Data
There are only a handful of quasars in Gaia DR1. A table was prepared as a result of the investigations by Mignard et. al. (arXiv:1609.07255) which contains 2191 quasars which were also used as the celestial reference frame (CRF) for Gaia DR1. This table is gaiadr1.aux_qso_icrf2_match and can be queried with
SELECT * FROM gaiadr1.aux_qso_icrf2_match
There is a discrepancy between the entries in this table and the data available in the official Gaia DR1 (table gaiadr1.gaia_source). If we try to find the quasars from gaiadr1.aux_qso_icrf2_match in gaiadr1.gaia_source using
SELECT * FROM gaiadr1.gaia_source AS dr1
WHERE dr1.source_id IN
(SELECT source_id FROM gaiadr1.aux_qso_icrf2_match)
we only find 2080 quasars, i.e., there are 111 entries which are not found in the official data release. According to private communication with François Mignard, this is because the CRF sample is prepared ahead of the official release, and some of the entires that were used for the CRF were removed from the release.
These discrepancies are not too disconcerting, since there is only 1 quasar (out of the 2080) for which proper motions data is available, so any analysis is not possible and this is included here only for completeness. Below is the distribution of the 2191 quasars from gaiadr1.aux_qso_icrf2_match on the sky.
The situation becomes more involved in Data Release 2, because there are more objects which were observed and therefore more data to deal with. All in all, there are around half a million QSOs in Gaia DR2 for which astrometric measurements are available.
The table gaiadr2.aux_allwise_agn_gdr2_cross_id contains 555934 Gaia DR2 objects which are part of the AllWISE AGN catalogue (see arXiv:1804.09377). The data can be extracted using the query
SELECT * FROM gaiadr2.gaia_source AS dr2 WHERE dr2.source_id IN (SELECT source_id FROM gaiadr2.aux_allwise_agn_gdr2_cross_id)
The table gaiadr2.aux_iers_gdr2_cross_id contains 2820 Gaia DR2 objects which are part of the ICRF3 database. All of these have frame_rotator_object_type = 2 (but are not all objects with this property in DR2). The data for these objects can be extracted using the query
SELECT * FROM gaiadr2.gaia_source AS dr2
WHERE dr2.source_id IN
(SELECT source_id FROM gaiadr2.aux_iers_gdr2_cross_id)
The Gaia CRF2 dataset (CRF stands for Celestial Reference Frame) is the union of the preceding 2 datasets and contains 556869 objects. The data can be obtained using the query
SELECT * FROM gaiadr2.gaia_source AS dr2
WHERE dr2.source_id IN (SELECT source_id FROM gaiadr2.aux_allwise_agn_gdr2_cross_id)
OR dr2.source_id IN (SELECT source_id FROM gaiadr2.aux_iers_gdr2_cross_id)
FROT23 is shorthand for "Frame Rotation Object Type 2 or 3". In general, these are objects which were used in some way to fix the Gaia reference frame. The dataset is comprised of the objects for which the column frame_rotator_object_type has value 2 or 3. In principle this dataset can be split into 2, however in practice the objects with frame_rotator_object_type = 2 are 2 orders of magnitude fewer than those with frame_rotator_object_type = 3 and therefore we consider only the full dataset.
As discussed above, there are 2191 QSOs in Gaia DR1. Out of them, 2080 can be found in the table gaiadr1.gaia_source. Furthermore, 2079 of these 2080 can be found in Gaia DR2. The data can be obtained using the query
SELECT * FROM gaiadr2.gaia_source AS dr2
WHERE dr2.source_id IN
(
SELECT dr2_source_id FROM gaiadr2.dr1_neighbourhood dr1_nbhd
WHERE dr1_nbhd.dr1_source_id IN (SELECT source_id FROM gaiadr1.aux_qso_icrf2_match)
)
In Gaia EDR3 there is a table gaiaedr3.agn_cross_id which contains 1614173 objects from the AllWISE AGN catalogue. This data can be obtained using the query
SELECT * FROM gaiaedr3.gaia_source AS edr3
WHERE edr3.source_id IN
(SELECT source_id FROM gaiaedr3.agn_cross_id)
As discussed above, there are 2191 QSOs in Gaia DR1. Out of them, 2080 can be found in the table gaiadr1.gaia_source and 2079 can be matched to objects in Gaia DR2. Because of possible duplicate sources when matching between data releases, 2096 matches for these objects can be found in Gaia EDR3. The data can be obtained using the query
SELECT * FROM gaiaedr3.gaia_source AS edr3
WHERE edr3.source_id IN
(
SELECT dr3_source_id FROM gaiaedr3.dr2_neighbourhood AS dr2_nbhd
WHERE dr2_nbhd.dr2_source_id IN
(
SELECT dr2_source_id FROM gaiadr2.dr1_neighbourhood AS dr1_nbhd
WHERE dr1_nbhd.dr1_source_id IN (SELECT source_id FROM gaiadr1.aux_qso_icrf2_match)
)
)
FROT23 is shorthand for "Frame Rotation Object Type 2 or 3". In general, these are objects which were used in some way to fix the Gaia reference frame. The dataset is comprised of the objects for which the column frame_rotator_object_type has value 2 or 3. In principle this dataset can be split into 2, however in practice the objects with frame_rotator_object_type = 2 are 2 orders of magnitude fewer than those with frame_rotator_object_type = 3 and therefore we consider only the full dataset.