If a galaxy cluster's X-ray gas distribution follows an isothermal polytropic β model, we may write the electron radial density distribution as; ne = ne0(1 + r2/rc2)–3/2β, rc being the core radius and ne0 the central electron density. This may be related to both an X-ray surface brightness distribution and a Sunyaev-Zel'dovich effect distribution (Sarazin 1986). Fitting to observational data then enables us to constrain the value of β. The normalisation value, ne0, to obtain a total mass estimate is calculated via the relationship between the X-ray and S-Z distribution normalisation constants, and the gas temperature and spectral emissivity parameters from fits to the X-ray spectrum. We are then in a position to evaluate ne(r) and its integral; the total electron gas mass. If we can further assume that there exists a simple ratio between the electron and proton number densities within the gas, we may straightforwardly posit a value for the total gas mass. An additional method of determining the polytropic gas index exists, with optical constraints on the galactic velocity dispersion, through the relation; β = μmHσz2/kBTe. Studies at optical, as well as X-ray and radio wavelengths are thus useful as a corroborative measure in determining the total gas mass.

The dominant baryonic component of clusters of galaxies is their X-ray emitting atmosphere. X-ray surveys have made it possible to examine this component in large samples of clusters. David et al. (1993) provided a catalog of X-ray temperatures. I have measured 79 clusters from that list using Einstein IPC data (Harris et al. 1990), determining several measures of the radius. The most useful turns out to be the Petrosian radius. Given a radius, one can measure the surface brightness at that position.

We have used the 1st revision of the WGACAT catalogue, generated from all ROSAT PSPC pointed observations from February 1991 to March 1994, available at the HEASARC public archive. This catalogue, described by White et al. (1994), contains 68,907 detections, with more than 62,000 individual sources.

Ultraluminous infrared galaxies (ULIRGs) have luminosities (1012L⊙) once exclusive to QSOs. This suggests they might be the early, dust-enshrouded stages of QSOs. ULIRGs have ∼ 3.5× the space density of QSOs at the present epoch. Quasars reached their peak space density at z ∼ 2, so if ULIRGs are QSO precursors, there should be a dramatic increase in their space density up to z ∼ 2. The small number of known ULIRGs makes it difficult to explore links between ULIRGs and QSOs, much less measure their evolution. To do so, a large sample of ULIRG candidates must be identified. The IRAS FSC contains ∼ 60,000 probable galaxies, of which 1%–3% should have LFIR > 1012L⊙. We discuss an efficient and reliable method which uses IRAS-VLA-APS cross-identification and flux ratios to mine the FSC for likely ULIRGs.

The intense study of interacting galaxies originated in large part with the discovery by IRAS that the most IR-luminous galaxies are nearly all products of collisions and may be the missing link in the chain of evolution from quasars to normal quiescent galaxies (Sanders et al. 1988a,b). These galaxies (with LIR > 1012L⊙) are considered to be the most strongly starbursting of all galaxies in the local universe, have a higher space density than quasars, emit >90% of their power in the IR, are rich in the raw materials of star formation, and to a large extent owe their peculiar morphologies to encounters with other galaxies. The particular importance of IR-luminous galaxies in the grand scheme of cosmology and galaxy evolution has been underscored by the luminosity function studies of Soifer et al. (1986), which indicated that most galaxies have gone through a high-IR luminosity stage. We are using the Hubble Space Telescope to survey the fine-scale features that are associated with the interaction- and activity-related processes that are at work within the Ultraluminous IR Galaxy Sample. It is widely believed that these galaxies are undergoing star formation at a prodigious rate and are abnormally dust-enshrouded. An alternative to the starburst hypothesis is that these galaxies' IR luminosity is powered by a dust-hidden quasar at its center (Sanders et al. 1988a). It is important for our understanding of the evolution of galaxies and quasars to determine which of these two hypotheses is valid, or in which objects they are separately valid.

The luminosity function of galaxies is central to many problems in cosmology, including the interpretation of faint number counts. The near-infrared provides several advantages over the optical for statistical studies of galaxies, including smooth and well-understood K-corrections and expected luminosity evolution. The K–band is dominated by near-solar mass stars which make up the bulk of the galaxy. The absolute K magnitude is a measure of the visible mass in a galaxy, and thus the K–band luminosity function is an observational counterpart of the mass function of galaxies.

We have conducted a wide-field K-selected galaxy survey with complementary optical I- and B-band imaging in six fields with a total coverage of 9.8 square degrees (Huang et al. 1997). The observations were carried out on the UH 0.6m and the UH 2.2m telescopes. The purpose of this survey is to study the properties of the local galaxies and explore the evolution of K-selected galaxies at low redshifts. Star-galaxy discrimination is performed using both galaxy color properties and object morphologies, and 6264 galaxies are found. This survey establishes the bright-end K-band galaxy number counts in the magnitude range 13 < K < 16 with high precision. We find that our bright-end counts have a significantly steeper slope than the prediction of a no-evolution model, which cannot be accounted for by observational or theoretical error. Since it is very unlikely that there is sufficient evolution at such low redshifts to account for this effect, we argue that there is a local deficiency of galaxies by a factor of 2 on scale sizes of around 300 Mpc. This would imply that local measurements of Ω0 underestimate the true value of the cosmological mass density by this factor and that local measurements of H0 could be high by as much as 33%.

Near-infrared (NIR) emission in galaxies is mainly radiated by old population low temperature stars, which construct the basic stellar structure and keep the trails of past galaxy evolution. On the other hand, optical observations show recent star formation activity, especially in spiral galaxies. Therefore multi-color observations from optical to near-infrared wavelengths are very important to understand the past and recent star-formation history. Nearby large galaxies are well studied not only in optical but also in mid- and far-infrared by IRAS, CO and HI radio observations. However, the study in the near-infrared is still limited because large format arrays are not common. Here we show a wide-field, near-infrared imaging of nearby elliptical and spiral galaxies and discuss their star-formation history.

At present, the photometric data for clusters at z ≲ 0.2 mainly come from photographic photometry. The lack of CCD data for such clusters is simply due to the fact that no CCD camera had been available until recently that covers the wide extension of clusters within a reasonable amount of observing time. We have developed a large mosaic CCD camera and conducted multicolor imaging observations of z ≲ 0.2 clusters using the 40-inch Swope telescope at Las Campanas Observatory.

The Sloan Digital Sky Survey (SDSS) will image 10,000 square degrees in the north galactic cap in five filters. We hope to identify and obtain spectra for about 100,000 quasars brighter than 20th magnitude in this area. The selection will be primarily on the basis of point spread function and colors, but we will also identify quasars from a catalog of FIRST radio sources. The selection areas in color space must be determined during the testing period prior to the official start of the survey. This task may determine the length of the test period. In anticipation of this becoming the critical path, we have written a body of software that will allow us to quickly analyze a set of multicolor data and make a first cut at the selection limits.

The Calar Alto Deep Imaging Survey (CADIS) combines deep multicolor imaging in broad and narrow band filters with deep probing in several selected wavelength intervals through a Fabry-Pérot (FP) interferometer with a spectral resolution of ∼450.

The Calar Alto Deep Imaging Survey (CADIS) is a very deep emission line survey using a Fabry-Pérot (FP), combined with deep broad- and medium-band photometry (for an overview see Hippelein et al. 1996). This survey is specifically designed to detect primeval galaxies, but it will in addition produce a large data base for investigations of faint galaxies at intermediate redshifts (0.2 < z < 1.2). We present some first results from the initial data recorded with the CADIS strategy.

We present first results of the Hamburg/SAO Survey of emission-line galaxies (hereafter HSS, SAO—Special Astrophysical Observatory, Russia) initiated to search for extremely metal-deficient (Z < Z⊙/10) galaxies and to create a large sample of Blue Compact Galaxies (BCG). This “Northern BCG Sample,” will be assembled by merging the HSS with samples from the Second Byurakan Survey (SBS) (Stepanian et al. 1987) and the Case Low-Dispersion Northern Sky Survey (Pesch et al. 1991).

We have initiated a major new survey for emission-line galaxies (ELGs) which we call the KPNO International Spectroscopic Survey (KISS). Survey observations began in March 1994 with the 0.61-m Burrell Schmidt telescope. The technique we employ combines the benefits of a traditional photographic objective-prism survey with the advantages of using a CCD detector. The field of view of our CCD is 1.1° square, and the prism employed provides a dispersion of 19 Å/pixel at 5000 Å. The spectral range covered (4800-5500 Å) is restricted by a specially designed filter that transmits from rest-frame Hβ to just shortward of the strong night-sky line at 5577 Å; this greatly reduces the sky background. We expect KISS to be sensitive to galaxies with magnitudes as faint as B = 20m–21m, much deeper than existing photographic surveys. Our initial pilot project covers 100 square degrees (Salzer et al. 1994) and overlaps the CfA/Dartmouth Century Redshift Survey (α = 8h30m–16h45m and δ = 29°–30°) in the North Galactic cap.

The correlation of an X-ray survey like the ROSAT All-Sky Survey (RASS) with optical surveys like the Hamburg Quasar Survey (HQS) can significantly increase the number of known BL Lac objects.

We have embarked on a survey of ROSAT PSPC archival data searching for all detected surface brightness enhancements due to sources in the innermost R ≤ 15′ of the PSPC field of view in the energy band 0.5–2.0 keV. This project is part of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarily to measure the low luminosity, high redshift, X-ray luminosity function of galaxy clusters and groups. Accurate measurements of the high redshift XLF would allow the form of the XLF evolution to be determined via the position of the Schechter function break. This would help discriminate between luminosity and density evolution, and discriminate between different hierarchical models, e.g., those including a different mix of fundamental particles, a flat power spectrum of the initial fluctuations, and reheating of the intracluster gas at high redshifts.

The WARPS (Wide-Angle ROSAT Pointed Survey) blazar survey is a deep X-ray search for BL Lac objects and flat-radio-spectrum quasars (FRSQs), drawn from a cross-correlation of serendipitous sources in the ROSAT PSPC database WGACAT (White et al. 1994) with the Green Bank 6 cm and 20 cm (Condon et al. 1989, Condon & Broderick 1985), the Parkes radio (Bolton et al. 1979), and the Parkes-MIT-NRAO (Griffith & Wright 1993, Wright et al. 1994, Griffith et al. 1994, 1995) catalogs. Our sample contains 165 new blazar candidates and 95 previously known blazars.

The origin of the Cosmic X-ray Background (CXB) radiation has been investigated extensively by soft X-ray deep survey imaging observations with Einstein and ROSAT. In contrast, the lack of telescopes capable of detecting hard X-rays has prevented us from extensive study of the nature of the CXB in the energy range above 2 keV before ASCA.

How is the universe organized on large scales? How did this structure evolve from the unknown initial conditions of a rather smooth early universe to the present time? The answers to these questions will shed light on the cosmology we live in, the amount, composition and distribution of matter in the universe, the initial spectrum of density fluctuations that gave rise to this structure, and the formation and evolution of galaxies, lusters of galaxies, and larger scale structures.

To address these fundamental questions, large and accurate sky surveys are needed—in various wavelengths and to various depths. In this presentation I review current observational studies of large scale structure, present the constraints these observations place on cosmological models and on the amount of dark matter in the universe, and highlight some of the main unsolved problems in the field of large-scale structure that could be solved over the next decade with the aid of current and future surveys. I briefly discuss some of these surveys, including the Sloan Digital Sky Survey that will provide a complete imaging and spectroscopic survey of the high-latitude northern sky, with redshifts for the brightest ∼ 106 galaxies, 105 quasars, and 103.5 rich clusters of galaxies. The potentialities of the SDSS survey, as well as of cross-wavelength surveys, for resolving some of the unsolved problems in large-scale structure and cosmology are discussed.

We have conducted a survey of the Lyα forest in the redshift domain 2.15 < z < 3.37 in front of nine QSOs within a 1o field to probe spatial structure along planes perpendicular to the line-of-sight. We find evidence for correlations of the Lyα absorption line wavelengths in the whole redshift range, and, at z > 2.8, of their equivalent widths. Such a correlation is consistent with the emerging picture that Lyα lines arise in filaments or large, flattened structures.