arXiv subjects

James G. Bartlett

Publications and source records attributed to James G. Bartlett.

45 records · Page 3Linked to original sources

arXiv2000

The ALADIN Interactive Sky Atlas

The Aladin interactive sky atlas, developed at CDS, is a service providing simultaneous access to digitized images of the sky, astronomical catalogues, and databases. The driving motivation is to facilitate direct, visual comparison of observational data at any wavelength with images of the optical sky, and with reference catalogues. The set of available sky images consists of the STScI Digitized Sky Surveys, completed with high resolution images of crowded regions scanned at the MAMA facility in Paris. A Java WWW interface to the system is available at: http://aladin.u-strasbg.fr/

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arXiv2000

Sunyaev-Zel'dovich Surveys: Analytic treatment of cluster detection

Thanks to advances in detector technology and observing techniques, true Sunyaev-Zel'dovich (SZ) surveys will soon become a reality. This opens up a new window into the Universe, in many ways analogous to the X-ray band and inherently well-adapted to reaching high redshifts. I discuss the nature, abundance and redshift distributions of objects detectable in ground-based searches with state-of-the-art technology. An advantage of the SZ approach is that the total SZ flux density depends only on the thermal energy of the intracluster gas and not on its spatial or temperature structure, in contrast to the X-ray luminosity. Because ground-based surveys will be characterized by arcminute angular resolution, they will resolve a large fraction of the cluster population. I quantify the resulting consequences for the cluster selection function; these include less efficient cluster detection compared to idealized point sources and corresponding steeper integrated source counts. This implies, contrary to expectations based on a point source approximation, that deep surveys are better than wide ones in terms of maximizing the number of detected objects. At a given flux density sensitivity and angular resolution, searches at millimeter wavelengths (bolometers) are more efficient than centimeter searches (radio), due to the form of the SZ spectrum. Possible ground-based surveys could discover up to $\sim 100$ clusters per square degree at a wavelength of 2 mm and $\sim 10$/sq. deg. at 1 cm, modeling clusters as a simple self-similar population.

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arXiv1999

The Standard Cosmological Model and CMB Anisotropies

This is a course on cosmic microwave background (CMB) anisotropies in the standard cosmological model, designed for beginning graduate students and advanced undergraduates. ``Standard cosmological model'' in this context means a Universe dominated by some form of cold dark matter (CDM) with adiabatic perturbations generated at some initial epoch, e.g., Inflation, and left to evolve under gravity alone (which distinguishes it from defect models). The course is primarily theoretical and concerned with the physics of CMB anisotropies in this context and their relation to structure formation. Brief presentations of the uniform Big Bang model and of the observed large--scale structure of the Universe are given. The bulk of the course then focuses on the evolution of small perturbations to the uniform model and on the generation of temperature anisotropies in the CMB. The theoretical development is performed in the (pseudo--)Newtonian gauge because it aids intuitive understanding by providing a quick reference to classical (Newtonian) concepts. The fundamental goal of the course is not to arrive at a highly exact nor exhaustive calculation of the anisotropies, but rather to a good understanding of the basic physics that goes into such calculations.

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arXiv1998

The mean density of the Universe from cluster evolution

The determination of the mean density of the Universe is a long standing problem of modern cosmology. The number density evolution of x-ray clusters at a fixed temperature is a powerful cosmological test, new in nature (Oukbir and Blanchard, 1992), somewhat different from standard analyses based on the dynamical measurement of individual objects. However, the absence of any available sample of x-ray selected clusters with measured temperatures at high redshift has prevented this test from being applied earlier. Recently, temperature measurements of ten EMSS clusters at $0.3 \le z \le 0.4$ have allowed the application of this test (Henry, 1997). In this work, we present the first results of a new analysis we have performed of this data set as well as a new estimation of the local temperature distribution function of clusters: a likelihood analysis of the temperature distribution functions gives a preferred value for the mean density of the universe which corresponds to 75% of the critical density. An open model with a density smaller than 30% of the critical density is rejected with a level of significance of 95%.

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arXiv1997

Constraints on Hubble's Constant, Omega_baryon and Lambda from Cosmic Microwave Background Observations

In this paper we compare data to theory. We use a compilation of the most recent cosmic microwave background (CMB) measurements to constrain Hubble's constant h, the baryon fraction Omega_b, and the cosmological constant lambda. We fit h-, Omega_b- and lambda-dependent power spectra to the data. The models we consider are flat cold dark matter (CDM) dominated universes with flat (n=1) power spectra, thus the results obtained apply only to these models. CMB observations can exclude more than half of the h - Omega_b parameter space explored. The CMB data favor low values of Hubble's constant; h \approx 0.35. Low values of Omega_b are preferred (Omega_b ~ 0.03) but the chi-squared minimum is shallow and we obtain Omega_b < 0.28. A model with h \approx 0.40, Omega_b \approx 0.15 and Omega_CDM \approx 0.85 is permitted by constraints from the CMB data, BBN, cluster baryon fractions and the shape parameter Gamma derived from the mass density power spectra of galaxies and clusters. For flat-lambda models, the CMB data, combined with BBN constraints exclude most of the h - lambda plane. Models with Omega_o \approx 0.3, lambda \approx 0.7 with h \approx 0.75 are fully consistent with the CMB data but are excluded by the strict new q_{o} limits from supernovae (Perlmutter et al. 1996). A combination of CMB data goodness-of-fit statistics, BBN and supernovae constraints in the h-lambda plane, limits Hubble's constant to the interval 0.23 < h < 0.72.

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arXiv1997

Galaxy Clusters in Cosmology: Cluster Abundance as a Probe of Structure Formation

In gaussian theories of structure formation, the galaxy cluster abundance is an extremely sensitive probe of the density fluctuation power spectrum and of the density parameter, $Ω$. We develop this theme by deriving and studying in detail the mass function of collapsed objects and its relation to these quantities. Application to current data yields constraints which are degenerate between the amplitude of the perturbations and the density parameter; we nevertheless obtain an important limit on the present day mass perturbation amplitude as a function of $Ω$ and can rule-out the `standard' cold dark matter (CDM) model. Future observations of the evolution of the cluster abundance will break the degeneracy and provide important constraints on both the power spectrum and the density parameter, individually. We focus primarily on X-ray clusters in the discussion, and finish with a presentation of the promising new field of Sunyaev-Zel'dovich observations of cluster evolution.

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arXiv1996

Cross Wavelength Comparison of Images and Catalogs

We discuss some aspects of data management important to cross-wavelength comparison of images and catalogs. We then consider the application of statistics to the general problem of object cross-identification. Finally, two software systems developed as tools to aid the comparison of catalogs and images are briefly presented.

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arXiv1996

The Sunyaev-Zel'dovich Effect as a Probe of $Ω_{o}$

We examine how the Sunyaev-Zel'dovich (SZ) effect of clusters can probe $Ω_{o}$. Using self-consistent models of X-ray clusters in the context of hierarchical models of structure formation, we show that both the mean Compton {\it y} parameter and the number of clusters observed via the SZ effect strongly depend on $Ω_{o}$. These quantities are higher in open cosmological models due to the earlier epoch of structure formation. Specifically, we compare two models which are able to reproduce the present abundance of X-ray clusters, one for $Ω_{o}=0.2$ and the other $Ω= 1$. For $Ω_{o}=0.2$, ${\ymean}$ exceeds the current FIRAS limit of $1.5\times10^{-5}$. Also, the SZ redshift source distribution considerably differs in the two cosmological models. Thus, these results show that both an improvement in our knowledge of CMB spectral distortions and the realization of millimeter surveys to look for SZ sources over a large area of the sky should produce interesting constraints on $Ω_{o}$ and on the evolution of the baryonic fractionof virialized structures.

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arXiv1995

The Significance of the Cosmic Virial Theorem

We discuss the development and physics of the cosmic virial theorem and its traditional application as an indicator of the cosmic mean density. The standard result that the density must be sub-critical relies on the assumption that the galaxy three-point correlation function accurately describes the mass distribution around a typical pair of galaxies. To challenge this assumption, we develop a simple model of this mass distribution based on the extension of galactic halos. In such a model, one sees clearly the importance of the local mass distribution around pairs to the prediction of the galaxy pair-wise velocity dispersion. The model provides insight into the mechanics of the theorem and demonstrates that a flat universe is {\it consistent} with the observations. More generally, we conclude that the relative pair-wise velocity dispersion probes the mass clustered around galaxies and that the observations, viewed in this light, indicate a lower bound of $\sim 300\; h^{-1}$kpc on the radius of galactic halos.

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