In a nutshell, the basic vision of the `virtual observatories' (VO) involves interconnected, globally distributed archives from observatories and large-scale sky surveys which are federated using common database query standards and data interchange protocols, combined with user interfaces and data mining tools that integrate and analyze the fused, multi-wavelength data sets to facilitate making new discoveries about the Universe (regardless of the location of the data or the investigators). A popular level description of the concept is given in an article by Cowen . The term `virtual observatory' is new, but the concept is essentially the same as that outlined a decade ago for the original Astrophysics Data System (ADS) . Portions of the VO concept also overlap, to some degree, the functionality of NED with respect to the production of multi-wavelength catalog cross-identifications and statistical associations, queries that join distributed archives, and support for studies of fused panchromatic data for extragalactic sources. It is noteworthy that Helou et al.  pointed out in 1991 that there is a dual challenge presented by the explosive growth in astronomical data: "dealing with the sheer volume, but also inter-connecting intelligently the huge variety of information available." Yet there is much work left to be done on all fronts by a broad community of astronomers teamed with computer scientists and programmers. The NED team shares a common vision regarding what a VO can enable for all fields of astrophysics, and we are actively involved in numerous collaborations and proposals designed to lay the foundation of the VO and extend its functionality. It is useful to summarize the role of NED in the emerging global VO in the context of current capabilities and logical future enhancements that will support related projects.
4.1. Current Capabilities
As the primary astrophysics data integration service of NASA's Space Science Data System (13), in cooperation with the Astrophysics Data Center Coordinating Council (ADCCC) (14) and new VO partners, NED will continue to establish and improve high fidelity relationships between multi-wavelength data with anchors to the literature for millions of extragalactic objects, using a combination of computer software that utilizes positional uncertainties and astronomical source properties (e.g., fluxes, redshifts, sample source densities), followed by human inspection to resolve important, complex cases that cannot be fully automated. In addition, through collaboration on a number of proposals to NASA and the National Science Foundation, the NED team is committed to participation in the collaborative development, testing, and deployment of the next generation of catalog cross-identification software tool-kits. The idea is that new tools that may be developed for individual researchers to construct dynamic catalog cross-identifications in a VO framework, for example enabling Bayesian type probabilities of association that incorporate prior astrophysical knowledge about a sample , will also be useful for performing rapid, bulk cross-identifications of new surveys against the data synthesis in NED. The advantages of this approach (over repeatedly performing dynamic cross-comparisons against a plethora of original catalogs) for many types of studies were reviewed in Section 2.
NED will also continue to maintain its object-based portal into distributed data sets (Section 3.1.2, Figure 3), in order to make this innovative service even more useful for a wide cross-section of the extragalactic research community. This work will involve keeping up with evolving technology for interoperability between archive query services, primarily XML and its associated family of protocols (e.g., XLink, Namespaces, DTD, Schema, CSS, XSL, SOAP) (15) as they are adopted by the community of VO developers (along with the much broader Internet industry).
4.2. Future Enhancements
In addition to staying current with the literature and extragalactic source observations in modern large-scale sky surveys, the NED team is committed to providing new functionality to extend the usefulness of NED as a research tool for astronomers. The newer NED capabilities with direct relevance to the VO concept were reviewed above. Over the next few years NED will provide the following enhanced capabilities for even tighter integration with other VO initiatives: (1) development of a spectral archive for extragalactic sources, focusing on reduced, calibrated spectra contributed by authors and links to spectra in mission/observatory archives (very similar to the NED image archive); (2) enhancements to the `By Parameters' tool to support all-sky queries based on multi-wavelength flux and color criteria; (3) upgrades to the `Batch Mode' to support larger result sets with output content and formats that can be configured by users for input into data mining applications; (4) integration of the `By Parameter' service on the Web interface with the Batch Mode to support queries that run too long to maintain connectivity with a Web browser; (5) production of an XML server mode, with the many benefits summarized above (Section 3.4) to support people who want to write software to analyze NED data or synthesize the query results into new VO interfaces and services.
13 http://ssds.nasa.gov Back.
14 http://www.adccc.org Back.
15 See http://www.w3.org for descriptions of these and other XML protocols put forth by the World Wide Web Consortium. Back.