Y.-I. Byun

Yonsei University, Seoul, Seoul, South Korea

Are you Y.-I. Byun?

Claim your profile

Publications (21)97.54 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We compare the dust attenuation properties of two samples of galaxies purely selected in the Galaxy Evolution Explorer (GALEX) near-ultraviolet band (NUV; 1750-2750 Å, λmean = 2310 Å) and in the far-infrared (FIR) at 60 μm. These samples are built using the GALEX and IRAS sky surveys over ~600 deg2. The NUV-selected sample contains 95 galaxies detected down to NUV = 16 mag (AB system). Eighty-three galaxies in this sample are spiral or irregular, and only two of them are not detected at 60 μm. The FIR-selected sample is built from the IRAS PSCz survey, which is complete down to 0.6 Jy. Among the 163 sources, we select 118 star-forming galaxies that are well measured by IRAS; all but one are detected in NUV, and 14 galaxies are not detected in the far-ultraviolet band (FUV; 1350-1750 Å, λmean = 1530 Å). The dust-to-ultraviolet (NUV and FUV) flux ratio is calibrated to estimate the dust attenuation at both wavelengths. The median value of the attenuation in NUV is found to be ~1 mag for the NUV-selected sample, versus ~2 mag for the FIR-selected one. Within both samples, the dust attenuation is found to correlate with the luminosity of the galaxies. Almost all the NUV-selected galaxies and two-thirds of the FIR-selected sample exhibit a lower dust attenuation than expected from the tight relation found previously for starburst galaxies between dust attenuation and the slope of the ultraviolet continuum. The situation is reversed for the remaining third of the FIR-selected galaxies: their extinction is higher than that deduced from their FUV - NUV color and the relation for starbursts.
    The Astrophysical Journal 12/2008; 619(1):L51. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In a companion Letter (Arnouts et al.) we present new measurements of the galaxy luminosity function at 1500 Å out to z ~ 1 using Galaxy Evolution Explorer VIMOS-VLT Deep Survey observations (1039 galaxies with NUV ≤ 24.5 and z > 0.2) and at higher z using existing data sets. In this Letter we use the same sample to study evolution of the FUV luminosity density ρ1500. We detect evolution consistent with a (1 + z)2.5±0.7 rise to z ~ 1 and (1 + z)0.5±0.4 for z > 1. The luminosity density from the most UV-luminous galaxies (UVLGs) is undergoing dramatic evolution (30 times) between 0 < z < 1. UVLGs are responsible for a significant fraction (>25%) of the total far-ultraviolet luminosity density at z ~ 1. We measure dust attenuation and star formation rates of our sample galaxies and determine the star formation rate density () as a function of redshift, both uncorrected and corrected for dust. We find good agreement with other measures of in the rest ultraviolet and Hα given the still significant uncertainties in the attenuation correction.
    The Astrophysical Journal 12/2008; 619(1):L47. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This work presents the main ultraviolet (UV) and far-infrared (FIR) properties of two samples of nearby galaxies selected from the GALEX (λ = 2315 Å, hereafter NUV) and IRAS (λ = 60 μm) surveys, respectively. They are built in order to obtain detection at both wavelengths for most of the galaxies. Star formation rate (SFR) estimators based on the UV and FIR emissions are compared. Systematic differences are found between the SFR estimators for individual galaxies based on the NUV fluxes corrected for dust attenuation and on the total IR luminosity. A combined estimator based on NUV and IR luminosities seems to be the best proxy over the whole range of values of SFR. Although both samples present similar average values of the birthrate parameter b, their star-formation-related properties are substantially different: NUV-selected galaxies tend to show larger values of b for lower masses, SFRs, and dust attenuation, supporting previous scenarios of star formation history (SFH). Conversely, about 20% of the FIR-selected galaxies show high values of b, SFR, and NUV attenuation. These galaxies, most of them being LIRGs and ULIRGs, break down the downsizing picture of SFH; however, their relative contribution per unit volume is small in the local universe. Finally, the cosmic SFR density of the local universe is estimated in a consistent way from the NUV and IR luminosities.
    The Astrophysical Journal Supplement Series 12/2008; 164(1):38. · 16.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have matched 358,046 objects in 143 deg2 of overlap between the Galaxy Evolution Explorer (GALEX) and the Sloan Digital Sky Survey (SDSS). This Letter provides matching statistics at Medium Imaging Survey and All-sky Imaging Survey depths for SDSS-classified stars and galaxies as well as spectroscopic star, galaxy, and QSO subsamples. Distributions of magnitudes, redshifts, and colors are provided. Near-ultraviolet (NUV)-based color-color diagrams reveal (1) the segregation of main-sequence, horizontal-branch, subdwarf, white dwarf, M dwarf, and M dwarf-white dwarf binary stellar populations, (2) a bimodal distribution of galaxies, and (3) the higher signal quality of the GALEX NUV compared to the SDSS u band.
    The Astrophysical Journal 12/2008; 619(1):L23. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: New Galaxy Evolution Explorer (GALEX) observations have detected significant far-UV (FUV; 1530 Å) and near-UV (NUV; 2310 Å) emission from stellar substructures within the tidal tails of four ongoing galaxy mergers. The UV-bright regions are optically faint and are coincident with H I density enhancements. FUV emission is detected at any location where the H I surface density exceeds ~2 M☉ pc-2, and it is often detected in the absence of visible wavelength emission. UV luminosities of the brighter regions of the tidal tails imply masses of 106 to ~109 M☉ in young stars in the tails, and H I luminosities imply similar H I masses. UV-optical colors of the tidal tails indicate stellar populations as young as a few megayears, and in all cases ages under 400 Myr. Most of the young stars in the tails formed in single bursts, rather than resulting from continuous star formation, and they formed in situ as the tails evolved. Star formation appears to be older near the parent galaxies and younger at increasing distances from the parent galaxy. This could be because the star formation occurs progressively along the tails, or because the star formation has been inhibited near the galaxy/tail interface. The youngest stellar concentrations, usually near the ends of long tidal tails, have masses comparable to confirmed tidal dwarf galaxies and may be newly forming galaxies undergoing their first burst of star formation.
    The Astrophysical Journal 12/2008; 619(1):L91. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have used the Galaxy Evolution Explorer UV photometric data to construct a first near-UV (NUV) color-magnitude relation (CMR) for the galaxies preclassified as early-type by Sloan Digital Sky Survey studies. The NUV CMR is a powerful tool for tracking the recent star formation history in early-type galaxies, owing to its high sensitivity to the presence of young stellar populations. Our NUV CMR for UV-weak galaxies shows a well-defined slope and thus will be useful for interpreting the rest-frame NUV data of distant galaxies and studying their star formation history. Compared to optical CMRs, the NUV CMR shows a substantially larger scatter, which we interpret as evidence of recent star formation activities. Roughly 15% of the recent epoch (z < 0.13) bright [M(r) < -22] early-type galaxies show a sign of recent (1 Gyr) star formation at the 1%-2% level (lower limit) in mass compared to the total stellar mass. This implies that low-level residual star formation was common during the last few billion years even in bright early-type galaxies.
    The Astrophysical Journal 12/2008; 619(1):L111. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We test the empirical relation between ultraviolet color and attenuation as derived for starburst galaxies with a wide assortment of galaxy types detected by the Galaxy Evolution Explorer and find that it systematically overestimates the far-ultraviolet attenuation of our sample by ~0.5 mag. Our efforts to find an additional parameter that could improve the starburst reddening relation were unsuccessful. In particular, UV - Ks colors (in nonmatching apertures) show no correlation with the offset from the starburst reddening relation, suggesting either that UV - Ks colors are a poor tracer of present to past average star formation history (the "b" parameter) or that the intrinsic dust distribution/geometry may be responsible for moving galaxies off the correlation. It is possible to reduce the systematic overestimate of AFUV by using the linear correlation derived from our sample, which simply lowers the starburst predicted values of AFUV by 0.58 mag. The scatter, however, remains large at 0.89 mag.
    The Astrophysical Journal 12/2008; 619(1):L55. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present GALEX near-ultraviolet (2310 Å) and far-ultraviolet (1530 Å) images of the interacting galaxy NGC 4438 (Arp 120) in the center of the Virgo Cluster. These images show an extended (20 kpc) tidal tail at the northwest edge of the galaxy that was previously undetected at other wavelengths; this tail is 15-25 kpc from NGC 4438's nucleus. Except for in the nucleus, the UV morphology of NGC 4438 is totally different from the Hα + [N II] morphology, which is more similar to the X-ray emission, confirming its gas cooling origin. We study the star formation history of NGC 4438 by combining spectrophotometric data in the UV-visible-near-IR wavelength range with population synthesis and galaxy evolution models. The data are consistent with a recent (~10 Myr), instantaneous burst of star formation in the newly discovered UV northwestern tail that is significantly younger than the age of the tidal interaction with NGC 4435, dated by dynamical models at ~100 Myr ago. Recent star formation events are also present at the edge of the northern arm and in the southern tail, while totally lacking in the other regions, which are dominated by the old stellar population that was perturbed during the dynamical interaction with NGC 4435. The contribution of this recent starburst to the total galaxy stellar mass is lower than 0.1%, an extremely low value for such a violent interaction. High-velocity, off-center tidal encounters such as that observed in Arp 120 are thus not sufficient to significantly increase the star formation activity of cluster galaxies.
    The Astrophysical Journal 12/2008; 623(1):L13. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the Galaxy Evolution Exlorer (GALEX) near-ultraviolet (2310 Å) and far-ultraviolet (1530 Å) galaxy luminosity functions of the nearby cluster of galaxies A1367 in the magnitude range -20.3 ≤ MAB ≤ -13.3. The luminosity functions are consistent with previous (~2 mag shallower) estimates based on the FOCA and FAUST experiments, but they display a steeper faint-end slope than the GALEX luminosity function for local field galaxies. Using spectrophotometric optical data, we select star-forming systems from quiescent galaxies and study their separate contributions to the cluster luminosity function. We find that the UV luminosity function of cluster star-forming galaxies is consistent with the field. The difference between the cluster and field luminosity functions is entirely due to the contribution at low luminosities (MAB > -16 mag) of non-star-forming, early-type galaxies that are significantly overdense in clusters.
    The Astrophysical Journal 12/2008; 623(1):L17. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present the first measurement of the galaxy luminosity function (LF) at 1500 Å in the range 0.2 ≤ z ≤ 1.2 based on Galaxy Evolution Explorer VIMOS-VLT Deep Survey observations (~1000 spectroscopic redshifts for galaxies with NUV ≤ 24.5) and at higher z using existing data sets. Our main results are summarized as follows: (1) Luminosity evolution is observed with ΔM* ~ -2.0 mag between z = 0 and z = 1 and ΔM* ~ -1.0 mag between z = 1 and z = 3. This confirms that the star formation activity was significantly higher in the past. (2) The LF slopes vary in the range -1.2 ≥ α ≥ -1.65, with a marginally significant hint of increase at higher z. (3) We split the sample in three rest-frame (B - I) intervals, providing an approximate spectral type classification: Sb-Sd, Sd-Irr, and unobscured starbursts. We find that the bluest class evolves less strongly in luminosity than the two other classes. On the other hand, their number density increases sharply with z (~15% in the local universe to ~55% at z ~ 1), while that of the reddest classes decreases.
    The Astrophysical Journal 12/2008; 619(1):L43. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The ELAIS S1 field was observed by GALEX in both its Wide Spectroscopic and Deep Imaging Survey modes. This field was previously observed by the Infrared Space Observatory and we made use of the catalogue of multi-wavelength data published by the ELAIS consortium to select galaxies common to the two samples. Among the 959 objects with GALEX spectroscopy, 88 are present in the ELAIS catalog and 19 are galaxies with an optical spectroscopic redshift. The distribution of redshifts covers the range $0<z<1.6$. The selected galaxies have bolometric IR luminosities $10<Log(L_{IR})<13$ (deduced from the $15 \mu m$ flux using ISOCAM) which means that we cover a wide range of galaxies from normal to Ultra Luminous IR Galaxies. The mean ($\sigma$) UV luminosity (not corrected for extinction) amounts to $Log(\lambda.L_{1530}) = 9.8 (0.6)$ $L_\sun$ for the low-z ($z \le 0.35$) sample. The UV slope $\beta$ (assuming $f_\lambda \propto \lambda^\beta$) correlates with the GALEX FUV-NUV color if the sample is restricted to galaxies below $z < 0.1$. Taking advantage of the UV and IR data, we estimate the dust attenuation from the IR/UV ratio and compare it to the UV slope $\beta$. We find that it is not possible to uniquely estimate the dust attenuation from $\beta$ for our sample of galaxies. These galaxies are highly extinguished with a median value $A_{FUV} = 2.7 \pm 0.8$. Once the dust correction applied, the UV- and IR-based SFRs correlate. For the closest galaxy with the best quality spectrum, we see a feature consistent with being produced by a bump near 220nm in the attenuation curve.
    12/2004;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We use the far-UV (FUV) and near-UV (NUV) images of M83 obtained by GALEX to compute the radial profile of the UV spectral slope in the star forming disk. We briefly present a model of its chemical evolution which allows us to obtain realistic intrinsic properties of the stellar populations. Using corollary data, we also compute the profiles of H-alpha/H-beta and of the total IR (TIR) to FUV ratio. Both data and model are used to estimate and compare the extinction gradients at the FUV wavelength obtained from these various indicators. We discuss the implications for the determination of the star formation rate. Comment: This paper will be published as part of the Galaxy Evolution Explorer (GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of papers will be available at http://www.galex.caltech.edu/PUBLICATIONS/ after November 22, 2004. A high resolution version of figure 1 is available at http://www.ociw.edu/~boissier/M83/fig1.ps
    The Astrophysical Journal 11/2004; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We present measurements of the FUV (1530A) and NUV (2310A) galaxy luminosity functions (LF) at low redshift (z<0.2) from GALEX observations matched to the 2dF Galaxy Redshift Survey. We split our FUV and NUV samples into two UV-bj color bins and two redshift bins. As observed at optical wavelengths, the local LF of the bluest galaxies tend to have steeper faint end slopes and fainter characteristic magnitudes M* than the reddest subsamples. We find evidence for color dependent evolution at very low redshift in both bands, with bright blue galaxies becoming dominant in the highest redshift bin. The evolution of the total LF is consistent with an 0.3 magnitude brightening between z=0 and 0.13, in agreement with the first analysis of deeper GALEX fields probing adjacent and higher redshifts. Comment: This paper will be published as part of the Galaxy Evolution Explorer (GALEX) Astrophysical Journal Letters Special Issue. Links to the full set of papers will be available at : http://www.galex.caltech.edu/PUBLICATIONS/ after November 22, 2004
    The Astrophysical Journal 11/2004; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the first year on-orbit performance results for the Galaxy Evolution Explorer (GALEX), a NASA Small Explorer that is performing a survey of the sky in two ultraviolet bands. The instrument comprises a 50 cm diameter modified Ritchey-Chretien telescope with a 1.25 degree field of view, selectable imaging and objective grism spectroscopic modes, and an innovative optical system with a thin-film multilayer dichroic beam splitter that enables simultaneous imaging by a pair of photon counting, microchannel plate, delay line readout detectors. Initial measurements demonstrate that GALEX is performing well, meeting its requirements for resolution, efficiency, astrometry, bandpass definition and survey sensitivity.
    The Astrophysical Journal 11/2004; 619(1). · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Far UV observations from UIT mission found strong radial gradients in UV colors within several nearby ellipticals and spiral bulges. Population models suggest this is due to the radial gradient in either age or metallicity of the underlying stellar populations. GALEX AIS mode will provide higher S/N data for a larger sample of nearby galaxies for this phenomenon. When combined with the metallicity gradient data from the HST and ground-based telescopes, these data will provide strong constraint on the formation history of the early-type systems.
    05/2001;
  • [Show abstract] [Hide abstract]
    ABSTRACT: The star formation history in elliptical galaxies has been a focus of many debates lately. Whether elliptical galaxies formed in a monolithic collapse or via more recent interaction/merger activities between disk galaxies holds a clue to the study of the galaxy formation history and even to cosmology. Globular cluster systems in them are arguably the best probes, because they are bright for detection and feasible for analysis because of their simple nature. Visible data have shown that they often have bimodal metallicity distributions, indicating an episodic formation history. However, the ages of these formation epochs are uncertain, while accurate determination of their ages is critical in distinguishing different formation scenarios. The models of Yi and collaborators have shown that their ages can be estimated if multi-band photometric data, in particular including UV, are available. This is because the UV flux of an intermediate-to-old population is sensitive to its HB morphology, which is a good age indicator for the population. Such UV data are difficult to collect in large quantity because observing time with space telescopes with UV capability is highly competitive. NASA's SMEX program, GALEX, will collect exactly this type of data not only of Local Group galaxies but also of external galaxies. Its AIS modes will detect thousands of globular clusters in the Local Group, and its DIS modes, if properly pointing, will collect numerous clusters in the galaxies in other clusters of galaxies.
    05/2001;
  • [Show abstract] [Hide abstract]
    ABSTRACT: Although there is a consensus that hot horizontal-branch (HB) and post-HB stars are the dominant UV sources in giant ellipticals, it is still debated whether the UV sources are metal-poor or metal-rich. The problem is that these two scenarios predict significantly different cosmological models. For example, if the metal-poor model is correct, then giant ellipticals are the oldest population in the Universe and would imply a non-zero cosmological constant from time scale test. We found that these two models predict substantially different UV-to-V flux ratios as functions of redshift (look-back time). We estimate this effect is detectable using GALEX by observing several dozens of not-so-dense clusters in the redshift range of 0.1 < z < 0.3 with the exposure times comparable to the GALEX DIS mode.
    05/2001;
  • H.-S. Yim, M.-S. Chun, Y.-I. Byun
    [Show abstract] [Hide abstract]
    ABSTRACT: Deep BV photometry for about 15,000 stars in the globular cluster M71 has been obtained from the 2.2 m University Hawaii Telescope using 2k CCD. The frames cover a 7.5' times 7.5' region of the cluster center through 220'' (about 7 core radii). A complete color-magnitude diagram(CMD) from the upper red-giant branch to the lower main sequence(Vlimit ~22 approximately 5 mag below the main-sequence turnoff) has been constructed. From the fitting of the CMD fiducial sequence to the New Yale Isochrone and the similar metallicity globular cluster 47 Tuc, we estimated the absolute and relative ages of this ``disk population'' globluar cluster. A luminosity function to Mv ~8.3 for the cluster main-sequence has been derived. Applying several theoretical mass-luminosity relations, we derived mass function of M71. We discuss the implications of this on recent HST results about low mass main-sequence stars' lumisoty function and on dynamical evolution of the globular cluster.
    01/2000;
  • Source
    H.-S. Yim, Y.-I. Byun, M.-S. Chun
    [Show abstract] [Hide abstract]
    ABSTRACT: Recent deep HST observations of a few selected globular clusters show that the number of faint main sequence stars diminishes rather quickly beyond a peak around M_v ~ 9 mag. Previous ground-based studies on M13, however, have indicated an opposite trend, i.e. rapidly increasing population at the faint end of luminosity function. Based on our ground based but deep BV CCD photometry, we examine the true shape of M13 luminosity function down to V ~ 25. In contrast to other studies, we do not see the sudden increase in the number of stars at these faint magnitudes. The main cause of this discrepancy appears to be the inappropriate application of standard galactic models when estimating the field star contamination. We also examine the radial dependency of luminosity function.
    08/1999; 31:1243.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The first large-format CCD color-magnitude diagram (CMD) in the B and V passbands is presented for the Galactic globular cluster M53 (NGC 5024). The observations consist of 40 pairs of B and V CCD frames with FWHM <1″2. These frames were used to produce color-magnitude data for more than 20000 stars from the tip of the red giant-branch (RGB) to about two magnitudes below the main-sequence turnoff (V ≡ 22). The CMD reveals a classic example of a metal-poor stellar system, with the horizontal-branch (HB) predominantly blueward of the RR Lyrae instability strip and the relatively steep RGB. The authors have discovered 117 new blue straggler stars (BSSs) in the field of M53. The analysis of bright (V < 19.39) BSSs clearly shows a bimodal radial distribution, with a high frequency in the inner and outer regions, but a distinct dip in the intermediate region. The distribution is similar to that found in M3, a globular cluster with similar central density and concentration. The relative age dating based on the color difference between the turnoff and the base of RGB reveals no significant age difference (Δt < 1 Gyr) between M53 and M92.
    Journal- Korean Physical Society 12/1998; · 0.51 Impact Factor

Publication Stats

464 Citations
97.54 Total Impact Points

Institutions

  • 2004–2008
    • Yonsei University
      • Center for Space Astrophysics
      Seoul, Seoul, South Korea
    • Johns Hopkins University
      • Department of Physics and Astronomy
      Baltimore, Maryland, United States