arXiv subjects

Marc Postman

Publications and source records attributed to Marc Postman.

103 records · Page 6Linked to original sources

arXiv2001

The Angular Correlation Function of Galaxies from Early SDSS Data

The Sloan Digital Sky Survey is one of the first multicolor photometric and spectroscopic surveys designed to measure the statistical properties of galaxies within the local Universe. In this Letter we present some of the initial results on the angular 2-point correlation function measured from the early SDSS galaxy data. The form of the correlation function, over the magnitude interval 18<r*<22, is shown to be consistent with results from existing wide-field, photographic-based surveys and narrower CCD galaxy surveys. On scales between 1 arcminute and 1 degree the correlation function is well described by a power-law with an exponent of ~ -0.7. The amplitude of the correlation function, within this angular interval, decreases with fainter magnitudes in good agreement with analyses from existing galaxy surveys. There is a characteristic break in the correlation function on scales of approximately 1-2 degrees. On small scales, < 1', the SDSS correlation function does not appear to be consistent with the power-law form fitted to the 1'< theta <0.5 deg data. With a data set that is less than 2% of the full SDSS survey area, we have obtained high precision measurements of the power-law angular correlation function on angular scales 1' < theta < 1 deg, which are robust to systematic uncertainties. Because of the limited area and the highly correlated nature of the error covariance matrix, these initial results do not yet provide a definitive characterization of departures from the power-law form at smaller and larger angles. In the near future, however, the area of the SDSS imaging survey will be sufficient to allow detailed analysis of the small and large scale regimes, measurements of higher-order correlations, and studies of angular clustering as a function of redshift and galaxy type.

astro-ph↗

arXiv2001

A Study of Nine High-Redshift Clusters of Galaxies: IV. Photometry and Sp ectra of Clusters 1324+3011 and 1604+4321

New photometric and spectroscopic observations of galaxies in the directions of three distant clusters are presented as part of our on-going high-redshift cluster survey. The clusters are CL1324+3011 at z = 0.76, CL1604+4304 at z = 0.90, and CL1604+4321 at z = 0.92. The observed x-ray luminosities in these clusters are at least a factor of 3 smaller than those observed in clusters with similar velocity dispersions at z <= 0.4. These clusters contain a significant population of elliptical-like galaxies, although these galaxies are not nearly as dominant as in massive clusters at z <= 0.5. We also find a large population of blue cluster members. Defining an active galaxy as one in which the rest equivalent width of [OII] is greater than 15 Angstroms, the fraction of active cluster galaxies, within the central 1.0 Mpc, is 45%. In the field population, we find that 65% of the galaxies with redshifts between z = 0.40 and z = 0.85 are active, while the fraction is 79% for field galaxies at z > 0.85. The star formation rate normalized by the rest AB B-band magnitude, SFRN, increases as the redshift increases at a given evolving luminosity. At a given redshift, however, SFRN decreases linearly with increasing luminosity indicating a remarkable insensitivity of the star formation rate to the intrinsic luminosity of the galaxy over the range -18 >= ABB >= -22. Cluster galaxies in the central 1 Mpc regions exhibit depressed star formation rates. We are able to measure significant evolution in the B-band luminosity function over the range 0.1 <= z <= 1. The characteristic luminosity increases by a factor of 3 with increasing redshift over this range.

astro-ph↗

arXiv2000

The Infrared Surface Brightness Fluctuation Hubble Constant

We measured infrared surface brightness fluctuation (SBF) distances to an isotropically-distributed sample of 16 distant galaxies with redshifts reaching 10,000 km/s using the near-IR camera and multi-object spectrometer (NICMOS) on the Hubble Space Telescope (HST). The excellent spatial resolution, very low background, and brightness of the IR fluctuations yielded the most distant SBF measurements to date. Twelve nearby galaxies were also observed and used to calibrate the F160W (1.6 micron) SBF distance scale. Of these, three have Cepheid variable star distances measured with HST and eleven have optical I-band SBF distance measurements. A distance modulus of 18.5 mag to the Large Magellanic Cloud was adopted for this calibration. We present the F160W SBF Hubble diagram and find a Hubble constant Ho=76 +/- 1.3 (1-sigma statistical) +/- 6 (systematic) km/s/Mpc. This result is insensitive to the velocity model used to correct for local bulk motions. Restricting the fit to the six most distant galaxies yields the smallest value of Ho=72 +/- 2.3 km/s/Mpc consistent with the data. This 6% decrease in the Hubble constant is consistent with the hypothesis that the Local Group inhabits an under-dense region of the universe, but is also consistent with the best-fit value of Ho=76 km/s/Mpc at the 1.5-sigma level.

astro-ph↗

arXiv2000

Observational Constraints on Higher Order Clustering up to $z\simeq 1

Constraints on the validity of the hierarchical gravitational instability theory and the evolution of biasing are presented based upon measurements of higher order clustering statistics in the Deeprange Survey, a catalog of $\sim710,000$ galaxies with $I_{AB} \le 24$ derived from a KPNO 4m CCD imaging survey of a contiguous $4^{\circ} \times 4^{\circ}$ region. We compute the 3-point and 4-point angular correlation functions using a direct estimation for the former and the counts-in-cells technique for both. The skewness $s_3$ decreases by a factor of $\simeq 3-4$ as galaxy magnitude increases over the range $17 \le I \le 22.5$ ($0.1 \lesssim z \lesssim 0.8$). This decrease is consistent with a small {\it increase} of the bias with increasing redshift, but not by more than a factor of 2 for the highest redshifts probed. Our results are strongly inconsistent, at about the $3.5-4 σ$ level, with typical cosmic string models in which the initial perturbations follow a non-Gaussian distribution - such models generally predict an opposite trend in the degree of bias as a function of redshift. We also find that the scaling relation between the 3-point and 4-point correlation functions remains approximately invariant over the above magnitude range. The simplest model that is consistent with these constraints is a universe in which an initially Gaussian perturbation spectrum evolves under the influence of gravity combined with a low level of bias between the matter and the galaxies that decreases slightly from $z \sim 0.8$ to the current epoch.

astro-ph↗

arXiv1999

High Redshift Clusters and Protoclusters

Our understanding of the cosmic history of galaxy clusters has recently been enhanced due to an extensive series of observations including faint spectroscopic data (especially those obtained at the Keck Observatory), deep optical and NIR imaging from the ground and in space, morphological data from HST, and new constraints on the evolution of the intracluster medium from ROSAT and ASCA. When such observations are applied to complete, objectively derived catalogs of clusters, our constraints on cluster formation and evolution become quite confined. A picture is emerging in which the bulk of cluster formation starts at z >= 2, yielding cluster potentials that are well established by z ~ 1, and there is little substantial evolution of the cluster galaxy population since z ~ 0.4. This review talk will summarize the current observational constraints on the properties and evolution of high redshift clusters and protoclusters and their implications.

astro-ph↗

arXiv1998

Clusters as Tracers of Large-Scale Structure

By virtue of their high galaxy space densities and their large spatial separations, clusters are efficient and accurate tracers of the large-scale density and velocity fields. Substantial progress has been made over the past decade in the construction of homogeneous, objectively derived cluster catalogs and in characterizing the spatial distribution of clusters. Consequently, the constraints on viable models for the growth of structure have been refined. A review of the status of cluster-based observations of large-scale structure is presented here, including discussions of the second and higher order moments, the dependence of clustering on richness (mass), recent and new measurements of bulk flows, and a new constraint on the cluster mass function in the range 0.7 < z < 1.

astro-ph↗

arXiv1998

A Study of Nine High-Redshift Clusters of Galaxies: II. Photometry, Spectra, and Ages of Clusters 0023+0423 and 1604+4304

We present an extensive photometric and spectroscopic study of two high-redshift clusters of galaxies based on data obtained from the Keck 10m telescopes and the Hubble Space Telescope. The clusters CL0023+0423 (z=0.84) and CL1604+4304 (z=0.90) are part of a multi-wavelength program to study nine candidate clusters at z > 0.6 (Oke, Postman & Lubin 1998). Based on these observations, we study in detail both the field and cluster populations. From the confirmed cluster members, we find that CL0023+0423 actually consists of two components separated by ~2900 km/s. A kinematic analysis indicates that the two components are a poor cluster with ~3 x 10^{14} solar masses and a less massive group with 10^{13} solar masses. CL1604+4304 is a centrally concentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226 km/s and a mass of ~3 x 10^{15} solar masses. Approximately 57% and 50% of the galaxies show high levels of star formation in CL0023+0423 and CL1604+4304, respectively. These numbers are significantly larger than those found in intermediate redshift clusters. We also observe many old, red galaxies. Found mainly in CL1604+4304, they have spectra consistent with passive stellar evolution, typical of the early-type galaxies in low and intermediate-redshift clusters. We have calculated their ages by comparing their spectral energy distributions to standard Bruzual & Charlot evolutionary models. We find that their colors are consistent with models having an exponentially decreasing star formation rate with a time constant of 0.6 Gyr. We observe a significant luminosity brightening in our brightest cluster galaxies. Compared to BCGs at z < 0.1, we find a luminosity increase of ~1 mag in the rest B-band and ~0.8 mag in the rest V-band.

astro-ph↗

arXiv1998

Clustering at High Redshift: Precise Constraints from a Deep, Wide Area Survey

We present constraints on the evolution of large-scale structure from a catalog of 710,000 galaxies with I_AB <= 24 derived from a KPNO 4m CCD imaging survey of a contiguous 4 deg x 4 deg region. The advantage of using large contiguous surveys for measuring clustering properties on even modest angular scales is substantial: the effects of cosmic scatter are strongly suppressed. We provide highly accurate measurements of the two-point angular correlation function, w(theta), as a function of magnitude on scales up to 1.5 degrees. The amplitude of w(theta) declines by a factor of ~10 over the range 16 <= I <= 20 but only by a factor of 2 - 3 over the range 20 < I <= 23. For a redshift dependence of the spatial correlation function, xi(r), parameterized as xi(r,z)=(r/r_o)^(-gamma)(1 + z)^(-[3+epsilon]), we find r_o=5.2 +/- 0.4 Mpc/h, and epsilon >= 0 for I <= 20. This is in good agreement with the results from local redshift surveys. At I > 20, our best fit values shift towards lower r_o and more negative epsilon. A strong covariance between r_o and epsilon prevent us from rejecting epsilon > 0 even at faint magnitudes but if epsilon > 1, we strongly reject r_o <= 4/h Mpc (co-moving). The above expression for xi(r,z) and our data give a correlation length of r_o(z=0.5) approx 3.0 +/- 0.4 Mpc/h, about a factor of 2 larger than the correlation length at z = 0.5 derived from the Canada--France Redshift Survey (CFRS). The small volume sampled by the CFRS and other deep redshift probes, however, make these spatial surveys strongly susceptible to cosmic scatter and will tend to bias their derived correlation lengths low. Our galaxy counts agree well with those from the HDF survey and, thus, argue against a significant inclusion of sub-galactic components in the latter census for I < 24.

astro-ph↗

arXiv1998

A Study of Nine High-Redshift Clusters of Galaxies: I. The Survey

We present a description of the observations and data reduction procedures for an extensive spectroscopic and multi-band photometric study of nine high redshift, optically-selected cluster candidates. The primary goal of the survey is to establish new constraints on cluster and galaxy evolution, with specific emphasis on the evolution of galaxy morphology and on the star-formation history of the galaxies within and around distant clusters. We have measured 892 new redshifts for galaxies with R <= 23.3. The data will also serve as deep probes of the foreground and background large-scale structures. The observations include broad band optical imaging and spectroscopy with the Low Resolution Imaging Spectrograph at the 10 meter W. M. Keck Observatory telescope; K-band imaging with IRIM at the 4 meter Kitt Peak National Observatory telescope; and deep, high angular resolution imaging with the WFPC2 onboard the Hubble Space Telescope. We also describe the procedures used to obtain morphological information. We have established that six of the nine cluster candidates are indeed real space density enhancements and are representative of those typically associated with clusters of galaxies. The remaining three candidates appear to be projections of several smaller groups at widely separated distances. This success rate is consistent with estimates of the false positive rate in 2D optical high-z cluster searches.

astro-ph↗

arXiv1997

The Far Field Hubble Constant

We used HST to obtain surface brightness fluctuation (SBF) observations of four nearby brightest cluster galaxies (BCG) to calibrate the BCG Hubble diagram of Lauer & Postman (1992). This BCG Hubble diagram contains 114 galaxies covering the full celestial sphere and is volume limited to 15,000 km/s, providing excellent sampling of the far field Hubble flow. The SBF zero point is based on the Cepheid calibration of the ground I_KC method (Tonry et al. 1997) as extended to the WFPC2 F814W filter by Ajhar et al. (1997). The BCG globular cluster luminosity functions give distances essentially identical to the SBF results. Using the velocities and SBF distances of the four BCG alone gives H_0 = 82 +/- 8 km/s/Mpc in the CMB frame, valid on ~4,500 km/s scales. Use of BCG as photometric redshift estimators allows the BCG Hubble diagram to be calibrated independently of recession velocities, yielding a far field H_0 = 89 +/- 10 km/s/Mpc with an effective depth of ~11,000 km/s. The error in this case is dominated by the photometric cosmic scatter in using BCG as distance estimators. The concordance of the present results with other recent H_0 determinations, and a review of theoretical treatments on perturbations in the near field Hubble flow, argue that going to the far field removes an important source of uncertainty, but that there is not a large systematic error to be corrected for to begin with. Further improvements in H_0 depend more on understanding nearby calibrators than on improved sampling of the distant flow.

astro-ph↗

arXiv1997

Infall Regions of Galaxy Clusters

In hierarchical clustering, galaxy clusters accrete mass through the aggregation of smaller systems. Thus, the velocity field of the infall regions of clusters contains significant random motion superimposed on radial infall. Because the purely spherical infall model does not predict the amplitude of the velocity field correctly, methods estimating the cosmological density parameter Omega_0 based on this model yield unreliable biased results. In fact, the amplitude of the velocity field depends on local dynamics and only very weakly on the global properties of the universe. We use N-body simulations of flat and open universes to show that the amplitude of the velocity field of the infall regions of dark matter halos is a direct measure of the escape velocity within these regions. We can use this amplitude to estimate the mass of dark matter halos within a few megaparsecs from the halo center. In this region dynamical equilibrium assumptions do not hold. The method yields a mass estimate with better than 30% accuracy. If galaxies trace the velocity field of the infall regions of clusters reliably, this method provides a straightforward way to estimate the amount of mass surrounding rich galaxy clusters from redshift data alone.

astro-ph↗

arXiv1996

Brightest Cluster Galaxy Profile Shapes

We model the surface brightness profiles of a sample of 119 Abell Brightest Cluster Galaxies (BCG), finding a generalised deVaucouleurs R^{1/n} law, where n is a free parameter, to be appropriate. Departures from the R^{1/4} law are shown to be a real feature of galaxy profiles, not due to observational errors or coupling of n with the other model parameters. BCG typically have values of n greater than 4. The shape parameter n is shown to correlate with effective half-light radius, such that the larger BCG have larger values of n. This continues a trend noticed amongst ordinary elliptical galaxies and dwarf ellipticals, such that the fainter galaxies have smaller values of n.

astro-ph↗

arXiv1996

The Palomar Distant Cluster Survey : II. The Cluster Profiles

We present a study of the surface density profiles of the clusters of galaxies from the Palomar Distant Cluster Survey (Postman et al. 1996). The survey contains a total of 79 clusters of galaxies, covering the estimated redshift range of $0.2 \simless z \simless 1.2$. We have analyzed the richest clusters in this sample and find that the typical Palomar cluster has a surface density profile of $r^{-1.4}$ ($r \ge 0.10~h^{-1}~{\rm Mpc}$) and a core radius of $0.05~h^{-1}~{\rm Mpc}$. There may be an indication that the slope of the surface density profile steepens with increasing redshift, though the observational uncertainty is at present too large to be conclusive. Our cluster population is inconsistent at a 99.9\% confidence level with a population of azimuthally symmetric clusters.

astro-ph↗