arXiv subjects

Yun-Young Choi

Publications and source records attributed to Yun-Young Choi.

At least 37 records · Page 2Linked to original sources

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).

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arXiv2009

Isolated Galaxies and Isolated Satellite Systems

We search for isolated galaxies using a volume-limited sample of galaxies with 0.02< z < 0.04742 from SDSS DR7 supplemented by bright galaxies. We devise a diagnostic tool to select isolated galaxies in different environments using the projected separation (r_p) normalized by the virial radius of the nearest neighbor (r_{vir,nei}) and the local background density. We find that the isolation condition of r_p>r_{vir,nei} and ρ<\barρ well segregates the CIG galaxies. We confirm the morphology conformity between the host and their satellites, which suggests importance of hydrodynamic interaction among galaxies within their virial radii in galaxy evolution.

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arXiv2009

Alignments of Group Galaxies with Neighboring Groups

Using a sample of galaxy groups found in the Sloan Digital Sky Survey Data Release 4, we measure the following four types of alignment signals: (1) the alignment between the distributions of the satellites of each group relative to the direction of the nearest neighbor group (NNG); (2) the alignment between the major axis direction of the central galaxy of the host group (HG) and the direction of the NNG; (3) the alignment between the major axes of the central galaxies of the HG and the NNG; and (4) the alignment between the major axes of the satellites of the HG and the direction of the NNG. We find strong signal of alignment between the satellite distribution and the orientation of central galaxy relative to the direction of the NNG, even when the NNG is located beyond $3r_{\rm vir}$ of the host group. The major axis of the central galaxy of the HG is aligned with the direction of the NNG. The alignment signals are more prominent for groups that are more massive and with early type central galaxies. We also find that there is a preference for the two major axis of the central galaxies of the HG and NNG to be parallel for the system with both early central galaxies, however not for the systems with both late type central galaxies. For the orientation of satellite galaxies, we do not find any significant alignment signals relative to the direction of the NNG. From these four types of alignment measurements, we conclude that the large scale environment traced by the nearby group affects primarily the shape of the host dark matter halo, and hence also affects the distribution of satellite galaxies and the orientation of central galaxies. (Shortened version)

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arXiv2009

Environmental dependence of AGN activity. I.: the effects of host galaxy

Using a large sample of local galaxies (144,940) with -17.5< Mr <-22 and 0.025< z <0.107, selected from SDSS DR5, we compare AGN host galaxies with non-AGN galaxies at matched luminosity, velocity dispersion, color, color gradient, or concentration index, to investigate how AGN activity is related with galaxy properties. The AGN sample is composed of Type II AGNs identified with flux ratios of narrow-emission lines with S/N > 6. We find that the fraction of galaxies hosting an AGN (f_AGN) depends strongly on morphology together with color, and very weakly on luminosity or velocity dispersion of host galaxies. In particular, f_AGN of early-type galaxies is almost independent of luminosity nor velocity dispersion when color is fixed. The host galaxy color preferred by AGNs is u-r ~2.0 for early-type hosts and u-r=2.0-2.4 for late-type hosts. This trend suggests that AGNs are dominantly hosted by intermediate-mass late-type galaxies. We also investigate how the accretion power varies with galaxy properties. While the Eddington ratio ([OIII] line luminosity normalized by black hole mass) ranges over three orders of magnitude for both morphological types, late-type galaxies are the dominant hosts over all AGN power. Among late-type galaxies, bluer color galaxies host higher power AGNs. These results are consistent with a scenario that more massive and redder galaxies are harder to host AGNs since these galaxiesalready consumed gas at the center or do not have sufficient gas supply to feed the black hole. In contrast, intermediate-mass, intermediate-color, and more concentrated late-type galaxies are more likely to host AGNs, implying that perhaps some fraction of low-mass, blue, and less concentrated lat e-type galaxiesmay not host massive black holes or may host very low-power AGNs.

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arXiv2009

3D Genus Topology of Luminous Red Galaxies

We measure the 3D genus topology of large scale structure using Luminous Red Galaxies (LRGs) in the Sloan Digital Sky Survey and find it consistent with the Gaussian random phase initial conditions expected from the simplest scenarios of inflation. This studies 3D topology on the largest scales ever obtained. The topology is sponge-like. We measure topology in two volume-limited samples: a dense shallow sample studied with smoothing length of 21h^{-1}Mpc, and a sparse deep sample studied with a smoothing length of 34h^{-1}Mpc. The amplitude of the genus curve is measured with 4% uncertainty. Small distortions in the genus curve expected from non-linear biasing and gravitational effects are well explained (to about 1-σaccuracy) by N-body simulations using a subhalo-finding technique to locate LRGs. This suggests the formation of LRGs is a clean problem that can be modeled well without any free fitting parameters. This bodes well for using LRGs to measure the characteristic scales such as the baryon oscillation scale in future deep redshift surveys.

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arXiv2008

Combined Effects of Galaxy Interactions and Large-Scale Environment on Galaxy Properties

We inspect the coupled dependence of physical parameters of the SDSS galaxies on the small-scale (distance to and morphology of the nearest neighbor galaxy) and the large-scale (background density smoothed over 20 nearby galaxies) environments. The impacts of interaction on galaxy properties are detected at least out to the neighbor separation corresponding to the virial radius of galaxies, which is typically between 200 and 400 h^{-1}kpc for the galaxies in our sample. To detect these long-range interaction effects it is crucial to divide galaxy interactions into four cases dividing the morphology of target and neighbor galaxies into early and late types. We show that there are two characteristic neighbor-separation scales where the galaxy interactions cause abrupt changes in the properties of galaxies. The first scale is the virial radius of the nearest neighbor galaxy r_{vir,nei}. Many physical parameters start to deviate from those of extremely isolated galaxies at the projected neighbor separation r_p of about r_{vir,nei}. The second scale is at r_p \approx 0.05 r_{vir,nei} = 10--20 h^{-1} kpc, and is the scale at which the galaxies in pairs start to merge. We find that late-type neighbors enhance the star formation activity of galaxies while early-type neighbors reduce it, and that these effects occur within r_{vir,nei}. The hot halo gas and cold disk gas must be participating in the interactions at separations less than the virial radius of the galaxy plus dark halo system. Our results also show that the role of the large-scale density in determining galaxy properties is minimal once luminosity and morphology are fixed. We propose that the weak residual dependence of galaxy properties on the large-scale density is due to the dependence of the halo gas property on the large-scale density.

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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.

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arXiv2008

Galactic Satellite Systems: Radial Distribution and Environment Dependence of Galaxy Morphology

We have studied the radial distribution of the early (E/S0) and late (S/Irr) types of satellites around bright host galaxies. We made a volume-limited sample of 4,986 satellites brighter than M_r = -18.0 associated with 2,254 hosts brighter than M_r =-19.0 from the SDSS DR5 sample. The morphology of satellites is determined by an automated morphology classifier, but the host galaxies are visually classified. We found segregation of satellite morphology as a function of the projected distance from the host galaxy. The amplitude and shape of the early-type satellite fraction profile are found to depend on the host luminosity. This is the morphology-radius/density relation at the galactic scale. There is a strong tendency for morphology conformity between the host galaxy and its satellites. The early-type fraction of satellites hosted by early-type galaxies is systematically larger than that of late-type hosts, and is a strong function of the distance from the host galaxies. Fainter satellites are more vulnerable to the morphology transformation effects of hosts. Dependence of satellite morphology on the large-scale background density was detected. The fraction of early-type satellites increases in high density regions for both early and late-type hosts. It is argued that the conformity in morphology of galactic satellite system is mainly originated by the hydrodynamical and radiative effects of hosts on satellites.

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arXiv2008

A Subhalo-Galaxy Correspondence Model of Galaxy Formation

We propose a model of allocating galaxies in cosmological N-body simulations. we identify each subhalo with a galaxy, and assign luminosity and morphological type assuming that the galaxy luminosity is a monotonic function of its host subhalo mass. The morphology assignment is made by using two simple relations between subhalo mass and galaxy luminosity of different types. One is using a constant ratio in luminosity of early (E/SO) and late (S/Irr) type galaxies at a fixed subhalo mass. And the other assumes that galaxies of different morphological types but having an equal luminosity have a constant ratio in their subhalo masses. We made a series of comparisons of the properties of these simulated galaxies with those of the SDSS galaxies. The resulting simulated galaxy sample is found to successfully reproduce the observed local number density distribution except for in high density regions. The luminosity function is studied as a function of local density. It was found that the observed luminosity functions in different local density environments are overall well-reproduced by the simulated galaxies. Discrepancy is found at the bright end of the luminosity function of early types in the underdense regions and at the faint end of both morphological types in very high density regions. A significant fraction of the observed early type galaxies in voids seems to have undergone a relatively recent star formation and became brighter. The lack of faint simulated galaxies in dense regions may be due to the strong tidal force of the central halo which destroys less massive satellite subhalos around in the simulation. The mass-to-light ratio is found to depend on the local density in the way similar to that observed in the SDSS sample. (abridged)

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arXiv2007

Transformation of Morphology and Luminosity Classes of the SDSS Galaxies

We present a unified picture on the evolution of galaxy luminosity and morphology. Galaxy morphology is found to depend critically on the local environment set up by the nearest neighbor galaxy in addition to luminosity and the large scale density. When a galaxy is located farther than the virial radius from its closest neighbor, the probability for the galaxy to have an early morphological type is an increasing function only of luminosity and the local density due to the nearest neighbor ($ρ_n$). The tide produced by the nearest neighbor is thought to be responsible for the morphology transformation toward the early type at these separations. When the separation is less than the virial radius, i.e. when $ρ_n > ρ_{\rm virial}$, its morphology depends also on the neighbor's morphology and the large-scale background density over a few Mpc scales ($ρ_{20}$) in addition to luminosity and $ρ_n$. The early type probability keeps increasing as $ρ_n$ increases if its neighbor is an early type. But the probability decreases as $ρ_n$ increases when the neighbor is a late type. The cold gas streaming from the late type neighbor can be the reason for the morphology transformation toward late type. The overall early-type fraction increases as $ρ_{20}$ increases when $ρ_n > ρ_{\rm virial}$. This can be attributed to the hot halo gas of the neighbor which is confined by the pressure of the ambient medium held by the background mass. We have also found that galaxy luminosity depends on $ρ_n$, and that the isolated bright galaxies are more likely to be recent merger products. We propose a scenario that a series of morphology and luminosity transformation occur through distant interactions and mergers, which results in the morphology--luminosity--local density relation.

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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.

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arXiv2007

Environmental Dependence of Properties of Galaxies in the Sloan Digital Sky Survey

We investigate the dependence of physical properties of galaxies brighter than M_r=-18.0 in the SDSS on environment, as measured by local density using an adaptive smoothing kernel. We find that variations of galaxy properties with environment are almost entirely due to the dependence of morphology and luminosity on environment. Because galaxy properties depend not only on luminosity but also on morphology, it is clear that galaxy properties cannot be determined solely by dark halo mass. When morphology and luminosity are fixed, other physical properties, such as color, color-gradient, concentration, size, velocity dispersion, and star formation rate, are nearly independent of local density. The only feature is the sharp decrease of late type fraction above the critical luminosity of about M_r=-21.3 in the morphology versus luminosity relation. Weak residual dependences on environment include that of the color of late-types (bluer at lower density) and of the L-sigma relation of early-types (larger dispersion at higher density for bright galaxies). The early-type fraction is a monotonically increasing function of local density and luminosity. The morphology-density- luminosity relation should be a key constraint on galaxy formation models. We demonstrate that the dependence on environment of the morphology of galaxie originates from variations in density on effective Gaussian smoothing scales much smaller than 12 h^{-1}Mpc. We find that galaxy morphology varies both with density measured on an effective Gaussian smoothing scale of 4.7 h^{-1} Mpc and on distance to the nearest bright galaxy (particularly, for about 0.2 h^{-1}Mpc). We propose a mechanism that the morphology of galaxies in galaxy systems is transformed by the tidal force.

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arXiv2007

Internal and Collective Properties of Galaxies in the Sloan Digital Sky Survey

We examine volume-limited samples from the SDSS galaxies to look for relations among internal and collective physical parameters of galaxies as faint as M_r=- 17.5, which include morphology, luminosity, color, color gradient, concentration, size, velocity dispersion, equivalent width (EW) of H_alpha line,axis ratio, the luminosity and velocity dispersion functions. At fixed morphology and luminosity, we find that bright (M_r<-20) early-types show very small dispersions in color, color gradient, concentration, size, and velocity dispersion. These dispersions increase at fainter magnitudes, where the fraction of blue early-types increases. Late-types show wider dispersions in all physical parameters compared to early types at the same luminosity. Concentration indices of early-types are well-correlated with velocity dispersion, but are insensitive to luminosity and color for bright galaxies. The slope of the Faber-Jackson relation continuously changes from 4.6 +- 0.4 to 2.7+- 0.2 when luminosity changes from M_r = -22 to -20. The size of early- types is well-correlated with stellar velocity dispersion (for >100 km/s). We find that passive spirals are well-separated from star-forming late-types at EW (H_alpha) of about 4. An interesting finding is that many physical parameters of galaxies manifest different behaviors across the absolute magnitude of about M_* +- 1. The morphology fraction as a function of luminosity depends less sensitively on large scale structure than the luminosity function (LF) does, and thus seems to be more universal. The effects of internal extinction in late-types on the completeness of volume limited samples and on the LF and morphology fraction are found to be very important. (abridged)

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arXiv2006

Empirical distributions of galactic $λ$ spin parameters from the SDSS

Using simple dimensional arguments for both spiral and elliptical galaxies, we present formulas to derive an estimate of the halo spin parameter $λ$ for any real galaxy, in terms of common observational parameters. This allows a rough estimate of $λ$, which we apply to a large volume limited sample of galaxies taken from the SDSS data base. The large numbers involved (11,597) allow the derivation of reliable $λ$ distributions, as signal adds up significantly in spite of the errors in the inferences for particular galaxies. We find that if the observed distribution of $λ$ is modeled with a log-normal function, as often done for this distribution in dark matter halos that appear in cosmological simulations, we obtain parameters $λ_{0}=0.04 \pm 0.005$ and $σ_λ=0.51 \pm 0.05$, interestingly consistent with values derived from simulations. For spirals, we find a good correlation between empirical values of $λ$ and visually assigned Hubble types, highlighting the potential of this physical parameter as an objective classification tool.

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arXiv2006

Topology of structure in the Sloan Digital Sky Survey: model testing

We measure the three-dimensional topology of large-scale structure in the Sloan Digital Sky Survey (SDSS). This allows the genus statistic to be measured with unprecedented statistical accuracy. The sample size is now sufficiently large to allow the topology to be an important tool for testing galaxy formation models. For comparison, we make mock SDSS samples using several state-of-the-art N-body simulations: the Millennium run of Springel et al. (2005)(10 billion particles), Kim & Park (2006) CDM models (1.1 billion particles), and Cen & Ostriker (2006) hydrodynamic code models (8.6 billion cell hydro mesh). Each of these simulations uses a different method for modeling galaxy formation. The SDSS data show a genus curve that is broadly characteristic of that produced by Gaussian random phase initial conditions. Thus the data strongly support the standard model of inflation where Gaussian random phase initial conditions are produced by random quantum fluctuations in the early universe. But on top of this general shape there are measurable differences produced by non-linear gravitational effects (cf. Matsubara 1994), and biasing connected with galaxy formation. The N-body simulations have been tuned to reproduce the power spectrum and multiplicity function but not topology, so topology is an acid test for these models. The data show a ``meatball'' shift (only partly due to the Sloan Great Wall of Galaxies; this shift also appears in a sub-sample not containing the Wall) which differs at the 2.5σlevel from the results of the Millennium run and the Kim & Park dark halo models, even including the effects of cosmic variance.

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arXiv2005

Morphology Segregation of Galaxies in the Color-Color Gradient Space

We have found the u -r color versus g -i color gradient space can be used for highly successful morphology classification of galaxies in the Sloan Digital Sky Survey. In this space galaxies form early and late type branches well-separated from each other. The location of galaxies along the branches reflects the degree and locality of star formation activity, and monotonically corresponds to the sequence of morphological subclasses. When the concentration index is used together, the completeness and reliability of classification reaches about 91% for a training set of SDSS galaxies brighter than r~15.9. At faintest magnitudes (r~17.5) of the SDSS spectroscopic sample, the performance still remains at about 88%. The new classification scheme will help us find accurate relations of galaxy morphology with spatial and temporal environments, and help one understand the origin of morphology of galaxies.

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arXiv2005

Topology Analysis of the Sloan Digital Sky Survey: I. Scale and Luminosity Dependences

We measure the topology of volume-limited galaxy samples selected from a parent sample of 314,050 galaxies in the Sloan Digital Sky Survey(SDSS), which is now complete enough to describe the fully three-dimensional topology and its dependence on galaxy properties. We compare the observed genus statistic Gν_f) to predictions for a Gaussian random field and to the genus measured for mock surveys constructed from new large-volume simulations of the LambdaCDM cosmology. In this analysis we carefully examine the dependence of the observed genus statistic on the Gaussian smoothing scale R_G from 3.5 to 11 h^-1Mpc and on the luminosity of galaxies over the range -22.50<M_r<-18.5. The void multiplicity A_V is less than unity at all smoothing scales. Because A_V cannot become less than 1 through gravitational evolution, this result provides strong evidence for biased galaxy formation in low density environments. We also find clear evidence of luminosity bias of topology within the volume-limited sub-samples. The shift parameter Delta nu indicates that the genus of brighter galaxies shows a negative shift toward a ``meatball'' (i.e. cluster-dominated) topology, while faint galaxies show a positive shift toward a ``bubble'' (i.e. void-dominated) topology. The transition from negative to positive shift occurs approximately at the characteristic absolute magnitude M_{r*}=-20.4. Even in this analysis of the largest galaxy sample to date, we detect the influence of individual large-scale structures, as the shift parameter Delta nu and cluster multiplicity A_C reflect (at ~3 sigma) the presence of the Sloan Great Wall and a X-shaped structure which runs for several hundred Mpc across the survey volume.

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arXiv2002

Chandra ACIS-S Observations of Abell 4059: Signs of Dramatic Interaction Between a Radio Galaxy and a Galaxy Cluster

We present Chandra observations of the galaxy cluster A4059. We find strong evidence that the FR-I radio galaxy PKS 2354-35 at the center of A4059 is inflating cavities with radii ~20 kpc in the intracluster medium, similar to the situation seen in Perseus A and Hydra A. We also find evidence for interaction between the intracluster medium and PKS 2354-35 on small scales in the very center of the cluster. Arguments are presented suggesting that this radio galaxy has faded significantly in radio power (possibly from an FR-II state) over the past 10^8 yrs.

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