arXiv subjects

A. S. Szalay

Publications and source records attributed to A. S. Szalay.

63 records · Page 4Linked to original sources

arXiv1998

Evolution of the Angular Correlation Function

For faint photometric surveys our ability to quantify the clustering of galaxies has depended on interpreting the angular correlation function as a function of the limiting magnitude of the data. Due to the broad redshift distribution of galaxies at faint magnitude limits the correlation signal has been extremely difficult to detect and interpret. We introduce a new technique for measuring the evolution of clustering. We utilize photometric redshifts, derived from multicolor surveys, to isolate redshift intervals and calculate the evolution of the amplitude of the angular 2-pt correlation function. Applying these techniques to the the Hubble Deep Field we find that the shape of the correlation function, at z=1, is consistent with a power law with a slope of -0.8. For z>0.4 the best fit to the data is given by a model of clustering evolution with a comoving r0 = 2.37 Mpc and eps = -0.4 +/- 0.5, consistent with published measures of the clustering evolution. To match the canonical value of r0 = 5.4 Mpc, found for the clustering of local galaxies, requires a value of eps = 2.10 +/- 0.5 (significantly more than linear evolution). The log likelihood of this latter fit is 4.15 less than that for the r0 = 2.37 Mpc model. We, therefore, conclude that the parameterization of the clustering evolution of (1+z)^-(3+eps) is not a particularly good fit to the data.

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arXiv1997

Effects of Sampling on Statistics of Large Scale Structure

The effects of sampling are investigated on measurements of counts-in-cells in three-dimensional magnitude limited galaxy surveys, with emphasis on moments of the underlying smooth galaxy density field convolved with a spherical window. A new estimator is proposed for measuring the k-th order moment < rho^k >: the weighted factorial moment F_k[w], corrected for the effects of the varying selection function. The cosmic error on the measurement of F_k[w] is computed via the the formalism of Szapudi & Colombi (1996), which is generalized to include selection effects. The integral equation for finding the minimum variance weight is solved numerically, and an intuitive analytical approximation is derived. The resulting estimator is more accurate than the traditional method of counts-in-cells in volume limited samples, which discards useful information. As a practical example we consider the case of the future Sloan Digital Sky Survey. Optimal (sparse) sampling strategies for designing magnitude limited redshift surveys are investigated as well. It is found that the optimal strategy depends greatly on the statistics and scales considered. Finally we consider the issue of designing the geometry of a catalog, when it covers only a small fraction of the sky.

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arXiv1997

The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field

The Hubble Deep Field (HDF) is the deepest set of multicolor optical photometric observations ever undertaken, and offers a valuable data set with which to study galaxy evolution. Combining the optical WFPC2 data with ground-based near-infrared photometry, we derive photometrically estimated redshifts for HDF galaxies with J<23.5. We demonstrate that incorporating the near-infrared data reduces the uncertainty in the estimated redshifts by approximately 40% and is required to remove systematic uncertainties within the redshift range 1 2, and bridge the redshift gap between those two samples. The overall star formation or metal enrichment rate history is consistent with the predictions of Pei and Fall (1995) based on the evolving HI content of Lyman-alpha QSO absorption line systems.

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arXiv1997

A Search For Large-Scale Structure at High Redshift

We present new and exciting results on our search for large-scale structure at high redshift. Specifically, we have just completed a detailed analysis of the area surrounding the cluster CL0016+16 (z=0.546) and have the most compelling evidence yet that this cluster resides in the middle of a supercluster. From the distribution of galaxies and clusters we find that the supercluster appears to be a sheet of galaxies, viewed almost edge-on, with a radial extent of 31 Mpc, transverse dimension of 12 Mpc, and a thickness of ~4 Mpc. The surface density and velocity dispersion of this coherent structure are consistent with the properties of the ``Great Wall'' in the CfA redshift survey.

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arXiv1996

Superclustering at Redshift Z=0.54

We present strong evidence for the existence of a supercluster at a redshift of z=0.54 in the direction of Selected Area 68. From the distribution of galaxies with spectroscopic redshifts we find that there is a large over-density of galaxies (a factor of four over the number expected in an unclustered universe) within the redshift range 0.530 < z < 0.555. By considering the spatial distribution of galaxies within this redshift range (using spectroscopic and photometric redshifts) we show that the galaxies in SA68 form a linear structure passing from the South-West of the survey field through to the North-East (with a position angle of approximately 35 deg East of North). This position angle is coincident with the positions of the X-ray clusters CL0016+16, RX J0018.3+1618 and a new X-ray cluster, RX J0018.8+1602, centered near the radio source 54W084. All three of these sources are at a redshift of approximately z=0.54 and have position angles, derived from their X-ray photon distributions, consistent with that measured for the supercluster. Assuming a redshift of 0.54 for the distribution of galaxies and a FWHM dispersion in redshift of 0.020 this represents a coherent structure with a radial extent of 31 Mpc, transverse dimension of 12 Mpc, and a thickness of approximately 4 Mpc. The detection of this possible supercluster demonstrates the power of using X-ray observations, combined with multicolor observations, to map the large scale distribution of galaxies at intermediate redshifts.

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arXiv1996

Luminosity Functions from Photometric Redshifts I: Techniques

The determination of the galaxy luminosity function is an active and fundamental field in observational cosmology. In this paper we propose a cost effective way of measuring galaxy luminosity functions at faint magnitudes. Our technique employs the use of galaxy redshifts estimated from their multicolor photometry (Connolly et al. 1995). Associated with the redshift estimate is a well defined error distribution. We have derived a variant of Lynden-Bell's (1971) C--method that considers, for each galaxy, the probability distribution in absolute magnitude resultant from the redshift error. This technique is tested through simulations and potential biases are quantified. We then apply the technique to a sample of galaxies with multicolor photometric data at moderately faint (B=23) limits, and compare the results to a subset of these data with spectroscopic redshifts. We find that the luminosity function derived from the photometric redshifts is consistent with that determined from spectroscopic redshifts.

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arXiv1996

A Medium-Deep Survey of a Mini-Slice at the North Galactic Pole. II. The Data

We report 328 redshifts, b$_J$ magnitudes and b$_J$--r$_F$ colors of galaxies measured in a redshift survey of a 4$^\circ$ $\times$ 0.67$^\circ$ slice close to the north Galactic pole. The faintest galaxies in this survey have a magnitude of b$_J$ $\sim$ 20.5. The redshifts present external errors of the order of 70 \kms, and we estimate that the mean photometry errors are $\sim$ 0.2 for magnitudes and $\sim$ 0.3 for colors. The redshift completeness level of the sample is of the order of $\sim$ 35\% at b$_J$=20, and part of this rather low completeness is the result of the combination of limitations imposed by the multifiber system with the clustering of galaxies, and an insufficient number of configurations. At the nominal magnitude limit of the survey, we were able to measure redshifts for $\sim$ 70 \% of the galaxies we observed. From the correlation between observed properties of the galaxies in this sample, we demonstrate that the mean surface brightness is a major limiting factor in our ability to measure redshifts of faint objects.

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arXiv1995

Slicing Through Multicolor Space: Galaxy Redshifts From Broadband Photometry

As a means of better understanding the evolution of optically selected galaxies we consider the distribution of galaxies within the multicolor space $U$, $B_J$, $R_F$ and $I_N$. We find that they form an almost planar distribution out to $B_J =22.5$ and $z<0.3$. The position of a galaxy within this plane is dependent on its redshift, luminosity and spectral type. While in the original $U$, $B_J$, $R_F$ and $I_N$ space these properties are highly correlated we can define an optimal rotation of the photometric axes that makes much of this information orthogonal. Fitting the observed spectroscopic redshifts with a quadratic function of the four magnitudes we show that redshifts for galaxies can be estimated to an accuracy better than $Δz =0.05$. This dispersion is due to the photometric uncertainties within the photographic data. Assuming no galaxy evolution we derive a set of simulated galaxy fluxes in the U, J, F and N passbands. Using these data we investigate how the redshift is encoded within the broadband magnitudes and the intrinsic dispersion of the photometric-redshift relation. We find that the signal that defines a galaxy's photometric redshift is not related to specific absorption or emission lines but comes from the break in the overall shape of the galaxy continuum at around 4000 Å. Using high signal-to-noise photometric data we estimate that it is possible to achieve an intrinsic dispersion of less than $Δz =0.02$.

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arXiv1994

Spectral Classification of Galaxies: An Orthogonal Approach

Classification of galaxy spectral energy distributions in terms of orthogonal basis functions provides an objective means of estimating the number of significant spectral components that comprise a particular galaxy type. We apply the Karhunen-Loève transform to derive a spectral eigensystem from a sample of ten galaxy spectral energy distributions. These spectra cover a wavelength range of 1200 Å to 1 $μ$m and galaxy morphologies from elliptical to starburst. We find that the distribution of spectral types can be fully described by the first two eigenvectors (or eigenspectra). The derived eigenbasis is affected by the normalization of the original spectral energy distributions. We investigate different normalization and weighting schemes, including weighting to the same bolometric magnitude and weighting by the observed distribution of morphological types. Projecting the spectral energy distributions on to their eigenspectra we find that the coefficients define a simple spectral classification scheme. The galaxy spectral types can then be described in terms of a one parameter family (the angle in the plane of the first two eigenvectors). We find a strong correlation in the mean between our spectral classifications and those determined from published morphological classifications.

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