20.4.3 allwise_neighbourhood

Table allwise_neighbourhood includes all good neighbours for each matched Gaia object. A good neighbour for a given Gaia source is a nearby object in the external catalogue whose position is compatible (within position errors) with the Gaia target. The cross-match algorithm is not symmetric and searches for Gaia source counterparts in AllWISE. The cross-match algorithm is positional and exploits the full 5 parameter covariance matrix of the Gaia astrometric solution when available and the external catalogue positions and position errors. In addition it takes into account the external catalogue environment using the local density.
Please note that the cross-match algorithm is a trade-off between multiple requirements, in particular between completeness and correctness. It is thus not limited to a simple cone search.

Reference papers:
Marrese et al. (2017)
Marrese et al. (2019)

Columns description:

source_id : Unique Gaia source identifier (long)

A unique single numerical identifier of the source (for a detailed description see gaia_source.source_id).

original_ext_source_id : Original External Catalogue source identifier (string)

The unique source identifier in the original external catalogue.

angular_distance : Angular Distance between the two sources (float, Angle[arcsec])

Angular distance between a Gaia source and its good neighbours in the external catalogue.

score : Score of neighbours (double)

Score of a given neighbour. The score is a figure of merit based on geometric distance and local density of the external catalogue: the higher the score, the more probable the match is.

xm_flag : Cross-match algorithm flag (short)

This flag is a bitmask indicating the details of the cross-match algorithm used for the source.

xm_flag values and descriptions:

• •

  0 = Initial value; resets all bits.

• •

  1 = The external catalogue object has one or more multiples. This means that there is at least another object with exactly the same astrometry.

• •

  2 = The external catalogue object has one or more suspected duplicates. This means that there is at least another object much closer than the catalogue angular resolution.

• •

  4 = The external catalogue object is resolved in Gaia.

• •

  8 = The Gaia object has a five parameters astrometric solution.

• •

  16 = The Gaia object has a two parameters astrometric solution.

• •

  32 = The external catalogue object is matched only after the special treatment for sources with under-estimated position errors.

• •

  64 = The external catalogue object is matched only after the special treatment for Gaia sources with large values of ipd_gof_harmonic_amplitude or ruwe.

For detailed documentation about xm_flag and the cross-match algorithm, see Chapter 15.

allwise_oid : External Catalogue source identifier (int)

The additional numeric unique source identifier of the external catalogue, increasing with Declination.