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gaia early data release 3 documentation

12.3 Crossmatches

12.3.1 hipparcos2_best_neighbour

Hipparcos2 BestNeighbour table lists each matched external catalogue object with its best neighbour in Gaia. The cross-match algorithm is not symmetric and searches Hipparcos2 sources counterparts in Gaia.
The best neighbour is chosen among good neighbours as the one with the highest value of the figure of merit, which evaluates the ratio between two opposite models/hypotheses: the counterpart candidate is a match or it is found by chance. Good neighbours are nearby objects in Gaia whose position is compatible within position errors with the external catalogue target.
The cross-match algorithm is positional and exploits the full 5 parameters covariance matrix of Gaia astrometric solution when available and the external catalogue positions and position errors. In addition it takes into account the Gaia 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)

Unique identifier of the Gaia source, the attribute corresponds to gaia_source.source_id

original_ext_source_id : Original External Catalogue source identifier (int)

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 best neighbour in the External Catalogue

number_of_neighbours : Number of neighbours in Gaia Catalogue (byte)

Number of sources in the Gaia Catalogue which match the External Catalogue source within position errors.
The identifiers of all the neighbours can be found in the Neighbourhood table.

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 8. For a detailed description of gaia_source columns ipd_gof_harmonic_amplitude and ruwe, see Chapter 4.