4.1. Object Databases
Object databases are understood here as those which gather both bibliographical references and measured quantities on Galactic and/or extragalactic objects. There are three prime ones: SIMBAD, NED and LEDA; the latter two are limited to extragalactic objects only. A comparison of their extragalactic content has been given by [Andernach (1995)]. All three involve an astronomical ``object-name resolver'', which accepts and returns identifiers; it also permits retrieval of all objects within a stated radius around coordinates in various different systems or equinoxes. For large lists of objects the databases can also support batch jobs, which are prepared according to specific formats and submitted via email. The results can either be mailed back or be retrieved by the user via anonymous ftp. While SIMBAD and NED allow some limited choice of output format, LEDA is the only one that delivers the result in well-aligned tables with one object per line.
SIMBAD (Set of Identifications, Measurements, and Bibliography for Astronomical Data) is a database of astronomical objects outside the solar system, produced and maintained by CDS. Presently SIMBAD contains 1.54 million objects under 4.4 million identifying names, cross-indexed to over 2200 catalogues. It provides links to 95,700 different bibliographical references, collected for stars systematically since 1950. Presently over 90 journals are perused for SIMBAD, which is the most complete database for Galactic objects (stars, HII regions, planetary nebulae, etc.), but since 1983 it has included galaxies and other extragalactic objects as well. SIMBAD is not quite self-explanatory; its user's guide can be retrieved from ftp://cdsarc.u-strasbg.fr/pub/simbad/guide13.ps.gz, or may be consulted interactively at simbad.u-strasbg.fr/guide/guide.html.
Access to SIMBAD requires a password, and applications may be sent by email to email@example.com. By special agreement, access is free for astronomers affiliated to institutions in Europe, USA and Japan, while users from other countries are charged for access. The telnet address of SIMBAD is simbad.u-strasbg.fr and its web address in Europe is simbad.u-strasbg.fr/Simbad. It has a mirror site in USA, at simbad.harvard.edu.
The ``NASA/IPAC Extragalactic Database'' (NED, [Helou et al. (1995)]) currently contains positions, basic data, and over 1,275,000 names for 767,000 extragalactic objects, nearly 880,000 bibliographic references to 33,000 published papers, and 37,000 notes from catalogues and other publications, as well as over 1,200,000 photometric measurements, and 500,000 position measurements. NED includes 15,500 abstracts of articles of extragalactic interest that have appeared in A&A, AJ, ApJ, MNRAS, and PASP since 1988, and from several other journals in more recent years. Although NED is far more complete in extragalactic objects than is SIMBAD, it is definitely worthwhile consulting SIMBAD to cover the extragalactic literature for the five years in its archives before NED commenced in 1988. Samples of objects may be extracted from NED through filters set by parameters like position in the sky, redshift, object type, and many others. NED has started to provide digital images, including finding charts from the ``Digitized Sky Survey'' (DSS) for some 120,000 of their objects. A unique feature of NED is that the photometric data for a given object may be displayed in a plot of the ``Spectral Energy Distribution'' (SED). NED is accessible without charge at nedwww.ipac.caltech.edu or via telnet to ned.ipac.caltech.edu (login as ned).
The ``Lyon-Meudon Extragalactic Database'' (LEDA), created in 1983 and maintained at Lyon Observatory, offers free access to the main (up to 66) astrophysical parameters for about 165,000 galaxies in the ``nearby'' Universe (i.e. typically z < 0.3). All raw data as compiled from literature are available, from which mean homogenized parameters are calculated according to reduction procedures refined every year ([Paturel et al. (1997)]). Finding charts of galaxies, at almost any scale, with or without stars from the GSC, can be created and ~ 74,000 images of part of these galaxies can be obtained in PS format. These images were taken with a video camera from the POSS-I for identification purposes only and are of lower quality than those from the Digitized Sky Survey (5). However, they have been used successfully by the LEDA team to improve positions and shape parameters of the catalogued galaxies. LEDA also incorporates the galaxies (20,000 up to now) which are being detected in the ongoing ``Deep Near-IR Survey of the Southern Sky'' (DENIS; www.strw.leidenuniv.nl/denis or denisexg.obspm.fr/denis/denis.html). A flexible query language allows the user to define and extract galaxy samples by complex criteria. LEDA can be accessed at www-obs.univ-lyon1.fr/ or via telnet to lmc.univ-lyon1.fr (login as leda). The homogenized part of LEDA's data, together with simple interrogation software, is being released on ``PGCROMs''s every four years (1992, 1996, 2000...).