IntroductionMany questions concerning the structure and evolution of galaxies are framed in terms of a natural tension between "Nature" and "Nurture". By "Nature" we mean those innate or originally defining properties of a galaxy that are in place at the time that galaxies decouple from the expansion of the universe and are then more or less constant over time. Examples of these properties are: total baryonic mass, specific angular momentum, mean density, baryonic-to-dark-matter ratio, etc. "Nurture" is synonymous with "Environment" as defined by the local density of other self-gravitating, interacting systems, be they individual galaxies, nearby groups of galaxies or surrounding clusters.
The taxonomy of objects defining the environment of a galaxy is rich and hierarchical. Individual galaxies can have companions; they can be in pairs, triples and quartets; they can be members of groups, clusters and even paradoxically so, galaxies can even be found residing in voids. Each of these types of associations can be further qualified by simple descriptors such as "close", "wide", "apparent", and even theory-laden terms such as "interacting" and "merging" are applied by many practitioners in the field. Chance alignments abound, and traditionally, redshift measurements of differential radial velocities are used to discriminate line-of-sight accidents from physically meaningful associations.
Scales matter. For small numbers of galaxies (pairs and triples, or galaxies with companions) found at small separations (up to a few 100 kpc, say), these systems, if physically bound, will have lower differential velocities (around +/- 500 km/s) than galaxies seen in populous groups and clusters where relative velocities may reach +/- several 1,000 km/s. Before reasonable probabilities of cluster/group membership or physical association can be assigned, the apparent angular separation of galaxies on the sky must be supplemented with (differential) velocity information, set in the context of the overall mean ambient density of galaxies in that same region of space.
Over the last century many catalogs of galaxy pairs, triples, groups and clusters have been assembled and published based on whatever positional and velocity data were available at the time of writing. All of these efforts were necessarily incomplete and subject to revision as soon as new surveys to fainter limits (in both new object detection and in follow-up radial velocity determinations) were made.
NED is the world's most comprehensive and up-to-date assimilation of extragalactic objects and their ancillary data, including especially their radial velocities. As such NED implicitly contains all of the currently available data necessary to define a given galaxy's environment based on its measured radial velocity and the radial velocities of other galaxies seen generally along and around that same line of sight. This service enables real-time exploration of a galaxy's environment for objects with available spectroscopic redshifts in NED.
As already mentioned, there exists a hierarchy of scales and a corresponding range of differential velocities that quantitatively define the more colloquially expressed concepts of companions, pairs, triples, groups, clusters, and even voids. To compactly capture and re-express both of these views (the quantitative and qualitative ways of expressing environment) we have created an Environmental Index made up of five (monotonically increasing) integers; an example of which might be [3, 7, 7, 47, 406]. At a glance the Environmental Index tells one what the hierarchically ordered environment is around any given galaxy of interest. Each of the elements of this vector correspond to the cumulative numbers of galaxies in concentric spheres surrounding the target galaxy (in both metric and velocity space).
The first element in the vector making up the Environmental Index corresponds simply to the number of galaxies (intentionally and explicitly including the target galaxy itself) that are found in a "sphere" having a transverse (metric) size of +/- 0.5 Mpc and a "back-to-front" (differential radial-velocity) "size" of +/- 250 km/s. Subsequent elements of the vector respectively sum up the numbers of galaxies known to NED, that falls in progressively larger spheres of 1 Mpc in radius and +/- 500 km/s in velocity separation, 2 Mpc and +/- 1000 km/s, 5 Mpc and +/- 2,500 km/s and, finally, 10 Mpc radius and +/- 5,000 km/s in line-of-sight velocity difference. These individual elements and their defining properties were deliberately chosen to quantitatively capture the more qualitative and colloquial concepts of "close pairs and galaxies with companions", "wider pairs, triple and quartets", "groups", "clusters" and finally "superclusters" and their low-density counterparts, "voids".
A galaxy with an Environmental Index of [1,1,1,1,1] would clearly be described as being a very "isolated galaxy" on all scales. A galaxy with an Environmental Index of [2, 10, 20, 64, 747] could well be described as being "in a pair, embedded in a major cluster of galaxies". An Environmental Index of [2,5,5,5,5] would then describe "a pair of galaxies in a relatively sparse but somewhat isolated group".
The obvious (and inevitable) shortcoming of this, and, in fact, any other compilation-based attempt to quantify the environment of a randomly selected galaxy, is that our census of galaxies is patchy across the sky, and the follow-up radial-velocity catalogs based on these surveys are even more incomplete and additionally subject to a wide range of other selection biases. However, since NED contains the most complete and up-to-date census of extragalactic objects with published radial velocities, and it follows an association strategy not unlike the strategies adopted by most of the past catalogers of clusters, groups, and pairs, the outputs of this service will simply be as incomplete as the published surveys in the extant literature are incomplete. But this situation is self-correcting and is automatically and continuously updating itself as new information flows into NED.
In its initial incarnation, this service definitively answers the following question: "What is the environment around a given target galaxy, defined by positional and radial-velocity data derived from the published literature as currently known to the NED database?" In future updates, we plan to enhance this service to provide environment parameters computed within well-known large area sky surveys, thus constraining the system to well-established selection effects.
Sample DefinitionsThe current sample contains 3,218,233 unique Classified Extragalactic Objects, with good spectroscopic redshift (zflag of: "" or "SPEC"). Following is a break down of the object types in the sample.