We describe a major new Anglo-Australian proposal for a U.K. Schmidt Telescope (UKST) Hα survey of the Southern Galactic plane, Magellanic clouds and selected regions. The agreed survey will use a new 12 × 12 inch monolithic Hα interference filter of very high specification in combination with Tech Pan film. Tech Pan offers significant advantages for this work due to an inherent sensitivity at Hα and its extremely fine grain, high resolution, exceptional DQE, excellent imaging and low noise (e.g., Parker et al. 1994). The combination of Tech Pan and a narrow band Hα filter will provide a survey of unprecendented area coverage, depth and resolution, superior to any previous optical survey of ionized gas in the galaxy. It should to lead to exciting new discoveries and avenues of research.

We are presently carrying out a northern hemisphere survey of the Galactic Hα emission. Our instrument, the Virginia Tech Spectral Line Imaging Camera (SLIC) utilizes a fast objective lens (f/1.2) with a cryogenically-cooled TK 512×512 CCD. A filter wheel in front of the lens allows us to select interference filters, including a narrowband Hα filter and a broader bandpass continuum filter in a line free part of the spectrum. The fast optics in combination with the low noise CCD result in sub-Rayleigh sensitivity at confusion limited levels. (1 Rayleigh = 106/4π photons cm–2 s–1 sr–1.) This corresponds to an emission measure sensitivity of ≈ 1 pc cm–6. Parameters of our system are given in Table 1.

We present Hα images with 0.1° resolution and fields of view larger than 10°. In some regions of the sky, the Balmer Hα emission is correlated positively with IRAS 100 μm emission. Observations of such sensitivity and angular scale as these provide a new view of the interstellar medium of our Galaxy (see also Dennison et al., this volume, p. 182) and may allow us to distinguish between Galactic foreground and cosmic background for both the free-free emission and the thermal dust emission associated with the warm ionized medium of the Milky Way.

The Marseille Observatory Hα survey supplies Hα velocities of the ionized hydrogen over large zones of the sky towards the galactic plane. This survey, led at the ESO La Silla, uses a 36 cm telescope equiped with a scanning Fabry-Perot interferometer and a photon counting camera (Le Coarer et al. 1992). About 250 fields (39′×39′) toward the galactic plane have already been covered (see Figure 1) with a spatial resolution of 9″×9″ and a spectral resolution of 5 km s–1. This allows us to observe the discrete HII regions and the diffuse ionized gas widely distributed between them and to separate the distinct layers found along the line of sight. HII regions are often grouped on the molecular cloud surface, then CO, radio continuum and recombination lines surveys of the galactic plane are also essential to distinguish the HII region-molecular cloud complexes met on the line of sight, and in order to take dynamical effects into account, such as the champagne effect, for the kinematic distance determination. Indeed, the spiral structure pattern determination requires avoiding any artificial spread by clearly identifying the giant complexes composed of molecular clouds, HII regions, diffuse ionized hydrogen widely surrounding them, and exciting stars. On the other hand the ionized gas data (Hα and recombination lines) associated with IRAS data help us to study the nature of the young objects constituent of these complexes and to assess their detectability. We present two fields from the Hα survey and parallel large scale investigations.

Molecular gas in the Galactic center region is spatially and kinematically complex, and its physical conditions are distinctively different from those of molecular gas in the Galactic disk (e.g., Morris 1996). Relative paucity of current star formation activity, despite the abundance of dense molecular gas in this region, is one of the problem at issue.

The interstellar medium (ISM) is rich with structure on varying size scales, reflecting its diverse energetics and dynamics. A step toward understanding this structure is to enhance the visibility of the structure on finer size scales. We present maps of a section of the Galactic plane filtered with the Median Normalized Spatial Filter (MNSF, a median smoothed map is divided into the original map), which emphasizes higher latitude emission relative to that of the plane itself (see also Waller et al., this volume, p. 194). The maps also illustrate the spatial correlations between the interstellar dust and the various gas phases: ionized, atomic, and molecular. The dust is represented by the IR emission maps of the IRAS survey, the ionized gas by the radio continuum maps at 1.4 GHz (Reich 1978, northern sky) and 2.3 GHz (Jonas & Baart 1995, southern sky), the atomic gas by surveys of the HI 21 cm line emission (Heiles & Habing 1974, Weaver & Williams 1973, Cleary et al. 1979, Kerr et al. 1986), and the molecular gas by surveys of the CO J = 1 → 0 line emission (Dame et al. 1987).

What is the general morphology of the diffuse interstellar medium? Is it mostly uniform or clumpy? Are the clumps mostly in the form of spheroidal clouds, sinuous filaments, extended sheets, or discrete shells? And do the clumps or the voids better define the overall structure? By addressing these morphological questions, one can better constrain the dynamical processes that are most responsible for shaping and energizing the ISM.

To paint with broad brush strokes, the spectrum of Galactic formation models has as extrema what may be termed the “fast and smooth” and the “slow and lumpy” scenarios. Appropriate or not, to ascribe as champions of these views the works of, respectively, Eggen et al. (1962, ELS hereafter) and Searle & Zinn (1978, SZ hereafter) has become de rigueur—though it is prudent to bear in mind the soft focus of historical perspective, and the maxim that “no one remembers what one has said, only what others say one has said” (on this point, see Sandage 1990 and Majewski 1993, section 1).

The analysis of stellar kinematics and the Galactic potential is linked to the study of the spatial distribution of stars in the Galaxy since they are related through the Boltzmann and Poisson equations. Measuring all the visible density and mass distribution from general star counts and the gas cloud density gives only a small fraction of the total amount of the dynamical mass that is deduced from the kinematics of the galactic constituents. As in many spiral galaxies, most of the Galactic mass is unseen and unknown.

Our galaxy was first seen to be warped, similarly to other galaxies, in 21cm HI surveys (Kerr 1957). Since then a large database on our and other warps has been built but they remain a puzzle for theorists trying to construct a general warp theory. Warps are observed to be common rather than rare features of spiral galaxies, which implies they are long lived. They are observed in both isolated and multiple galaxy systems in the gas but the stellar warp is often difficult to detect. The galactic warp is always below b=10° making it difficult to observe because of absorption and confusion, to add to this problem the sun appears to lie near to the line of nodes and to view the warp at it's largest extent we must observe through a large part of the disk. A theoretical model must evoke driving forces that explain the longevity, commonality, and independence of environment that we see in warps.

We have carried out a sample survey in UBVR photometry and proper motions in various directions in the Galaxy. Three fields in the direction of galactic anticentre, centre, and antirotation have been surveyed. Using our new data together with wide-area surveys in other fields available to date, we discuss the radial and vertical structure of the Galaxy. Our results confirm that the thick disk population is distinct from other populations based on their kinematical and spatial distribution. The most probable value of scale height for the thick disk component is determined to be hz≃760±50 pc and a local density of ≃7.4+2.5−1.5% relative to the thin disk. The ratio of the number of thick disk stars in our galactic centre region to that in anticentre region yield hR≃3±1 kpc for the scale length of thick disk. These values are in perfect agreement with the recent determination given by Robin et al. (1996).

We are conducting the deepest and largest photographic proper-motion survey ever undertaken of the Galactic bulge. Our first-epoch plate material (from 1972-3) goes deep enough (Vlim ∼ 22) to reach below the bulge main-sequence turnoff. These plates cover an area of approximately 25′ × 25′ of the bulge in the low-extinction (Av ∼ 0.8 mag) Plaut field at l= 0°, b= −8°, approximately 1 kpc south of the nucleus. This is the point at which the transition between bulge and halo populations likely occurs and is, therefore, an excellent location to study the interface between the dense metal-rich bulge and the metal-poor halo.

The maturity of all-sky surveys hold great promise for galactic structure studies. Surveys such as the APS Catalog of POSS I allow large scale studies of stellar distributions where the data is very complete to a reasonably faint magnitude, uniformly reduced, and independently calibrated. The APS data covers a large portion of the sky, allowing studies where the primary source of uncertainty is no longer small-number statistics.

Faint surveys of accurate colors and proper motions are fundamental for studying the physical and kinematical properties of the stellar populations in the Galaxy. A program designed to address such issues has been initiated on selected fields utilizing prototype material from the GSC-II project (see McLean et al. in this volume, p. 431). Photographic photometry V, B–V, V–Rc, is derived down to V = 18.5 with a precision of about 0.1 mag, together with absolute proper motions to σμ ≃ 3 mas/yr.

Thanks to the DENIS program (Deep Near-Infrared Survey of the Southern Sky), relatively deep near-infrared star counts are now available for the first time on a large scale. The basic method to interpret star counts in terms of galactic structure is to compare them with predictions given by models of the point source sky. Of particular promise are studies with DENIS of the spatial distribution of evolved stars in our Galaxy, thanks to its high sensitivity to red giant and to the much lower interstellar extinction that hampers visual observations of far-away stars in the disc of our Galaxy. In this paper, I present a sample of extensive comparisons between two models of the Galaxy and DENIS star counts (Ruphy 1996). I will focus on the analysis of star counts in the anticenter direction, that leads to new values for the distance of the cutoff and the radial scale length of the stellar disc.

The Hubble Deep Field (HDF, Williams et al. 1996), recently acquired with the Hubble Space Telescope, is the deepest, most detailed optical view of the Universe. Even though intended primarily for the study of the high-redshift Universe, the HDF provides a unique opportunity to find faint stellar objects in our Galaxy, set constraints on number of low-mass Halo stars, estimate the contribution of baryonic matter to the dark Halo, investigate the nature of microlensing sources, and to calibrate Galactic Structure models. The HDF has been an STScI initiative to provide the deepest exposures yet acquired with HST on a non-proprietary basis. Observations of an ‘anonymous field’ located at l = 125.9°, b = 54.8° were performed on the continuous viewing zone, for a total of 150 orbits (10 consecutive days) in the four HST passbands F300W, F450W, F606W and F814W. These observations reached 5σ magnitude limits (for Galaxies) of roughly (STMAG) 30th mag in all these passbands (except F300W which reaches 27th mag). We have used a novel software detection & classification algorithm to create a sample of point-like objects in the HDF. We have also compared the observed stellar counts to constrain the faint end of the Halo field luminosity function by using a recent Galactic Structure model. The unexpected appearance of a faint-blue group of very compact objects is discussed as well.

The Astrophysics Data Facility at NASA Goddard Space Flight Center supports the processing, management, and dissemination of data obtained by past, current, and future NASA and international astrophysics missions, and promotes the effective use of those data by the astrophysics community, educators, and the public. Our Multiwavelength Milky Way poster was printed for broad distribution. It depicts the Galaxy at radio, infrared, optical, X-ray, and gamma-ray wavelengths. In particular, the poster contains images of the Galactic 21-cm and CO (J = 1 → 0) line emission, and IRAS 12, 60, and 100 μm, COBE/DIRBE 1.25, 2.2, and 3.5 μm, Digitized Sky Survey optical wavelength, ROSAT/PSPC 0.25, 0.75, and 1.5 keV X-ray, and CGRO/EGRET E > 100 MeV gamma ray broadband emission. All of the data sets are publicly available. Captions describe the Milky Way and what can be learned about the Galaxy from measurements made in each segment of the electromagnetic spectrum. The poster is intended to be an educational tool, one that will stimulate heightened awareness by laypersons of NASA's contribution to modern astronomy.

Through an interface available on the World Wide Web at http://adf.gsfc.nasa.gov/adf/adf.html one may view the images that appear on the poster, read the poster captions, and locate the archived data and references.

Light rays from distant sources are deflected if they pass near an intervening matter inhomogeneity. This gravitational lens effect is responsible for the well-established lens systems like multiple-imaged QSOs, (radio) ‘Einstein’ rings, the giant luminous arcs in clusters of galaxies, and the flux variations of stars in the LMC and the Galactic bulge seen in the searches for compact objects in our Galaxy. These types of lensing events are nowadays called ‘strong lensing,’ to distinguish it from the effects discussed here: light bundles are not only deflected as a whole, but distorted by the tidal gravitational field of the deflector. This image distortion can be quite weak and can then not be detected in individual images. However, since we are lucky to live in a Universe where the sky is full of faint distant galaxies, this distortion effect can be discovered statistically. This immediately implies that weak lensing requires excellent and deep images so that image shapes (and sizes) can be accurately measured and the number density be as high as possible to reduce statistical uncertainties. Weak gravitational lensing can be defined as using the faint galaxy population to measure the mass and/or mass distribution of individual intervening cosmic structures, or the statistical properties of their mass distribution, or to detect them in the first place, independent of the physical state or nature of the matter, or the luminosity of these mass concentrations. In addition, weak lensing can be used to infer the redshift distribution of the faintest galaxies. After introducing the necessary concepts, I will list the main applications of weak lensing and discuss some of them in slightly more detail, stressing the need for very deep and wide-field images of the sky taken with instruments of excellent image quality.

George Djorgovski's message to me on behalf of the organizing committee suggested that I discuss 1) Evolution of the Quasar Luminosity Function; 2) Possible Differences among Radio-Loud, Radio-Quiet, and X-Ray Selected Samples; 3) Quasars as Probes of Clustering and Large-Scale Structure at High Redshift; 4) Searches for Extremely Distant Quasars; 5) Theoretical Implications; 6) Major Surveys in Progress and Planned; and 7) What Surveys Should We Do in the Future. To cover all these topics in a 25-minute talk and 7-page article is a challenge, but I am happy to try.

Active Galactic Nuclei (AGN) are ideal sources for multi-wavelength studies as their emission can cover almost 20 orders of magnitude in frequency from the radio to the γ-ray band. After reviewing their basic properties, I will assess how well we know the multifrequency spectra of AGN as a class. I will then briefly illustrate how currently available and forthcoming sky surveys will help in addressing some of the open questions of AGN studies. Finally, an analysis of the problem of the missing Type 2 QSO will exemplify the dangers of monochromatic sky surveys for AGN.