Changbom Park

Cosmological constraints from the SDSS luminous red galaxies

We measure the large-scale real-space power spectrum P(k) using luminous red galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) and use this measurement to sharpen constraints on cosmological parameters from the Wilkinson Microwave Anisotropy Probe (WMAP). We employ a matrix-based power spectrum estimation method using Pseudo-Karhunen-Loeve eigenmodes, producing uncorrelated minimum-variance measurements in 20 k-bands of both the clustering power and its anisotropy due to redshift-space distortions, with narrow and well-behaved window functions in the range 0.01h/Mpc 0.1h/Mpc and associated nonlinear complications, yet agree well with more aggressive published analyses where nonlinear modeling is crucial.

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

We investigate the dependence of physical properties of galaxies brighter than Mr = -18.0 + 5 log h in the Sloan Digital Sky Survey (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, without any break or feature. The only feature is the sharp decrease of the late-type fraction above the critical luminosity of about Mr = -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-σ relation of early types (larger dispersion at higher density for bright galaxies). The fraction of galaxies with early morphological type is a monotonically increasing function of local density and luminosity. The morphology-density-luminosity relation, as measured in this work, should be a key constraint on galaxy formation models. We demonstrate that the dependence on environment of the morphology of galaxies 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 with distance to the nearest bright galaxy, particularly when the distance is about 0.2 h-1 Mpc. We propose as a mechanism that the morphology of galaxies in galaxy systems is transformed by the tidal force.

Power spectrum, correlation function, and tests for luminosity bias in the CfA redshift survey

We describe and apply a method for directly computing the power spectrum for the galaxy distribution in the extension of the Center for Astrophysics Redshift Survey. Tests show that our technique accurately reproduces the true power spectrum for k greater than 0.03 h Mpc(exp -1). The dense sampling and large spatial coverage of this survey allow accurate measurement of the redshift-space power spectrum on scales from 5 to approximately 200 h(exp -1) Mpc. The power spectrum has slope n approximately equal -2.1 on small scales (lambda less than or equal 25 h(exp -1) Mpc) and n approximately -1.1 on scales 30 less than lambda less than 120 h(exp -1) Mpc. On larger scales the power spectrum flattens somewhat, but we do not detect a turnover. Comparison with N-body simulations of cosmological models shows that an unbiased, open universe CDM model (OMEGA h = 0.2) and a nonzero cosmological constant (CDM) model (OMEGA h = 0.24, lambda(sub zero) = 0.6, b = 1.3) match the CfA power spectrum over the wavelength range we explore. The standard biased CDM model (OMEGA h = 0.5, b = 1.5) fails (99% significance level) because it has insufficient power on scales lambda greater than 30 h(exp -1) Mpc. Biased CDM with a normalization that matches the Cosmic Microwave Background (CMB) anisotropy (OMEGA h = 0.5, b = 1.4, sigma(sub 8) (mass) = 1) has too much power on small scales to match the observed galaxy power spectrum. This model with b = 1 matches both Cosmic Background Explorer Satellite (COBE) and the small-scale power spect rum but has insufficient power on scales lambda approximately 100 h(exp -1) Mpc. We derive a formula for the effect of small-scale peculiar velocities on the power spectrum and combine this formula with the linear-regime amplification described by Kaiser to compute an estimate of the real-space power spectrum. Two tests reveal luminosity bias in the galaxy distribution: First, the amplitude of the pwer spectrum is approximately 40% larger for the brightest 50% of galaxies in volume-limited samples that have M(sub lim) greater than M*. This bias in the power spectrum is independent of scale, consistent with the peaks-bias paradigm for galaxy formation. Second, the distribution of local density around galaxies shows that regions of moderate and high density contain both very bright (M less than M* = -19.2 + 5 log h) and fainter galaxies, but that voids preferentially harbor fainter galaxies (approximately 2 sigma significance level).

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

We examine volume-limited samples of galaxies drawn from the Sloan Digital Sky Survey to look for relations among internal and collective physical parameters of galaxies as faint as Mr = -17.5. We morphologically classify galaxies using u - r color, g - i color gradient, and concentration index. At fixed morphology and luminosity, we find that bright (Mr 100 km s-1). We find that passive spirals are well separated from star-forming late types at Hα equivalent width 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. 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. An important improvement of our analyses over most previous works is that the extinction effects are effectively reduced by excluding the inclined late-type galaxies with axis ratios of b/a < 0.6.

Cosmic Voids in Sloan Digital Sky Survey Data Release 7

We study the distribution of cosmic voids and void galaxies using Sloan Digital Sky Survey Data Release 7 (SDSS DR7). Using the VoidFinder algorithm as described by Hoyle & Vogeley (2002), we identify 1054 statistically significant voids in the northern galactic hemisphere with radii > 10h 1 Mpc. The filling factor of voids in the sample volume is 62%. The largest void is just over 30h 1 Mpc in effective radius. The median effective radius is 17h 1 Mpc. The voids are found to be significantly underdense, with density contrast � < 0.85 at the edges of the voids. The radial density profiles of these voids are similar to predictions of dynamically distinct underdensities in gravitational theory. We find 8,046 galaxies brighter than Mr = 20.09 within the voids, accounting for 7% of the galaxies. We compare the results of VoidFinder on SDSS DR7 to mock catalogs generated from a SPH halo model simulation as well as other �-CDM simulations and find similar void fractions and void sizes in the data and simulations. This catalog is made publicly available at http://www.physics.drexel.edu/�pan/VoidCatalog for download.

Measures of galaxy environment – I. What is ‘environment’?

The influence of a galaxy’s environment on its evolution has been studied and compared extensively in the literature, although differing techniques are often used to define environment. Most methods fall into two broad groups: those that use nearest neighbours to probe the underlying density field and those that use fixed apertures. The differences between the two inhibit a clean comparison between analyses and leave open the possibility that, even with the same data, different properties are actually being measured. In this work we apply twenty published environment definitions to a common mock galaxy catalogue constrained to look like the local Universe. We find that nearest neighbour-based measures best probe the internal densities of high-mass haloes, while at low masses the inter-halo separation dominates and acts to smooth out local density variations. The resulting correlation also shows that nearest neighbour galaxy environment is largely independent of dark matter halo mass. Conversely, aperture-based methods that probe super-halo scales accurately identify high-density regions corresponding to high mass haloes. Both methods show how galaxies in dense environments tend to be redder, with the exception of the largest apertures, but these are the strongest at recovering the background dark matter environment. We also warn against using photometric redshifts to define environment in all but the densest regions. When considering environment there are two regimes: the ‘local environment’ internal to a halo best measured with nearest neighbour and ‘large-scale environment’ external to a halo best measured with apertures. This leads to the conclusion that there is no universal environment measure and the most suitable method depends on the scale being probed.

Morphology Segregation of Galaxies in 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 well-separated early- and late-type branches. 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 also used, the completeness and reliability of classification reaches about 91% for a training set of SDSS galaxies brighter than rpet ≈ 15.9. At the faintest magnitudes (rpet ≈ 17.5) of the SDSS spectroscopic sample, the accuracy 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 us to understand the origin of morphology of galaxies.

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

We inspect the coupled dependence of physical parameters of the Sloan Digital Sky Survey 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 rp of about r vir,nei. The second scale is at rp 0.05r 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.

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 Sloan Digital Sky Survey 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 1-3 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.

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

In support of the new 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 41203 = 69.9 billion particles, and covering a volume of (6.592 h –1 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. LRGs are selected by finding the most massive gravitationally bound, cold dark matter subhalos, not subject to tidal disruption, a technique that correctly reproduces the three-dimensional topology of the LRGs in the Sloan Survey. We have measured the covariance function, power spectrum, and the three-dimensional topology of the LRG distribution in our simulation and made 32 mock surveys along the past light cone to simulate the Sloan III survey. Our large N-body simulation is used to accurately measure the nonlinear 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. 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.