaCam at CFHT. The source catalog covers about 2 square degree area centered at the NEP and reaches depths of about 26 mag for u*, g, r bands, about 25 mag for i band, and about 24 mag for z band (4 sigma detection over a 100 aperture). The total number of cataloged sources brighter than r 23 mag is about 56,000 including both point sources and extended sources. From the investigation of photometric properties using the color-magnitude diagrams and color-color diagrams, we have found that the colors of extended sources are mostly (u*- r) 0.5. This can be used to separate the extended sources from the point sources reliably, even for the faint source domain where typical morphological classification schemes hardly work efficiently. We have derived an empirical color-redshift relation of the red sequence galaxies using the Sloan Digital Sky Survey data. By applying this relation to our photometry catalog and searching for any spatial overdensities, we have found two galaxy clusters and one nearby galaxy group.

H II REGION LUMINOSITY FUNCTION OF THE INTERACTING GALAXY M51

We present a study of H II regions in M51 using the Hubble Space Telescope Advanced Camera for Surveys images taken as part of the Hubble Heritage Program. We have cataloged about 19,600 H II regions in M51 with Hα luminosity in the range of L = 1035.5-1039.0xa0ergxa0s–1. The Hα luminosity function of H II regions (H II LF) in M51 is well represented by a double power law with its index α = –2.25 ± 0.02 for the bright part and α = –1.42 ± 0.01 for the faint part, separated at a break point L = 1037.1xa0ergxa0s–1. This break was not found in previous studies of M51 H II regions. Comparison with simulated H II LFs suggests that this break is caused by the transition of H II region ionizing sources, from low-mass clusters (with ~103 M ☉, including several OB stars) to more massive clusters (including several tens of OB stars). The H II LFs with L < 1037.1xa0ergxa0s–1are found to have different slopes for different parts in M51: the H II LF for the interarm region is steeper than those for the arm and the nuclear regions. This observed difference in H II LFs can be explained by evolutionary effects: H II regions in the interarm region are relatively older than those in the other parts of M51.

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.

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 2000 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 103 to 106 M ☉ and the mass function can be represented by a power law with an index ranging from α = –2.23 ± 0.34 for t 100 Myr. The mass function of star clusters older than 10 Myr also appears to display the steepest distribution with α ≈ –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 ± 0.02), and with cluster age (0.14 ± 0.03).

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 the UBV photometry of IC 10 from the Local Group Survey, obtaining E(B – V) = 0.52 ± 0.04 mag. We derive the total reddening including the internal reddening, E(B – V) = 0.98 ± 0.06 mag, using the UBV photometry of early-type stars in IC 10 and comparing the JHK S photometry of red giant branch stars in IC 10 and the Small Magellanic Cloud. Using the Two Micron All Sky Survey 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 ± 0.28. 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 ± 0.01. Based on the TRGB method, we estimate the distance modulus of IC 10 to be (m – M)0 = 24.27 ± 0.03(random) ± 0.18(systematic), corresponding to the distance of d = 715 ± 10 ± 60 kpc. This confirms that IC 10 is a member of the Local Group.

A deep survey of the AKARI north ecliptic pole field . I. WSRT 20 cm radio survey description, observations and data reduction

Aims. The Westerbork Radio Synthesis Telescope, WSRT, has been used to make a deep radio survey of an ∼1.7 degree 2 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. Methods. 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 μJy beam −1 at 1.4 GHz. Results. A catalogue containing 462 sources detected with a resolution of 17.0 �� × 15.5 �� 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. Conclusions. 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.

The evolved asymptotic giant branch stars in the central bar of the dwarf irregular galaxy NGC 6822

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