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gaia early data release 3 documentation

12.3 Crossmatches

12.3.2 hipparcos2_neighbourhood

Hipparcos2 Neighbourhood table includes all good neighbours for each matched Hipparcos2 object. A good neighbour for a given Hipparcos2 source is a nearby Gaia object whose position is compatible (within position errors) with the Hipparcos2 target.
The cross-match algorithm is not symmetric and searches Hipparcos2 sources counterparts in Gaia. 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 good neighbours in the External Catalogue

score : Score of neighbours (double)

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