arXiv subjects

Joon Hyeop Lee

Publications and source records attributed to Joon Hyeop Lee.

46 records · Page 3Linked to original sources

arXiv2012

Hubble Space Telescope Pixel Analysis of the Interacting S0 Galaxy NGC 5195 (M51B)

We report the properties of the interacting S0 galaxy NGC 5195 (M51B), revealed in a pixel analysis using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. We analyze the pixel color-magnitude diagram (pCMD) of NGC 5195, focusing on the properties of its red and blue pixel sequences and the difference from the pCMD of NGC 5194 (M51A; the spiral galaxy interacting with NGC 5195). The red pixel sequence of NGC 5195 is redder than that of NGC 5194, which corresponds to the difference in the dust optical depth of 2<Δτ_V<4 at fixed age and metallicity. The blue pixel sequence of NGC 5195 is very weak and spatially corresponds to the tidal bridge between the two interacting galaxies. This implies that the blue pixel sequence is not an ordinary feature in the pCMD of an early-type galaxy, but that it is a transient feature of star formation caused by the galaxy-galaxy interaction. We also find a difference in the shapes of the red pixel sequences on the pixel color-color diagrams (pCCDs) of NGC 5194 and NGC 5195. We investigate the spatial distributions of the pCCD-based pixel stellar populations. The young population fraction in the tidal bridge area is larger than that in other areas by a factor > 15. Along the tidal bridge, young populations seem to be clumped particularly at the middle point of the bridge. On the other hand, the dusty population shows a relatively wide distribution between the tidal bridge and the NGC 5195 center.

astro-ph.CO↗

arXiv2011

Ultraviolet color-color relation of early-type galaxies at 0.05 < z < 0.12

We present the ultraviolet (UV) color-color relation of early-type galaxies (ETGs) in the nearby universe (0.05 < z < 0.12) to investigate the properties of hot stellar populations responsible for the UV excess (UVX). The initial sample of ETGs is selected by the spectroscopic redshift and the morphology parameter from the Sloan Digital Sky Survey (SDSS) DR7, and then cross-matched with the Galaxy Evolution Explorer (GALEX) Far-UV (FUV) and Near-UV (NUV) GR6 data. The cross-matched ETG sample is further classified by their emission line characteristics in the optical spectra into quiescent, star-forming, and AGN categories. Contaminations from early-type spiral galaxies, mergers, and morphologically disturbed galaxies are removed by visual inspection. By drawing the FUV - NUV (as a measure of UV spectral shape) vs. FUV - r (as a measure of UVX strength) diagram for the final sample of ~3700 quiescent ETGs, we find that the "old and dead" ETGs consist of a well-defined sequence in UV colors, the "UV red sequence", so that the stronger UVX galaxies should have a harder UV spectral shape systematically. However, the observed UV spectral slope is too steep to be reproduced by the canonical stellar population models in which the UV flux is mainly controlled by age or metallicity parameters. Moreover, 2 mag of color spreads both in FUV - NUV and FUV - r appear to be ubiquitous among any subsets in distance or luminosity. This implies that the UVX in ETGs could be driven by yet another parameter which might be even more influential than age or metallicity.

astro-ph.CO↗

arXiv2011

Hubble Space Telescope Pixel Analysis of the Interacting Face-on Spiral Galaxy NGC 5194 (M51A)

A pixel analysis is carried out on the interacting face-on spiral galaxy NGC 5194 (M51A), using the HST/ACS images in the F435W, F555W and F814W (BVI) bands. After 4x4 binning of the HST/ACS images to secure a sufficient signal-to-noise ratio for each pixel, we derive several quantities describing the pixel color-magnitude diagram (pCMD) of NGC 5194: blue/red color cut, red pixel sequence parameters, blue pixel sequence parameters and blue-to-red pixel ratio. The red sequence pixels are mostly older than 1 Gyr, while the blue sequence pixels are mostly younger than 1 Gyr, in their luminosity-weighted mean stellar ages. The color variation in the red pixel sequence from V = 20 mag arcsec^(-2) to V = 17 mag arcsec^(-2) corresponds to a metallicity variation of Δ[Fe/H] ~ 2 or an optical depth variation of Δτ_V ~ 4 by dust, but the actual sequence is thought to originate from the combination of those two effects. At V < 20 mag arcsec^(-2), the color variation in the blue pixel sequence corresponds to an age variation from 5 Myr to 300 Myr under the assumption of solar metallicity and τ_V = 1. To investigate the spatial distributions of stellar populations, we divide pixel stellar populations using the pixel color-color diagram and population synthesis models. As a result, we find that the pixel population distributions across the spiral arms agree with a compressing process by spiral density waves: dense dust \rightarrow newly-formed stars. The tidal interaction between NGC 5194 and NGC 5195 appears to enhance the star formation at the tidal bridge connecting the two galaxies. We find that the pixels corresponding to the central active galactic nucleus (AGN) area of NGC 5194 show a tight sequence at the bright-end of the pCMD, which are in the region of R ~ 100 pc and may be a photometric indicator of AGN properties.

astro-ph.CO↗

arXiv2010

AKARI Near-Infrared Spectroscopy of SDSS-Selected Blue Early-Type Galaxies

A near-infrared (NIR; 2.5 - 4.5 micron) spectroscopic survey of SDSS(Sloan Digital Sky Survey)-selected blue early-type galaxies (BEGs) has been conducted using the AKARI. The NIR spectra of 36 BEGs are secured, which are well balanced in their star-formation(SF)/Seyfert/LINER type composition. For high signal-to-noise ratio, we stack the BEG spectra all and in bins of several properties: color, specific star formation rate and optically-determined spectral type. We estimate the NIR continuum slope and the equivalent width of 3.29 micron PAH emission. In the comparison between the estimated NIR spectral features of the BEGs and those of model galaxies, the BEGs seem to be old-SSP(Simple Stellar Population)-dominated metal-rich galaxies with moderate dust attenuation. The dust attenuation in the BEGs may originate from recent star formation or AGN activity and the BEGs have a clear feature of PAH emission, the evidence of current SF. BEGs show NIR features different from those of ULIRGs, from which we do not find any clear relationship between BEGs and ULIRGs. We find that Seyfert BEGs have more active SF than LINER BEGs, in spite of the fact that Seyferts show stronger AGN activity than LINERs. One possible scenario satisfying both our results and the AGN feedback is that SF, Seyfert and LINER BEGs form an evolutionary sequence: SF - Seyfert - LINER.

astro-ph.CO↗

arXiv2009

The nature of the Sloan Digital Sky Survey galaxies in various classes based on morphology, colour and spectral features - III. Environments

We present a study on the environments of the SDSS galaxies divided into fine classes based on their morphology, colour and spectral features. The SDSS galaxies are classified into early-type and late-type; red and blue; passive, HII, Seyfert and LINER, which returns a total of 16 fine classes of galaxies. We estimate the local number density, target-excluded local luminosity density, local colour, close pair fraction and the luminosity and colour of the brightest neighbour, which are compared between the fine classes comprehensively. The morphology-colour class of galaxies strongly depends on the local density, with the approximate order of high-density preference: red early-type galaxies (REGs) -- red late-type galaxies (RLGs) -- blue early-type galaxies (BEGs) -- blue late-type galaxies (BLGs). We find that high-density environments (like cluster environments) seem to suppress AGN activity. The pair fraction of HII REGs does not show statistically significant difference from that of passive REGs, while the pair fraction of HII BLGs is smaller than that of non-HII BLGs. HII BLGs show obvious double (red + blue) peaks in the distribution of the brightest neighbour colour, while red galaxies show a single red peak. The brightest neighbours of Seyfert BLGs tend to be blue, while those of LINER BLGs tend to be red, which implies that the difference between Seyfert and LINER may be related to the pair interaction. Other various environments of the fine classes are investigated, and their implication on galaxy evolution is discussed.

astro-ph.CO↗

arXiv2009

The nature of the SDSS galaxies in various classes based on morphology, colour and spectral features - II. Multi-wavelength properties

We present a multi-wavelength study of the nature of the SDSS galaxies divided into fine classes based on their morphology, colour and spectral features. The SDSS galaxies are classified into early-type and late-type; red and blue; passive, HII, Seyfert and LINER, which returns a total of 16 fine classes of galaxies. The properties of galaxies in each fine class are investigated from radio to X-ray, using 2MASS, IRAS, FIRST, NVSS, GALEX and ROSAT data. The UV - optical - NIR colours of blue early-type galaxies (BEGs) seem to result from the combination of old stellar population and recent star formation (SF). Non-passive red early-type galaxies (REGs) have larger metallicity and younger age than passive REGs, which implies that non-passive REGs have suffered recent SF adding young and metal-rich stars to them. The radio detection fraction of REGs strongly depends on their optical absolute magnitudes, while that of most late-type galaxies does not, implying the difference in their radio sources: AGN and SF. The UV - optical colours and the radio detection fraction of passive RLGs show that they have properties similar to REGs rather than non-passive RLGs. Dust extinction may not be a dominant factor making RLGs red, because RLGs are detected in the mid- and far-infrared bands less efficiently than blue late-type galaxies (BLGs). The passive BLGs have very blue UV - optical - NIR colours, implying either recent SF quenching or current SF in their outskirts. Including star formation rate, other multi-wavelength properties in each fine class are investigated, and their implication on the identity of each fine class is discussed (abridged).

astro-ph.CO↗

arXiv2008

The nature of the SDSS galaxies in various classes based on morphology, colour and spectral features -- I. Optical properties

We present a comprehensive study of the nature of the SDSS galaxies divided into various classes based on their morphology, colour, and spectral features. The SDSS galaxies are classified into early-type and late-type; red and blue; passive, HII, Seyfert, and LINER, returning a total of 16 fine classes of galaxies. We examine the luminosity dependence of seven physical parameters of galaxies in each class. We find that more than half of red early-type galaxies (REGs) have star formation or AGN activity, and that these active REGs have smaller axis ratio and bluer outside compared to the passive REGs. Blue early-type galaxies (BEGs) show structural features similar to those of REGs, but their centres are bluer than REGs. HII BEGs are found to have bluer centres than passive BEGs, but HII REGs have bluer outside than passive REGs. Bulge-dominated late-type galaxies have red colours. Passive red late-types are similar to REGs in several aspects. Most blue late-type galaxies (BLGs) have forming stars, but a small fraction of BLGs do not show evidence for current star formation activity. Differences of other physical parameters among different classes are inspected, and their implication on galaxy evolution is discussed.

astro-ph↗

arXiv2007

The Connection between Star-Forming Galaxies, AGN Host Galaxies and Early-Type Galaxies in the SDSS

We present a study of the connection between star-forming galaxies, AGN host galaxies, and normal early-type galaxies in the Sloan Digital Sky Survey (SDSS). Using the SDSS DR5 and DR4plus data, we select our early-type galaxy sample in the color versus color-gradient space, and we classify the spectral types of the selected early-type galaxies into normal, star-forming, Seyfert, and LINER classes, using several spectral line flux ratios. We investigate the slope in the fundamental space for each class of early-type galaxies and find that there are obvious differences in the slopes of the fundamental planes (FPs) among the different classes of early-type galaxies, in the sense that the slopes for Seyferts and star-forming galaxies are flatter than those for normal galaxies and LINERs. This may be the first identification of the systematic variation of the FP slope among the subclasses of early-type galaxies. The difference in the FP slope might be caused by the difference in the degree of nonhomology among different classes or by the difference of gas contents in their merging progenitors. One possible scenario is that the AGN host galaxies and star-forming galaxies are formed by gas-rich merging and that they may evolve into normal early-type galaxies after finishing their star formation or AGN activities.

astro-ph↗

arXiv2006

The Nature of Blue Early-Type Galaxies in the Goods Fields

We present a study of the nature of the blue early-type galaxies (BEGs) in the GOODS north and south fields using the GOODS HST/ACS archival data. Using visual inspection, we have selected 58 BEGs and 113 normal red early-type galaxies (REGs) in the sample of 1,949 galaxies with spectroscopic redshifts. We find that the BEGs are generally bluer, fainter, and less-massive than the REGs, although a few BEGs are exceptionally bright and massive. The number fraction of the BEGs to total early-type galaxies is almost constant ($\sim0.3$) at $z \le 1.1$. In addition, we find that the size of the BEGs in given redshift bin decrease as redshift decreases. The BEGs look similar to the REGs in the images and surface brightness profiles. However, at least 27 BEGs show traces of tidal disturbances in their fine structures: elongated cores, off-centered cores, asymmetric internal color distributions, tidally distorted outer structures, collisional rings, or very nearby companions. Twenty-one BEGs are detected in the X-ray bands and eleven of them are as luminous as $L_{0.5-10 {\rm keV}} \ge 10^{43.5} {\rm erg~s}^{-1}$, indicating the existence of AGNs in their centers. These results show that at least a half of the BEGs may be descendants of mergers/interacting-galaxies and that at least a quarter of the BEGs may be AGN-host galaxies. The BEGs may evolve into REGs, and the size evolution of the BEGs is consistent with the galactic \emph{downsizing} scenario.

astro-ph↗

arXiv2003

Deep Wide Field BVI CCD Photometry of the Sextans Dwarf Spheroidal Galaxy

We present deep wide field $VI$ CCD photometry of the Sextans dwarf spheroidal galaxy (dSph) in the Local Group, covering a field of 42' x 28' located at the center of the galaxy. Color-magnitude diagrams of the Sextans dSph show well-defined red giant branch (RGB), blue horizontal branch (BHB), prominent red horizontal branch (RHB), asymptotic giant branch (AGB), about 120 variable star candidates including RR Lyraes and anomalous Cepheids, about 230 blue stragglers (BSs), and main sequence (MS) stars. The distance to the galaxy is derived using the $I$-band magnitude of the tip of the RGB at I(TRGB)=15.95+/-0.04: (m-M)_0=19.90+/-0.06 for an adopted reddening of E(B-V)=0.01. The mean metallicity of the RGB is estimated from the (V-I) color: [Fe/H]=-2.1+/-0.1(statistical error) +/-0.2(standard calibration error) dex, with a dispersion of sigma[Fe/H]=0.2 dex. The age of the MSTO of the main old population is estimated to be similar to that of M92, and there are seen some stellar populations with younger age. There is found to be one RGB bump at V=19.95+/-0.05 mag (M_V=0.03 mag), and a weak brighter bump at V=19.35+/-0.05 mag(Mv=-0.58 mag) which is probably an AGB bump. The V-band luminosity function of the RGB and MS stars is in general similar to that of M92, with a slight excess of stars in the magnitude range brighter than the MSTO with respect to that of M92. The bright BSs are more centrally concentrated than the faint BSs. Significant radial gradients are seen for several kinds of populations: the RHB, the red RGB, the red subgiant branch (SGB), and the bright BSs are more centrally concentrated toward the center of the galaxy, compared with the BHB, the blue RGB, the blue SGB, and the faint BSs, respectively.

astro-ph↗