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Explore Galaxy Environments via Radial Velocity Constrained Cone Searches

(Latest Revision: 13 December 2018)



Many 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 Definitions

The current sample contains 3,307,689 unique Classified Extragalactic Objects, with good spectroscopic redshift (zflag of: "" or "SPEC"). Following is a break down of the object types in the sample.

Object TypeObject Type Codecounts
Individual Galaxies G 2947388
Quasi-Stellar Object (Quasar) QSO 220458
Galaxy Clusters GClstr 91198
Galaxy Groups GGroup 29771
Galaxy Triples GTrpl 6060
Galaxy Pairs GPair 5564
Absorption-Line Systems AbLS 4594
Emission-Line Systems EmLS 1870
Gravitationally-Lensed Sources G_Lens 650
Gravitationally-Lensed QSOs Q_Lens 113
QSO Groups QGroup 23

We have decided to consider the following object types as being "virtual": GGroup, QGroup, GClstr, GTrpl, GPair, because in general there is no object at their position and furthermore there are usually physical objects that compose them. Therefore "Virtual Objects" are not counted in the Environmental Index mentioned in the Introductory section. Only those with preferred object types of G, QSO, AbLS, EmLS, G_Lens, Q_Lens, are counted as 1 object each.


  • Excluded Objects - In the current version, not all known NED objects within the searched volume are included, see Sample Definitions. Of particular note are objects with highly uncertain spectroscopic redshifts, and all objects with photometric redshifts (which have very large uncertainties); both classes of redshifts are currently being excluded.
  • Under-Represented Systems - NED is a collation of published data and in many occasions, systems of objects (e.g. Galaxy pairs, triples, groups and clusters) are published without listing of the individual components. This is the reason that there are such systems found in this service that do not contain any other objects nearby.
  • Duplicate Objects - In the process of collating a new dataset, the NED team merges incoming data with existing NED objects to the best of our ability. If you discover objects that should be merged, please contact us.
  • Uncorrected Redshifts - The distance estimates used in the current galaxy environment service are based on Heliocentric redshifts without any reference-frame corrections being applied. This feature will be addressed in a future update. Nevertheless, because all of the plots are produced using differential velocities with respect to the target galaxy the only penalty incurred is in the calculated transverse radii; these errors will drop with increasing redshift as the corrections for flows, etc. naturally decrease in their relative contribution to the cosmological redshift.
  • Non-Uniformity - NED being a collation of published surveys and literature, it is highly inhomogeneous in coverage and sensitivity. Extreme caution is warranted when inter comparing different regions of sky. NED compiles and reports what is currently in the published literature.
  • Sample Density - Owing to unknown contributions due to peculiar velocities, it is not possible to use a pure Hubble-flow approximation to measure the back-to-front distances between galaxies down to the tangential length scales we have decided to define our concentric bubbles, hence the normalizing volume used here is assumed to be spherical. Higher-velocity objects that may be physically in that volume will be displaced into other more inclusive, higher-velocity bubbles. Furthermore, we note that the actual volume sampled is a truncated conic section, rather than a strictly spherical volume. See Figure 1.
Figure 1: Object separation is estimated from angular separation (θ) scaled by the angular diameter distance of the central object. Hence the maximal search radius of 10 Mpc is defined by the redshift of the central object and the chosen cosmological parameters. In the figure, objects labeled A, B, and C are all assigned a projected separation of 10 Mpc. Similarly, those objects labeled D, E, and F all have projected separations of 5 Mpc. Whereas objects within the region of space demonstrated by objects labeled G and H are not found in the search even when they are physically within 10 Mpc of the input object. The orange and purple colored areas show the projected regions of space where objects are found for the 5 Mpc and 10 Mpc volumes respectively.

Query Interfaces

  • Search by environment parameters

    We have pre-generated a dataset of Environmental Indices centered at each of the objects within our current Sample Definitions, with a radial velocity of ≥ 3,000 km/s, assuming these cosmological parameters: Ho = 73 km/s/Mpc; Ωmatter = 0.27; Ωvacuum = 0.73 (flat universe). This interface allows you to search and browse the dataset based on any combination of the following: Object Type of central object; Radial Velocity of central object; and the number of object counts (or range of object counts) within any of the five designated concentric search radii (the Environmental Index discussed above).

    The query result tabulates the object name with a link to dynamically extract (through the interface described below) the list of objects within the 10 Mpc search, as well as the Object Type and Radial Velocity of the central object, and the corresponding Environmental Index.

  • Search by position or object name

    This interface allows you to dynamically perform a search within a projected radius of 10 Mpc centered on a requested NED object, or centered on any position and radial velocity within the limits of the current Sample Definitions, with a chosen set of cosmological parameters.

    The resultant page presents:

    1. A quantitative summary of the searched environment, listing the Environmental Index and the corresponding Sample Densities (see Caveats section);
    2. A tabulation of the Redshift References for the objects found. The origin of the redshift provides a crude, first attempt in determining the general survey coverage of the searched area;
    3. Graphics showing the projected 2-D spatial distribution (left) and velocity distribution (right) of the objects found;
    4. A detail listing of all the objects found ordered in increasing spatial and velocity separations from the search center, color-coded (to match those in the quantitative summary) according to which of the five concentric search radii the objects falls within. The link on the left-most column under Recenter allows you a quick way to resubmit a search to recenter the search at the new object's position and radial velocity.


    • As described in the Sample Definitions section, the Environmental Index does not count "virtual objects" of the following types: GGroup, GClstr, GTrpl, and GPair. However, these objects are counted in the Redshift Reference tabulation, they are plotted in the diagrams and they are shown in the detail listing.
    • If the user provides a NED recognized object name without providing a chosen radial velocity, and that object has a redshift in NED (irrespective of redshift type or quality), that redshift will be used in the search. However, any object that does not satisfy Sample Definitions will not be found (nor counted) in the search (see Excluded Objects in the Caveats section).

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