arXiv subjects

Asantha Cooray

Publications and source records attributed to Asantha Cooray.

At least 271 records · Page 15Linked to original sources

arXiv2005

Halo Model at Its Best: Constraints on Conditional Luminosity Functions from Measured Galaxy Statistics

(Summary) Using the conditional luminosity function (CLF) -- the luminosity distribution of galaxies in a dark matter halo -- as the fundamental building block, we present an empirical model for the galaxy distribution. The model predictions are compared with the published luminosity function and clustering statistics from Sloan Digital Sky Survey (SDSS) at low redshifts, and galaxy correlation functions from COMBO-17 survey at a redshift of 0.6, DEEP2 survey at a redshift of unity, and Great Observatories Deep Origins Survey (GOODS) at a redshift around 3. The comparison with statistical measurements allows us to constrain certain parameters related to analytical descriptions that describe the relation between a dark matter halo and its central galaxy luminosity, its satellite galaxy luminosity, and the fraction of early- and late-type galaxies of that halo. The single parameter best constrained by clustering measurements is the total satellite galaxy luminosity averaged over the halo mass distribution probed by the galaxy sample. For SDSS, $ =(2.1^{+0.8}_{-0.4}) \times 10^{10}$ h${-2}$ L$_{\sun}$, while for GOODS at $z \sim 3$, $ < 2 \times 10^{11}$ h$^{-2}$ L$_{\sun}$. Quantities such as the fraction of satellite galaxies at a given galaxy luminosity are not strongly constrained with SDSS data. We use our CLFs to establish probability distribution of halo mass in which galaxies of a given luminosity could be found either at halo centers or as satellites. Finally, to help establish further properties of the galaxy distribution, we propose the measurement of cross-clustering between galaxies divided into two distinctly different luminosity bins.

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arXiv2005

Did WMAP see Moving Local Structures?

The divergence of the momentum density field of the large scale structure generates a secondary anisotropy contribution to the Cosmic Microwave Background (CMB). While the effect is best described as a non-linear extension to the well-known integrated Sachs-Wolfe effect, due to mathematical coincidences, the anisotropy contribution is also described as the lensing of the dipole seen in the rest-frame of a moving mass. Given the closeness, there is a remote possibility that local concentrations of mass in the form of the Great Attractor and the Shapley concentration generate large angular scale fluctuations in CMB and could potentially be responsible, at least partly, for some of the low-multipole anomalies in WMAP data. While the local anisotropy contribution peaks at low multipoles, for reasonable models of the mass and velocity distributions associated with local super structures we find that the amplitude of temperature anisotropies is at most at a level of 10$^{-2}$ $μ$K and is substantially smaller than primordial fluctuations. It is extremely unlikely that the momentum density of local mass concentrations is responsible for any of the large angular scale anomalies in WMAP data.

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arXiv2005

Cluster Merger Variance and the Luminosity Gap Statistic

The presence of multiple luminous galaxies in clusters can be explained by the finite time over which a galaxy sinks to the center of the cluster and merges with the the central galaxy. The simplest measurable statistic to quantify the dynamical age of a system of galaxies is the luminosity (magnitude) gap, which is the difference in photometric magnitude between the two most luminous galaxies. We present a simple analytical estimate of the luminosity gap distribution in groups and clusters as a function of dark matter halo mass. The luminosity gap is used to define "fossil" groups; we expect the fraction of fossil systems to exhibit a strong and model-independent trend with mass: ~1-3% of massive clusters and ~5-40% of groups should be fossil systems. We also show that, on cluster scales, the observed intrinsic scatter in the central galaxy luminosity-halo mass relation can be ascribed to dispersion in the merger histories of satellites within the cluster. We compare our predictions to the luminosity gap distribution in a sample of 730 clusters in the Sloan Digital Sky Survey C4 Catalog and find good agreement. This suggests that theoretical excursion set merger probabilities and the standard theory of dynamical segregation are valid on cluster scales.

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arXiv2005

Cosmology from supernova magnification maps

High-z Type Ia supernovae are expected to be gravitationally lensed by the foreground distribution of large-scale structure. The resulting magnification of supernovae is statistically measurable, and the angular correlation of the magnification pattern directly probes the integrated mass density along the line of sight. Measurements of cosmic magnification of supernovae therefore complements galaxy shear measurements in providing a direct measure of clustering of the dark matter. As the number of supernovae is typically much smaller than the number of sheared galaxies, the two-point correlation function of lensed Type Ia supernovae suffers from significantly increased shot noise. Neverthless, we find that the magnification map of a large sample of supernovae, such as that expected from next generation dedicated searches, will be easily measurable and provide an important cosmological tool. For example, a search over 20 sq. deg. over five years leading to a sample of ~ 10,000 supernovae would measure the angular power spectrum of cosmic magnification with a cumulative signal-to-noise ratio of ~20. This detection can be further improved once the supernova distance measurements are cross-correlated with measurements of the foreground galaxy distribution. The magnification maps made using supernovae can be used for important cross-checks with traditional lensing shear statistics obtained in the same fields, as well as help to control systematics. We discuss two applications of supernova magnification maps: the breaking of the mass-sheet degeneracy when estimating masses of shear-detected clusters, and constraining the second-order corrections to weak lensing observables.

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arXiv2005

Problems with Pencils: Lensing Covariance of Supernova Distance Measurements

While luminosity distances from Type Ia supernovae (SNe) provide a powerful probe of cosmological parameters, the accuracy with which these distances can be measured is limited by cosmic magnification due to gravitational lensing by the intervening large-scale structure. Spatial clustering of foreground mass fluctuations leads to correlated errors in distance estimates from SNe. By including the full covariance matrix of supernova distance measurements, we show that a future survey covering more than a few square degrees on the sky, and assuming a total of ~2000 SNe, will be largely unaffected by covariance noise. ``Pencil beam'' surveys with small fields of view, however, will be prone to the lensing covariance, leading to potentially significant degradations in cosmological parameter estimates. For a survey with 30 arcmin mean separation between SNe, lensing covariance leads to a ~45% increase in the expected errors in dark energy parameters compared to fully neglecting lensing, and a ~20% increase compared to including just the lensing variance. Given that the lensing covariance is cosmology dependent and cannot be mapped out sufficiently accurately with direct weak lensing observations, surveys with small mean SN separation must incorporate the effects of lensing covariance, including its dependence on the cosmological parameters.

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arXiv2005

A Brighter Past: Galaxy Luminosity Function At High Redshifts

Using the conditional luminosity function -- the luminosity distribution of galaxies in a dark matter halo as a function of the halo mass -- we present an empirical model to describe the redshift evolution of the rest B-band galaxy luminosity function (LF). The model is compared to measured LFs out to a redshift of 3.5, including LFs of galaxy types separated to red and blue galaxies. The increase in the number density of luminous galaxies, at the bright-end of the LF, can be explained as due to a brightening of the luminosity of galaxies present in dark matter halo centers, relative to the luminosity of central galaxies in similar mass halos today. The lack of strong evolution in the faint-end of the LF, however, argues against a model involving pure luminosity evolution at all halo mass scales. The increase in luminosity at the bright-end compensates the rapid decline in the number density of massive halos as the redshift is increased. The decline in group to cluster-mass dark matter halos out to a redshift of ~ 2 is not important as the central galaxy luminosity flattens at halo masses around 10^13 M_sun At redshifts ~ 2 to 3, however, the density of bright galaxies begins to decrease due to the rapid decline in the number density of dark matter halos at mass scales around and below 10^13 M_sun. We compare our predictions to the UV LF of galaxies at redshifts 3 to 6 and the galaxy clustering bias measurements at redshifts ~ 3, and use our models to establish the dark matter halo mass scales of galaxies observed at high redshifts. In general, to explain high-redshift LFs, galaxies in dark matter halos around 10^12 M_sun must increase in luminosity by a factor of ~ 4 to 6 between today and redshift of 6.

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arXiv2005

The Rise of Dwarfs and the Fall of Giants: Galaxy Formation Feedback Signatures in the Halo Satellite Luminosity Function

The observed luminosity function (LF) of satellite galaxies shows several interesting features that require a better understanding of gas-thermodynamic processes and feedback effects related to reionization and galaxy formation. In galaxy clusters, the abundance of dwarf galaxies is in good agreement with the expectation based on the subhalo mass function, whereas in galaxy groups, the relatively small abundance of dwarfs conflicts with theoretical expectations. In all halo systems, there is a dip in the abundance of galaxies with luminosities in the range ~ 2x10^8 L_sun to 10^10 L_sun, corresponding to subhalo mass scales between ~ 5x10^10 M_sun to few times 10^11 M_sun. Photoionization from reionization has been used to explain statistics of the dwarf population, with larger systems forming prior to, and smaller systems forming subsequent to, reionization. The observed dip in the LF is an imprint of small dwarf galaxies (<2x10^8 L_sun) that formed prior to reionization. The galactic winds powered by supernovae in these dwarf galaxies propagate energy and metals to large distances such that the intergalactic medium is uniformly enriched to a level of 10^-3 zsun, as observed in the low-redshift Ly-alpha forest. The associated energy related to this metallicity raises the intergalactic medium temperature and the Jeans mass to a range 10^10-10^11 M_sun at z ~ 3.4-6.0. Because the epoch of nonlinearity for halos in this mass range is at z > 3.4-4.4, their gas content, hence star formation, is greatly suppressed on average and leads to a dip in the observed LF at $z=0$. Larger halos (M > 10^11 M_sun), becoming nonlinear at z < 3.4-4.4, have masses topping the Jeans mass, where subhalo mass function based LF is again in agreement with observations.

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arXiv2005

A Divided Universe: Red and Blue Galaxies and their Preferred Environments

(Summary) Making use of scaling relations between the central and the total galaxy luminosity of a dark matter halo as a function of the halo mass, and the scatter in these relations, we present an empirical model to describe the luminosity function (LF) of galaxies. We extend this model to describe relative statistics of early-type, or red, and late-type, or blue, galaxies, with the fraction of early type galaxies at halo centers, relative to the total sample, determined only by the halo mass and the same fraction in the case of satellites is taken to be dependent on both the halo mass and the satellite galaxy luminosity. This simple model describes the conditional luminosity functions, LF of galaxies as a function of the halo mass, measured with the 2dF galaxy group catalog from cluster to group mass scales. Using 2dF measurements, we extract information related to conditional mass function for halos from extreme voids to dense regions in terms of the galaxy overdensity. We also calculate the probability distribution function of halo mass, as a function of the galaxy overdensity, and use these probabilities to address preferred environments of red and blue galaxies. Our model also allow us to make predictions, for example, galaxy bias as a function of the galaxy type and luminosity, the void mass function, and the average galaxy luminosity as a function of the density environment.

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arXiv2005

What is L*?: Anatomy of the Galaxy Luminosity Function

Using the empirical relations between the central galaxy luminosity and the halo mass, and between the total galaxy luminosity in a halo and the halo mass, we construct the galaxy luminosity function (LF). To the luminosity of the central galaxy in a halo of a given mass we assign log-normal scatter with a mean calibrated against the observations. In halos where the total galaxy luminosity exceeds that of the central galaxy, satellite galaxies are distributed as a power-law in luminosity. Combined with the halo mass function, this description reproduces the observed characteristics of the galaxy LF, including a shape consistent with the Schechter function. When all galaxies are included, regardless of the environment or the Hubble type, the Schechter L* is the luminosity scale above which the central galaxy luminosity-halo mass relation flattens; L* corresponds to ~10^{13}Msun on the halo mass scale. In surveys where central galaxies in massive clusters are neglected, either by design or because of the cosmic variance, L* is simply the mean luminosity of central galaxies in halos at the upper end of the selected mass range. The smooth, exponential decay of the Schechter function toward high luminosities reflects the intrinsic scatter in the central galaxy luminosity-halo mass relation. In addition to the LF, the model successfully reproduces the observed dependence of galaxy clustering bias on luminosity.

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arXiv2005

Dissipationless Merging and the Assembly of Central Galaxies

We reanalyze the galaxy-mass correlation function measured by the Sloan Digital Sky Survey to obtain host dark matter halo masses at galaxy and galaxy group scales. We extend the data to galaxy clusters in the 2MASS catalog and study the relation between central galaxy luminosity and halo mass. While the central galaxy luminosity scales as ~M^{0.7-0.8} at low masses, the relation flattens to ~M^{<0.3} above ~4x10^{13} M_sun. The total luminosity of galaxies in the halo, however, continues to grow as a power-law ~M^{0.8-0.9}. Starting from the hypothesis that the central galaxies grow by merging ("galactic cannibalism"), we develop a simple model for the evolution of their luminosities as a consequence of the accretion of satellite galaxies. The luminosity-mass relation flattens when the time scale on which dynamical friction induces orbital decay in the satellite galaxies exceeds the age of the dark matter halo. Then, the growth of the central galaxy is suppressed as it can cannibalize only the rare, massive satellite galaxies. The model takes the dependence of the total luminosity of galaxies in a halo on its mass and the global galaxy luminosity function as input, and reproduces the observed central galaxy luminosity-mass relation over three decades in halo mass, (10^{12}-10^{15}) M_sun. The success of the model suggests that gas cooling and subsequent star formation did not play an important role in the final assembly of central galaxies from sub-L_star precursors.

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arXiv2005

Primordial Gravitational Waves and Inflation: CMB and Direct Detection With Space-Based Laser Interferometers

The curl-modes of Cosmic Microwave Background (CMB) polarization probe horizon-scale primordial gravitational waves related to inflation. A significant source of confusion is expected from a lensing conversion of polarization related to density perturbations to the curl mode, during the propagation of photons through the large scale structure. Either high resolution CMB anisotropy observations or 21 cm fluctuations at redshifts 30 and higher can be used to delens polarization data and to separate gravitational-wave polarization signature from that of cosmic-shear related signal. Separations based on proposed lensing reconstruction techniques for reasonable future experiments allow the possibility to probe inflationary energy scales down to 10^15 GeV. Beyond CMB polarization, at frequencies between 0.01 Hz to 1 Hz, space-based laser interferometers can also be used to probe the inflationary gravitational wave background. The confusion here is related to the removal of merging neutron star binaries at cosmological distances. Given the low merger rate and the rapid evolution of the gravitational wave frequency across this band, reliable removal techniques can be constructed. We discuss issues related to joint constraints that can be placed on the inflationary models based on CMB polarization information and space-based interferometers such as the Big Bang Observer.

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arXiv2005

Cosmic Shear of the Microwave Background: The Curl Diagnostic

Weak-lensing distortions of the cosmic-microwave-background (CMB) temperature and polarization patterns can reveal important clues to the intervening large-scale structure. The effect of lensing is to deflect the primary temperature and polarization signal to slightly different locations on the sky. Deflections due to density fluctuations, gradient-type for the gradient of the projected gravitational potential, give a direct measure of the mass distribution. Curl-type deflections can be induced by, for example, a primordial background of gravitational waves from inflation or by second-order effects related to lensing by density perturbations. Whereas gradient-type deflections are expected to dominate, we show that curl-type deflections can provide a useful test of systematics and serve to indicate the presence of confusing secondary and foreground non-Gaussian signals.

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arXiv2005

Cosmic 21-cm Delensing of Microwave Background Polarization and the Minimum Detectable Energy Scale of Inflation

The curl (B) modes of cosmic microwave background (CMB) polarization anisotropies are a unique probe of the primordial background of inflationary gravitational waves (IGWs). Unfortunately, the B-mode polarization anisotropies generated by gravitational waves at recombination are confused with those generated by the mixing of gradient-mode (E-mode) and B-mode polarization anisotropies as CMB photons propagate through the Universe and are gravitationally lensed. We describe here a method for delensing CMB polarization anisotropies using observations of anisotropies in the cosmic 21-cm radiation emitted or absorbed by neutral hydrogen atoms at redshifts 10 to 200. While the detection of cosmic 21-cm anisotropies at high resolution is challenging, a combined study with a relatively low-resolution (but high-sensitivity) CMB polarization experiment could probe inflationary energy scales well below the Grand Unified Theory (GUT) scale of 10^{16} GeV -- constraining models with energy scales below 10^{15} GeV (the detectable limit derived from CMB observations alone). The ultimate theoretical limit to the detectable inflationary energy scale via this method may be as low as 3 \times 10^{14} GeV.

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arXiv2005

Can LISA Resolve Distance to the Large Magellanic Cloud?

The Laser Interferometer Space Antenna (LISA) is expected to detect N ~ 22 [x10^(+/- 1)] close white dwarf binaries in the Large Magellanic Cloud (LMC) through their gravitational radiation with signal-to-noise ratios greater than 10 in observational durations of 3 years or more. In addition to chirp mass, location on the sky, and other binary parameters, the distance to each binary is an independent parameter that can be extracted from an analysis of gravitational waves from these binaries. Using a sample of binaries, one can establish the mean distance to the LMC as well as the variance of this distance. Assuming no confusion noise at frequencies above 2 mHz, LISA might determine the LMC distance to ~ 4.5 (N/22)^(1/2)% and the line of sight extent of LMC to ~ 15 (N/22)^(1/4)$%, relative to its distance, at the one-sigma confidence. These estimates are competitive to some of the proposed direct geometric techniques to measure LMC distance in future with missions such as SIM and GAIA.

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arXiv2005

Cosmological Constraints on the Very Low Frequency Gravitational-Wave Background

While there are no strong observational constraints on the gravitational wave background across six or more orders of magnitude between 10^{-16} Hz and 10^{-10} Hz and it is difficult to get a constraint below 10^{-12}Hz using objects in our Galaxy, we suggest that the anisotropy pattern of time variation of the redshift related to a sample of high redshift objects can be used to constrain the gravitational wave background around 10^{-12} Hz. Useful observations for the monitoring of an anisotropy signal in a global redshift change include spectroscopic observations of the Ly-alpha forest in absorption towards a sample of quasars, redshifted 21 cm line observations either in absorption or emission towards a sample of neutral HI regions before or during reionization, and high frequency (0.1 Hz to 1 Hz) gravitational wave analysis of a sample of neutron star--neutron star binaries detected with gravitational wave instruments such as the Decihertz Interferometer Gravitational Wave Observatory (DECIGO). The low frequency background can also be constrained by arcsecond-scale anisotropy observations of the CMB. For reasonable observations in the future involving extragalactic sources, we find best limits at the level of Ω_{GW} < 10^{-5} at a frequency around 10^{-12} Hz while the eventual ultimate limit one cannot beat is Ω_{GW} < 10^{-11}.

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arXiv2004

Large-Scale Non-Gaussianities in the 21 cm Background Anisotropies From the Era of Reionization

The brightness temperature fluctuations in the 21 cm background related to neutral Hydrogen distribution provide a probe of physics related to the era of reionization when the intergalactic medium changed from a completely neutral to a partially ionized one. We formulate statistics of 21 cm brightness temperature anisotropies in terms of the angular power spectrum, the bispectrum, and the trispectrum. Using the trispectrum, we estimate the covariance related to the power spectrum measurements and show that correlations resulting from non-Gaussianities are below a percent, at most. While all-sky observations of the 21 cm background at arcminute-scale resolution can be used to measure the bispectrum with a cumulative signal-to-noise ratio of order a few ten, in the presence of foregrounds and instrumental noise related to first-generation interferometers, the measurement is unlikely to be feasible. For most purposes, non-Gaussianities in 21 cm fluctuations can be ignored and the distribution can be described with Gaussian statistics. Since 21 cm fluctuations are significantly contaminated by foregrounds, such as galactic synchrotron or low-frequency radio point sources, the lack of a significant non-Gaussianity in the signal suggests that any significant detection of a non-Gaussianity can be due to foregrounds. Similarly, in addition to frequency information that is now proposed to separate 21 cm fluctuations from foregrounds, if the non-Gaussian structure of foregrounds is a priori known, one can potentially use this additional information to further reduce the confusion.

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arXiv2004

Statistical Imprints of SZ Effects in the Cosmic Microwave Background

We review several aspects of the Sunyaev-Zel'dovich (SZ) effect associated with the large scale baryon distribution and its characteristic signatures in the statistics of cosmic microwave background (CMB) anisotropies. We discuss (1) the contributions to the angular power spectrum from the thermal and kinetic SZ effects, (2) the effect of SZ non-Gaussianities on cosmological parameter estimation, and (3) the SZ-induced CMB polarization towards galaxy clusters. We also discuss the extent to which recent measurements of small angular scale CMB anisotropy can be accounted for by SZ clusters and the potential use of SZ polarization towards large samples of galaxy clusters as a probe of dark energy parameters.

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arXiv2004

Polarization Signals of the 21 cm Background from the Era of Reionization

While emission and absorption lines of the 21 cm spin-flip transition of neutral Hydrogen are intrinsically unpolarized, a magnetic field creates left- and right-handed polarized components through the Zeeman effect. Here we consider the resulting polarization of the redshifted 21 cm background from the intergalactic medium before reionization. The polarization is detectable in regions with a strong gradient in the mean brightness temperature. In principle, this can open a new window on the evolution of intergalactic magnetic fields. One possible approach is an extended integration of an individual target during this era, such as the Mpc-scale HII regions inferred to surround quasars at z ~ 6.5. The differential intensity between the two polarization states can be used as a probe of the magnetic field at the edge of the HII region. We estimate that the SKA could (ignoring systematics) detect B \~ 200 (10) \muG coherent over several kiloparsecs with an observational bandwidth of 100 (2) kHz. Beyond individual sources, the statistical properties of wide-field 21 cm polarization maps, such as the angular power spectrum, can be used to constrain the large-scale magnetic field. In this case, the SKA can detect $B \sim 100 μ$G fields coherent over many megaparsecs. The magnetic field can be measured in any epoch over which the 21 cm background changes rapidly (for example because the ionized fraction or spin temperature change). Although the resulting constraints with SKA are relatively weak compared to theoretical expectations, they nevertheless offer a unique direct probe of magnetic fields in the high-redshift universe.

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