arXiv subjects

Changbom Park

Publications and source records attributed to Changbom Park.

At least 181 records · Page 10Linked to original sources

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.

astro-ph.CO↗

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)

astro-ph↗

arXiv2009

Statistical nature of non-Gaussianity from cubic order primordial perturbations: CMB map simulations and genus statistic

We simulate CMB maps including non-Gaussianity arising from cubic order perturbations of the primordial gravitational potential, characterized by the non-linearity parameter $g_{NL}$. The maps are used to study the characteristic nature of the resulting non-Gaussian temperature fluctuations. We measure the genus and investigate how it deviates from Gaussian shape as a function of $g_{NL}$ and smoothing scale. We find that the deviation of the non-Gaussian genus curve from the Gaussian one has an antisymmetric, sine function like shape, implying more hot and more cold spots for $g_{NL}>0$ and less of both for $g_{NL}<0$. The deviation increases linearly with $g_{NL}$ and also exhibits mild increase as the smoothing scale increases. We further study other statistics derived from the genus, namely, the number of hot spots, the number of cold spots, combined number of hot and cold spots and the slope of the genus curve at mean temperature fluctuation. We find that these observables carry signatures of $g_{NL}$ that are clearly distinct from the quadratic order perturbations, encoded in the parameter $f_{NL}$. Hence they can be very useful tools for distinguishing not only between non-Gaussian temperature fluctuations and Gaussian ones but also between $g_{NL}$ and $f_{NL}$ type non-Gaussianities.

astro-ph.CO↗

arXiv2009

Non-Gaussian Distribution and Clustering of Hot and Cold Pixels in the WMAP Five-Year Sky

We present measurements of the clustering of hot and cold patches in the microwave background sky as measured from the Wilkinson Microwave Anisotropy Probe (WMAP) five-year data. These measurements are compared with theoretical predictions which assume that the cosmological signal obeys Gaussian statistics. We find significant differences from the simplest Gaussian-based prediction. However, the measurements are sensitive to the fact that the noise is spatially inhomogeneous (e.g., because different parts of the sky were observed for different lengths of time). We show how to account for this spatial inhomogeneity when making predictions. Differences from the Gaussian-based expectation remain even after this more careful accounting of the noise. In particular, we note that hot and cold pixels cluster differently within the same temperature thresholds at few-degree scales. While these findings may indicate primordial non-Gaussianity, we discuss other plausible explanations for these discrepancies. In addition, we find some deviations from Gaussianity at sub-degree scales, especially in the W band, whose origin may be associated with extragalactic dust emission.

astro-ph.CO↗

arXiv2009

The Horizon Run N-body Simulation: Baryon Acoustic Oscillations and Topology of Large Scale Structure of the Universe

In support of the new Sloan III survey, which will measure the baryon oscillation scale using the luminous red galaxies (LRGs), we have run the largest N-body simulation to date using $4120^3 = 69.9$ billion particles, and covering a volume of $(6.592 h^{-1} {\rm Gpc})^3$. This is over 2000 times the volume of the Millennium Run, and corner-to-corner stretches all the way to the horizon of the visible universe. LRG galaxies are selected by finding the most massive gravitationally bound, cold dark matter subhalos, not subject to tidal disruption, a technique that correctly reproduces the 3D topology of the LRG galaxies in the Sloan Survey.We have measured the covariance function, power spectrum, and the 3D topology of the LRG galaxy distribution in our simulation and made 32 mock surveys along the past lightcone to simulate the Sloan III survey. Our large N-body simulation is used to accurately measure the non-linear systematic effects such as gravitational evolution, redshift space distortion, past light cone space gradient, and galaxy biasing, and to calibrate the baryon oscillation scale and the genus topology. For example, we predict from our mock surveys that the baryon acoustic oscillation peak scale can be measured with the cosmic variance-dominated uncertainty of about 5% when the SDSS-III sample is divided into three equal volume shells, or about 2.6% when a thicker shell with $0.4<z<0.6$ is used. We find that one needs to correct the scale for the systematic effects amounting up to 5.2% to use it to constrain the linear theories. And the uncertainty in the amplitude of the genus curve is expected to be about 1% at 15 $h^{-1}$Mpc scale. We are making the simulation and mock surveys publicly available.

astro-ph↗

arXiv2009

Evidence for Morphology and Luminosity Transformation of Galaxies at High Redshifts

We study the galaxy morphology-luminosity-environmental relation and its redshift evolution using a spectroscopic sample of galaxies in the Great Observatories Origins Deep Survey (GOODS). In the redshift range of $0.4\leq z\leq1.0$ we detect conformity in morphology between neighboring galaxies. The realm of conformity is confined within the virialized region associated with each galaxy plus dark matter halo system. When a galaxy is located within the virial radius of its nearest neighbor galaxy, its morphology strongly depends on the neighbor's distance and morphology: the probability for a galaxy to be an early type ($f_E$) strongly increases as it approaches an early-type neighbor, but tends to decrease as it approaches a late-type neighbor. We find that $f_E$ evolves much faster in high density regions than in low density regions, and that the morphology-density relation becomes significantly weaker at $z\approx 1$. This may be because the rate of galaxy-galaxy interactions is higher in high density regions, and a series of interactions and mergers over the course of galaxy life eventually transform late types into early types. We find more isolated galaxies are more luminous, which supports luminosity transformation through mergers at these redshifts. Our results are consistent with those from nearby galaxies, and demonstrate that galaxy-galaxy interactions have been strongly affecting the galaxy evolution over a long period of time.

astro-ph.CO↗

arXiv2009

Large-Scale Structure of the Universe as a Cosmic Standard Ruler

We propose to use the large-scale structure of the universe as a cosmic standard ruler, based on the fact that the pattern of galaxy distribution should be maintained in the course of time on large scales. By examining the scale-dependence of the pattern in different redshift intervals it is possible to reconstruct the expansion history of the universe, and thus to measure the cosmological parameters governing the expansion of the universe. The features in the galaxy distribution that can be used as standard rulers include the topology of large-scale structure and the overall shapes of galaxy power spectrum and correlation function. The genus, being an intrinsic topology measure, is resistant against the non-linear gravitational evolution, galaxy biasing, and redshift-space distortion effects, and thus is ideal for quantifying the primordial topology of the large-scale structure. The expansion history of the universe can be constrained by comparing among the genus measured at different redshifts. In the case of initially Gaussian fluctuations the genus accurately recovers the slope of the primordial power spectrum near the smoothing scale, and the expansion history can be constrained by comparing between the predicted and measured genus.

astro-ph.CO↗

arXiv2009

Interactions of Galaxies in the Galaxy Cluster Environment

We study the dependence of galaxy properties on the clustercentric radius and the environment attributed to the nearest neighbor galaxy using the SDSS galaxies associated with the Abell galaxy clusters. We find that there exists a characteristic scale where the properties of galaxies suddenly start to depend on the clustercentric radius at fixed neighbor environment. The characteristic scale is $1\sim 3$ times the cluster virial radius depending on galaxy luminosity. Existence of the characteristic scale means that the local galaxy number density is not directly responsible for the morphology-density relation in clusters because the local density varies smoothly with the clustercentric radius and has no discontinuity in general. What is really working in clusters is the morphology-clustercentric radius-neighbor environment relation, where the neighbor environment means both neighbor morphology and the local mass density attributed to the neighbor. The morphology-density relation appears working only because of the statistical correlation between the nearest neighbor distance and the local galaxy number density. We find strong evidence that the hydrodynamic interactions with nearby early-type galaxies is the main drive to quenching star formation activity of late-type galaxies in clusters. The hot cluster gas seems to play at most a minor role down to one tenth of the cluster virial radius. We also find that the viable mechanisms which can account for the clustercentric radius dependence of the structural and internal kinematics parameters are harassment and interaction of galaxies with the cluster potential. The morphology transformation of the late-type galaxies in clusters seems to have taken place through both galaxy-galaxy hydrodynamic interactions and galaxy-cluster/galaxy-galaxy gravitational interactions.

astro-ph↗

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.

astro-ph↗

arXiv2008

Internal Extinction in the SDSS Late-Type Galaxies

We study internal extinction of late-type galaxies in the Sloan Digital Sky Survey. We find that the degree of internal extinction depends on both the concentration index c and K_s-band absolute magnitude M_K. We give simple fitting functions for internal extinction. In particular, we present analytic formulae giving the extinction-corrected magnitudes from the observed optical parameters. For example, the extinction-corrected r-band absolute magnitude can be obtained by M_{r,0}=-20.77 +(-1+\sqrt{1+4Δ(M_{r,obs}+20.77+4.93Δ)})/2Δ, where Δ= 0.236\{1.35(c-2.48)^2-1.14\}\log(a/b), c=R_{90}/R_{50} is the the concentration index, and a/b is the isophotal axis ratio of the 25 mag/arcsec^2 isophote in the i-band. The 1σerror in M_{r,0} is 0.21{\rm log}(a/b). The late-type galaxies with very different inclinations are found to trace almost the same sequence in the (u-r)-$M_r$ diagram when our prescriptions for extinction correction are applied. We also find that (u-r) color can be a third independent parameter that determines the degree of internal extinction.

astro-ph↗

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.

astro-ph↗

arXiv2008

Environment and mass dependencies of galactic $λ$ spin parameter: cosmological simulations and observed galaxies compared

We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the $λ$ spin parameter and the environment and mass of galaxies. In order to calculate the total value of $λ$ for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies, which allows a rough estimate of the value of $λ$ using only readily obtainable observables from the luminous galaxies. Use of a large volume-limited sample (upwards of 11,000) allows reliable inferences of mean values and dispersions of $λ$ distributions. We find, in agreement with some N-body cosmological simulations, no significant dependence of $λ$ on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with $λ$, in the sense that low-mass galaxies present both higher mean values of $λ$ and associated dispersions, than high-mass galaxies. These results provide interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models.

astro-ph↗

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↗

arXiv2008

Environment and mass dependencies of galactic $λ$ spin parameter: cosmological simulations and SDSS galaxies compared

We use a sample of galaxies from the Sloan Digital Sky Survey (SDSS) to search for correlations between the $λ$ spin parameter and the environment and mass of galaxies. In order to calculate the total value of $λ$ for each observed galaxy, we employed a simple model of the dynamical structure of the galaxies which allows a rough estimate of the value of $λ$ using only readily obtainable observables from the luminous galaxies. Use of a large volume limited sample (upwards of 11,000) allows reliable inferences of mean values and dispersions of $λ$ distributions. We find, in agreement with some N-body cosmological simulations, no significant dependence of $λ$ on the environmental density of the galaxies. For the case of mass, our results show a marked correlation with $λ$, in the sense that low mass galaxies present both higher mean values of $λ$ and associated dispersions, than high mass galaxies. This last direct empirical result, at odds with expectations from N-body cosmological simulations, provides interesting constrain on the mechanisms of galaxy formation and acquisition of angular momentum, a valuable test for cosmological models.

astro-ph↗

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.

astro-ph↗

arXiv2008

Seoul National University Bright Quasar Survey in Optical (SNUQSO) I: First Phase Observations and Results

We present results from the first phase of the Seoul National University Bright Quasar Survey in Optical (SNUQSO) as well as its basic observational setup. Previous and current large-area surveys have been successful in identifying many quasars, but they could have missed bright quasars due to their survey design. In order to help complete the census of bright quasars, we have performed spectroscopic observations of new bright quasar candidates selected from various methods based on optical colors, near-infrared colors, radio, and X-ray data. In 2005/2006, we observed 55 bright quasar candidates using the Bohyunsan Optical Echelle Spectrograph (BOES) on the 1.8 m telescope at the Bohyunsan Optical Astronomy Observatory in Korea. We identify 14 quasars/Seyferts from our observation, including an optically bright quasar with i=14.98 mag at z=0.092 (SDSS J003236.59-091026.2). Non-quasar/Seyfert objects are found to be mostly stars, among which there are five M-type stars and one cataclysmic variable. Our result shows that there still exist bright quasars to be discovered. However, at the same time, we conclude that finding new bright quasars in high Galactic latitude regions is very challenging and that the existing compilation of optically bright quasars is nearly complete in the northern hemisphere.

astro-ph↗

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)

astro-ph↗

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.

astro-ph↗