© The Authors 2024

1 Introduction

Since the adoption of the International Celestial Reference Frame (ICRF; Ma et al. 1998, 2009; Fey et al. 2015; Charlot et al. 2020), the instantiation of the International Celestial Reference System (ICRS; Arias et al. 1995) at the turn of the 21st century, the Celestial Reference Frame (CRF) has been defined by the positions of distant active galactic nuclei (AGNs), which do not have physical parallaxes and intrinsic proper motions detectable with current technology, given their cosmological distances. With the advent of ICRF3 (Charlot et al. 2020), the ICRF has become a multi-wavelength CRF, defined at X-band (8.4 GHz), K-band (24 GHz), and Ka-band (32 GHz), with Gaia CRF3 (Gaia Collaboration 2022) now the fundamental realisation of the ICRS at visual wavelengths¹.

One aspect that is critical for the construction of a catalogue of distant AGNs suitable for the CRF is that the catalogue must contain as few stellar contaminants as possible, which poses a challenge as they are typically difficult to distinguish from AGNs based on photometry at visual wavelengths. Photometric selection is dramatically more effective at infrared wavelengths, allowing for the construction of nearly all-sky samples of AGNs with a high level of purity (e.g. Secrest et al. 2015) suitable for CRF work (Lindegren et al. 2018), but a census of spectroscopically confirmed AGNs is critical for purposes of validation and improving the purity of the CRF where spectroscopic data are available. Additionally, AGNs in extended host galaxies at low redshift exhibit problematic Gaia astrometry, such as apparent but spurious proper motions (Souchay et al. 2022b; Makarov & Secrest 2022), so it is valuable to have spectroscopic redshifts for selection of compact, moderate-to-high-redshift AGNs (i.e. quasars or QSOs).

Over the last 14 yr, the Large Quasar Astrometric Catalogue (LQAC) has been developed specifically for this purpose (Souchay et al. 2009, 2012, 2015, 2019; Gattano et al. 2018), containing optimised positions, broad-band photometry, radio flux densities, and spectroscopic redshifts for hundreds of thousands of AGNs and quasars sourced from multiple catalogues. Starting in LQAC-4 (Gattano et al. 2018), the LQAC has listed positions from Gaia, where available, with Gaia-DR2 positions used for LQAC-5 (Souchay et al. 2019) and Gaia-EDR3 positions used for an LQAC-5+ version in the analysis by Souchay et al. (2022b). The LQAC has been important for a wide range of analyses, from earlier, pre-Gaia optical realisations of the ICRS (Andrei et al. 2009), to validating the Gaia catalogues (Arenou et al. 2017; Bailer-Jones et al. 2019; Gaia Collaboration 2023) and assessing completeness in other AGN catalogues (Secrest et al. 2015), constructing Gaia-CRF3 (Gaia Collaboration 2022), as well as finding utility in several fields of general research (e.g. Krogager et al. 2017).

The full third Gaia Data Release in 2022 contains numerous astrophysical parameters, in addition to the astrometric information from EDR3, such as stellar spectral types, BP/RP spectra, epoch photometry and variability information, extragalactic object type classifications, and redshifts for quasar candidates².

In this work, we describe the creation of the sixth release of the LQAC, LQAC-6. Using several spectroscopic redshift catalogues not previously incorporated, such as 6dF, GAMA, and LAMOST, we dramatically increased the total number of objects in LQAC and improved sky coverage. We include small area surveys such as the MQS to help fill gaps in the sky, and we also use redshifts from Gaia BP/RP spectra that are likely to be reliable. When available, we include radio flux density estimates from VLASS, in addition to objects with FIRST, and all ICRF objects are included as before. In Sect. 2, we describe the input catalogues used and quality criteria employed. In Sect. 3, we give the basic astrometric principles of determination of positional uncertainties, parallaxes and proper motions for objects present in the Gaia DR3 release. In Sect. 4, we explain ourmethods in detail to get photometric information at various bands, extending from UV to radio domains. Section 5 deals with the redshifts determination and some discrepancies in its evaluation between ground-based original catalogues and Gaia DR3. In Sect. 6, we recall the ways of calculations of absolute magnitudes and morpholgical indices already used in the previous LQAC releases. Section 7 summarizes the contents of this work.

2 Source catalogues

A total of 33 input catalogues are included in LQAC-6. They are listed in Table 1 with the number of sources, as well as the Gaia DR3 and GCRF3 counterparts for each of them. As was the case for previous versions of LQAC, four VLBI catalogues, which contain extragalactic sources with exquisite astrometric quality, are the first on the list. Naturally, ICRF3 takes first position. Each input catalogue is represented by a flag. The first nine catalogues, with flags”A” to”I” were already used for the LQAC-5 compilation, and they are taken exactly the same as in this release, except for the first catalogue which is ICRF3 instead of ICRF2. We note that the three following catalogues, with flags”J” to”L” do not correspond to quasars catalogues. They are included for information. The catalogue flagged”M” is the last update of the Véron-Cetty and Véron catalogue (Véron-Cetty & Véron 2010). We recall that it is another compilation of existing quasars catalogues. For this reason, we mention the flag”M” only for quasars which have no counterpart in any of the other contributing catalogues, except in the GCRF3, for which Véron-Cetty & Véron (2010) may provide the lacking value of the redshift.

A total of 7285 objects are present exclusively in Véron-Cetty & Véron (2010), which represents a mere 0.35% of the LQAC-6 catalogue, whereas 2608 objects are present exclusively in both Véron-Cetty & Véron (2010) and GCRF3 (Gaia Collaboration 2022).

There are 20 additional catalogues, with flags”N” to”g”, that were not taken into account in LQAC-5. For a majority of these contributing catalogues, the selected objects constitute a sub-sample of a much larger catalogue of galaxies, with a specific classification coming from their spectrum and a flag indicating that we are concerned with an AGN/QSO. In the following, we show one by one the characteristics of the catalogues taken in our LQAC-6 compilation.

2.1 2dF-SDSS LRG and QSO Survey

The aim of the 2dF-SDSS LRG and QSO Survey (da Ângela et al. 2008) is to combine the QSO samples from the 2dF QSO Survey and the 2dF SDSS luminous red galaxy (LRG) and QSO Survey to investigate the clustering of z ≈ 1.5 QSOs and to measure a correlation function. Assuming a range of relations between halo and black hole mass, investigations are made to evaluate how black hole mass correlates with luminosity and red-shift. It shows that QSO’s clustering does not depend strongly on luminosity for a given redshift. The calculations suggest that QSO of different luminosities may contain black holes of similar mass.

2.2 2SLAQ

The 2dF-SDSS LRG and QSO Survey (Croom et al. 2009), known as 2SLAQ is a deep sample of LRG (luminous red galaxies) and QSOs. it is a deep sample, with 18 < g < 21.85 when extinction has been corrected, aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at z < 2.6. The area covered by the sample is 191.9 deg² and contained new spectra of 16326 objects, of which 8764 are QSOs. The global QSO sample, including QSOs previously observed in the SDSS and 2QZ surveys, contains 12 702 QSOs. The presentation of the sample is accompanied by studies of completeness, as well as conclusion that colours are significantly affected by the presence of a host galaxy, up to z ≈ 1

2.3 6dF

The 6dF Galaxy Survey (Jones et al. 2009) is a near-infrared and optically selected redshift and peculiar velocity survey using a dedicated multi-object spectrograph at the United Kingdom Schmidt Telescope (UKST). Target fields covered the ≈ 17 000 deg² of southern sky more than 10° from the Galactic plane, which corresponds to 41% fraction of the sky. A total amount of 136 304 spectra have yielded 110256 new extragalactic redshifts and a new catalogue of 125 071 galaxies brighter than a given threshold in 5 bandwidths (K, H, J, r_F, b_J). The median redshift of the sample is 0.053. This survey has mapped the large structures of the local southern universe (z < 0.1) in unprecedented detail. The survey includes 318 sources with z > 1, mostly QSOs.

2.4 AGES

The AGN and Galaxy Evolution Survey (Kochanek et al. 2012) is a redshift survey with a global area of 7.7deg² inside the Bootes field of the NOAO Deep Wide-Field Survey. The final sample consists of 23 745 redshifts. The median redshift value is 0.31 and 90% of the redshifts verify 0.085 < z < 0.66 Finally red-shifts for 4764 quasars and galaxies with AGN signatures were obtained. A total of 21 749 objects are included in our LQAC-6 compilation. Among them, 4387 ones (20.2%) belong to the Gaia DR3.

2.5 COSMOS

The Cosmic Evolution Survey (COSMOS) is a Hubble Space Telesccope (HST) Treasury project aimed at obtaining spectroscopic redshifts for the first 466 X-ray and radio-selected AGN targets in a 2 deg² specific field (Trump et al. 2007). 86 new type 1 AGNs and 130 new type 2 AGNs were identified with high-confidence redshifts and reliable classification, together with red galaxies with no emission lines. According to the authors, a completness ratio is obtained at the level of 72% down to i_AB = 24 and 90% down to i_AB = 22. This redshift survey was consistent with an obscured AGN population that peaks at z ~ 0.7.

2.6 DEEP

Our DEEP sample comes from the DEEP2 (Newman et al. 2013) and DEEP3 (Cooper et al. 2012) Galaxy Redshift Surveys, that is specifically aimed at investigating the relationship between the environment and the structure of galaxies located in the red sequence at intermediate redshift. Correlations between galaxy densities and size are investigated in the frame of models of galaxy formation. Observations were done with the Keck telescope. The area covered by the DEEP2 is 2.8 deg² divided into four separate fields observed to a limiting magnitude of R_AB = 24.1, containing 53 000 spectra and more than 38 000 reliable redshift measurements. The fields covered by the DEEP3 is one of the four fields surveyed by the DEEP2 Galaxy Redhift Survey belonging to the EGS (Extended Groth Strip) which is a deep multiband zone explored by the Hubble Space Telescope (HST) and is by far the most complete fields with regard to spectroscopic coverage at intermediate redshifts. Among the recent generation of deep spectroscopic redshift surveys at z ~ 1, the combination of the DEEP2 and DEEP3 spectroscopic data provides one of the largest samples of accurate spectroscopic redshifts, displaying the highest precision velocity information and sampling density.

2.7 FIRST

In the LQAC-6, the FIRST catalogue (Becker et al. 1995) is identical to the one included in the previous LQACs. It provides faint images of the radio sky at 20 cm. A total of 2153 individual pointings yielded an image database containing 1039 merged images covering 300 deg².

2.8 GAMA

A sub-sample of the Galaxy And Mass assembly (GAMA) Data Release 4 (Driver et al. 2022) was incorporated to our LQAC-6 compilation. The regions explored cover ≈ 230 deg² containing 205 540 galaxies with r < 19.65, for which 95.1% have reliable spectroscopic redshift measurements. We note that the 791 GAMA objects flagged as quasars retained in our LQAC-6 compilation all belong to the Gaia DR3, whereas only 490 (61.9%) are taken into account in the GCRF3.

2.9 GCRF3

The third release of the Gaia Celestial Reference Frame, GCRF3, (Gaia Collaboration 2022) is by far the largest contributor to our LQAC-6 catalogue. It is defined by the positions and proper motions at epoch 2016.0 for a specific set of extragalactic sources already present in the (E)DR3 catalogue.

It was constructed from a cross-match of Gaia DR3 with 17 external catalogues of QSOs and AGNs, combined with astrometric filtering designed to remove stellar contaminants. When constructing the final sample, random and systematic errors in the proper motions were analysed, as well as radio-optical offsets in position for the sources of the ICRF3 (Charlot et al. 2020). The GCRF3 sample includes about 1.6 million QSO-like sources, of which 80% have a five-parameter astrometric solution, and 20% of the remaining ones have a six-parameter solution. The G magnitude interval is 13 < G < 21 with a peak density at G = 20.6, at which the positional uncertainty is about 1 mas. Radio-optical offsets for the 3142 sources with a counterpart in ICRF3 is typically of the order of 0.5 mas.

2.10 GPQ

The Galactic plane can be considered by far as the”zone of avoidance” for extragalactic astronomy, including quasar surveys. A recent study (Fu et al. 2021) was devoted to the search for Galactic Plane Quasars (GPQs) which is particularly difficult because of large extinctions and high source densities in the Galactic plane. Starting from a transfer-learning framework at both the data and algorithm levels, the authors above obtained a reliable catalogue with 60 946 sources located at | b | < 20°, with a wide redshift range (0 < z < 5). The sub-sample kept for our LQAC-6 compilation concerns only the objects with formal spectroscopic identification as those obtained with the 2 m telescopes base at Lijang and Xinglong in China and Siding Spring in Australia.

2.11 HES

The Hamburg/ESO survey (HES) for bright QSOs is a sample of around 400 bright QSOs and Seyfert 1 nuclei drawn from the Hamburg/ESO survey, which is spectroscopically complete at 99% and with accurate values of fluxes and redshifts (Wisotzki et al. 2000). Magnitudes of the objects are ranged in the interval 13 < B_J < 17.5 and redshifts range within 0 < z < 3.2. This flux-limited quasar sample, covering 3700 deg² in the sky is useful for a wide variety of statistical studies. Thanks to its completness, the HES sample greatly improved the statistics at very bright magnitudes.

2.12 Hewitt and Burbridge

Here also we keep exactly the same catalogue (Hewitt & Burbidge 1993) referred as HB, as in the previous LQACs. It was a compilation of all known QSOs with measured emission red-shifts and BL Lac objects complete to 1992 Dec. 31. The sample has 7315 objects, nearly all QSOs with 90 BL Lac objects. The catalogue contains an extensive information on names, positions, magnitudes, colours, emission-line redshifts, absorption, variability and polarisation, together with X-ray, radio and IR data.

2.13 HQS

The Hamburg Quasar Survey (HQS; Hagen et al. 1999) covers ≈13 600 deg² of the northern sky at galactic latitudes | b |> 20°. It was motivated by the search for bright quasars (B < 17.0) whose the expected density in a typical Schmidt field of 25 deg² is 5. Only 43 and 7 objects among the 196 HQS quasars taken into account in our compilation belong respectively to Gaia DR3 and GCRF which represent a mere 21.9% and 3.6%.

2.14 ICRF3

The third release of the International Celestial Reference Frame, ICRF3, (Charlot et al. 2020) refers to the latest realization in the radio domain, based on the work achieved by a dedicated working group of the International Astronomical Union (IAU). It is based on nearly 40 yr of data acquired by very long baseline interferometry (VLBI) at the standard S and X bands, respectively at 8.4 and 2.3 GHz, with a supplementary data obtained at higher radio frequencies (24 GHz and 32 GHz) from 2005. In all, ICRF3 comprises 4588 sources, with three frequencies positions available for 600 of these. A sample of 303 sources, uniformly distributed on the sky, have been adopted as”defining sources” and as such serve to define the axes of the frame. In our present LQAC-6 catalogue, we retain the 4536 extragalactic sources with positions available at 8.4 GHz in the ICRF3. We note that the previous LQAC-5 version (Souchay et al. 2019) included the ICRF2 catalogue (Fey et al. 2015), which contained only 3414 sources, corresponding to 75% of the ICRF3 that had not yet been published at that time.

2.15 LAMOST

The Large Sky Area Multi-Object Fiber Spectroscopic Catalog (LAMOST) is a Chinese national scientific research facility operated by the National Astronomical Observatories, Chinese Academy of Sciences. It is based on a specific Schmidt telescope with 4000 fibers in a field of view of 20 deg² in the sky. The regular survey was initiated in 2012 and ended in 2019. The Data Release 7 contains a total of 10 431 197 flux, wavelength calibrated sky-subtracted spectra concerning 9 846 793 stars, 198 272 galaxies. From LAMOST we included 57 014 objects flagged as quasars. Notice that 54 006 of them, that is to say a large majority (94.7%) belongs to Gaia DR3.

2.16 MGC

The Millennium Galaxy Catalogue (MGC) is a 37.55 deg², medium-deep, B-band imaging survey along the celestial equator, taken with the Wide-Field Camera (WFC) mounted at the prime focus on the 2.5-m Isaac Newton Telescope (INT) situated at La Palma (Liske et al. 2003). This survey provides a robust, well-defined catalogue of galaxies in the range 16 < B_MGC < 24. The galaxy counts were used to test various estimates of the galaxy luminosity function. MGC is the smallest contribution to the LQAC-6 with only 86 objects.

2.17 MQS

The MGS (Magellanic Quasars Survey), produced from the Anglo-Australian Telescope AAT and the AA Omega spectrograph (Kozlowski et al. 2013) has considerably increased the number of quasars known behind the Magellanic clouds. The whole Large Magellanic Cloud and about 70% of the Small Magellanic Cloud were explored. A total of 753 quasars (565 in the LMC, and 188 in the SMC) were spectroscopically detected and confirmed. The new objects correspond to 94% of the sample. The detection of quasars in both fields is crucial to measure the internal and bulk proper motions of the clouds. Moreover this survey is one of the best long-term, densely sampled light curves enabling one to carry out studies about quasar variability physics.

2.18 OCARS

OCARS (Optical Characteristics of Astrometric Radio Sources) (Malkin 2018) is a compiled catalogue including radio sources observed in different VLBI programs and experiments which give redshift and photometry in the visible and near-infrared bands. Although it is regularly updated, here we use a recent version published in 2018.

2.19 PRIMUS

Prism Multi-Object Survey (PRIMUS) is a spectroscopic faint galaxy redshift survey to z ~ 1 carried out using the IMACS (Inamori Magellan Arreal Camera and Spectrograph) device at the focus of the Magellan I Baade 6.5 m telescope at Las Campanas Observatory (Coil et al. 2011). The PRIMUS survey covers a total area of 9.1 deg² of the sky with a magnitude threshold i_ab ~ 23.5 and a high-redshift precision. The redshift distribution peaks at z ~ 0.6 and extends at z ~ 1.2 for galaxies and z ~ 5 for broad-line AGNs. 1431 objects are included from the PRIMUS survey.

2.20 QUBRICS

The aim of the QUBRICS project (Calderone et al. 2019) was to identify new bright QSOs (i < 18) at relatively high red-shifts (z < 2.5) in the southern hemisphere with a large success rate. The survey takes advantage of high quality photometric and astrometric databases provided by Skymapper, WISE, 2MASS and Gaia in order to remove contaminants (stars, low z QSOs, galaxies) with high probability.

2.21 RFC, VLA and JVAS radio catalogues

The VLBA, VLA and JVAS radio catalogues remain rigorously identical to those already included in the previous versions of the LQAC.

The VLB A catalogue can be found in RFCC 2015d that is publicly available³.

Recall that the Very Large Array (VLA) interferometer consists on 27 radio atennas in a Y shape configuration at San Augustin (New Mexico). The related quasar catalogue can be found in the VLA Calibrator manual⁴.

The Jodrell Bank-VLA Astrometric Survey (JVAS) contains 2117 sources with 8.4 GHz flux information (Browne et al. 1998; Patnaik et al. 1992). These sources were intended for use as phase calibration ones for the Multi-Element Radio Linked Interferometer Network (MERLIN).

2.22 SDSS

Since the publication of the LQAC-5 (Souchay et al. 2019) a new release of the Sloan Digital Sky Survey (SDSS), called DR16 was published. In the tradition of previous data releases, DR16 is accompanied by a quasar catalogue, named as DR16Q (Lyke et al. 2020). It is by far the largest original catalogue to date, containing data for 750 414 quasars, of which 225 082 are new. Naturally, we carried out a cross-match with the LQAC-5 compilation to add the quasars which were not previously included in it. We found 223 993 new objects, by adopting a cross-match threshold of 1 arcsecond. The difference of 1089 objects between the two last numbers correspond to the objects already present in the LQAC-5 but coming from other surveys than the previous SDSS catalogues. Including previous up-dates of the SDSS, the total number of SDSS quasars included in the LQAC-6 amounts to 772 245 objects, which represent 37.25% of the LQAC-6 compilation. Remark that 267 357 of these objects (34.6%) do not belong to the Gaia DR3 and 364 389 (47.2%) do not belong to the GCRF3. This fact is mainly due to the magnitude limit of the SDSS which is larger than the DR3.

2.23 VIPERS

The VIMOS Public Extragalactic Redshift Survey (VIPERS) performed at the ESO VLT (Scodeggio et al. 2018) release redshifts, spectra, CFHTLS magnitudes as well as additional information for a complete sample of 86 775 galaxies, filtered by the condition 0.5 < z < 1.2 in a total area of ≈23.5 deg² constructed starting from 288 VIMOS fields with small overlapping. The galaxies with z < 0.5 have been eliminated from a colour-colour pre-selection.

2.24 VVDS

The VVDS (Vimos VLT Deep Survey) is principally devoted to trace large-scale distribution of galaxies at z ~ 1 in the aim of giving unique information on the structure of the Universe and on the fundamental parameters of the cosmological model (Garilli et al. 2008). It provides a good control of cosmic variance over areas of a few square degrees. The observational strategy, starting from the VIsible Multi-Object Spectrograph (VIMOS) instrument alllows one to reach a spectroscopic rate of ~35% of all galaxies with I_AB < 22.5. The purely magnitude-limited VVDS Wide sample includes 19 977 galaxies, 304 type 1 AGNs, and 9913 stars.

2.25 WIGZ

The WiggleZ Dark Energy Survey (Drinkwater et al. 2018) measured the redshifts of over 200 000 ultraviolet selected galaxies, using the Anglo-Australian telescope, with a magnitude limit of NUV < 22.8. In particular when restricting to the cosmic range of 0.2 < z < 1.0 this progamm detected the Baryon Acoustic Oscillation (BAO).

2.26 ZCAT

The ZCAT database contains the CFA Redshift Catalog, which incorporates much of the latest velocity data from the Whipple Observatory as well as velocities from earlier compilations such as the”Second Reference Catalog”, the Index of Galaxy Spectra, and the Catalog of Radial Velocities of Galaxies. The data presented in ZCAT have primarily been assembled for the aim of studying the large scale structure of the universe.

3 Astrometry

As it is indicated in Table 1, 1 739 187 objects, which represent 83.9% of the total number of 2073 099 objects of the LQAC-6, have a Gaia DR3 counterpart. For this large amount of objects, accurate positional uncertainties, parallax and proper motions determinations are available. The postulate according to which parallaxes and proper motions should be equal to zero within the error bar estimation for these two parameters can be checked directly by the user.

One of the most important aims of the LQAC-6 as in the previous updates of the LQAC was to give the a priori most accurate determination of the equatorial coordinates of each object with respect to the ICRF3. The selection in decreasing order of priority according to the availability is: ICRF3 (Charlot et al. 2020), Gaia DR3, LQRF (Andrei et al. 2009), and finally, the original catalogue the object comes from.

3.1 Positional uncertainties

The formula to measure the positional uncertainty of the objects was taken from Lindegren et al. (2016), where the semi-major axis of the dispersion ellipse is taken into account, which was computed from a combination of the standard deviation in α and δ and of a correlation coefficient, namely ρ_α,δ. It is given by the following straightforward relationship:

$${\sigma }_{\text{pos}}^2=\frac{1}{2}\left({\sigma }_{\alpha \star }^2+{\sigma }_{\delta }^2+\sqrt{{\left({\sigma }_{\alpha \star }^2-{\sigma }_{\delta }^2\right)}^2+{\left(2{\sigma }_{\alpha \star }{\sigma }_{\delta }{\rho }_{\alpha ,\delta }\right)}^2}\right),$$(1)

Within the total amount of 2 073 099 objects of the LQAC-6, 1739 187 objects (83.89 %) are found with a Gaia DR3 counterpart. Within this last sub-sample, 1 738 907 objects (99.983%) own a ρ_α,δ coefficient which enables one to determine a global uncertainty σ_pos. This parameter is given in a specific column of the LQAC-6.

3.2 Parallaxes

When sources have either five-parameter or six-parameter Gaia astrometric solutions, we include their parallaxes, but note that a significant zeropoint offset has been detected in Gaia EDR3/DR3 astrometry (e.g., Lindegren et al. 2021) of the order of several tens of µas, and the parallax uncertainties are likely under-estimated by about 6% (e.g. Souchay et al. 2022b). These should be appropriately accounted for when assessing the significance of apparent parallax.

3.3 Proper motions

As with parallaxes, we include Gaia EDR3/DR3 proper motions when available. While zero-point offsets in proper motions have generally been found to be minimal, the dispersions of uncertainty-normalised proper motions in α cos δ (hereafter α_⋆) and δ have also been found to be somewhat larger than unity (e.g. Gaia Collaboration 2022), although in the case of proper motion this may have some contribution from astrophysical effects such as source structure or multiplicity. For convenience, we include the joint significance of the total proper motion:

$$X_{\mu }^2=\frac{1}{1-{\rho }^2}\left[{\left(\frac{{\mu }_{{\alpha }_{\star }}}{{\sigma }_{{\mu }_{{\alpha }_{\star }}}}\right)}^2+{\left(\frac{{\mu }_{\delta }}{{\sigma }_{{\mu }_{\delta }}}\right)}^2-2\rho \frac{{\mu }_{{\alpha }_{\star }}{\mu }_{\delta }}{{\sigma }_{{\mu }_{{\alpha }_{\star }}}{\sigma }_{{\mu }_{\delta }}}\right],$$(2)

where the distribution of $X_{\mu }^2$ is proportional to exp($-0.5X_{\mu }^2/s^2$), where s is a scaling factor to account for intrinsic astrophysical dispersion or under-estimated uncertainties (e.g., Gaia Collaboration 2022).

Among the 1 739 187 objects of the LQAC-6 with a Gaia DR3 counterpart, 1 627 864 (93.60 %) have a proper motion determination and, in addition, 1 627 648 (93.59 %) have a correlation coefficient, ρ, which enables one to evaluate the X_µ parameter. A column for the joint significance X_µ is included in the LQAC-6.

4 Photometry

We concatenated archival photometry from all large-scale sky survey programs available, spanning the far-UV (0.1 µm) through the mid-IR (22 µm) and into the radio (~1-3 GHz). These data archives were queried using the TOPCAT, version 4.8–6, CDS Upload X-Match function, where available. We used an initial match radius of 10” for completeness, but chose a smaller, catalog-dependent match radius to select reliable matches. All sources within 10″ were downloaded, removing repeats, and then LQAC-6 was matched symmetrically to the set of unique associations, so that no match is shared by multiple LQAC-6 objects.

The details of how we concatenated photometry from each wavelength regime are given in the following sub-sections, but we give summary statistics of LQAC-6 photometry in Tables 2 and 3. In optical wavelengths, the g,r,i,z magnitudes are available for more than 90% of the sample (Table 2), whereas in infrared this amount is attained onlt at the W₁ and W₂ bands. The GH, BP and RP magnitudes coming from Gaia DR3 are available for more than 83% of the LQAC-6. On average, objects in LQAC-6 have photometry from 9 out of 15 bands searched, which is also the median and mode number (Table 2). Only 1% of objects have no photometry, but only 2% have photometry in all bands. Twenty percent of LQAC-6 objects have the modal number of bands, and 90% of objects have between 4 and 13 bands. We note that in order for a photometric measurement to be considered valuable, its formal error must be less than 0.5 mag, which approximately corresponds to a > 2σ detection in that band. Finally, a photometry source column is included in LQAC-6, which contains a 15-character string containing the source catalog code for each band.

4.1 UV

UV photometry was obtained from the revised All-Sky Survey catalog of GALEX sources (GALEX GR6+7 AIS; Bianchi et al. 2017), using the”best” FUV (0.1–0.2 µm) and NUV (0.2–0.3 µm) magnitudes that are the default option in VizieR. These magnitudes are on the AB scale.

4.2 Optical

We obtained ugrizy photometry (0.3–41.0 µm) from a combination of SDSS DR16 (Ahumada et al. 2020), Dark Energy Survey DR2 (DES2; Abbott et al. 2021), Pan-STARRS DR1 (PS1; Chambers et al. 2016), and SkyMapper DR1.1 (Wolf et al. 2018, SMSS) photometry. For SDSS, we used the default model magnitudes, respectively adding −0.04 mag and +0.02 mag to the u and z photometry to bring them onto the AB system. For DES2 and PS1, we used the default MAG_AUTO and Kron magnitudes, respectively, which are on the AB scale. Similarly, we use the Petrosian magnitudes for SMSS, which are also on the AB system.

Where photometry is available from multiple catalogs for the same band, we gave priority to the catalog with the smallest formal photometric error in that band, so that a given object may have photometry from different surveys depending on the band. This maximizes the availability of photometry for LQAC-6 objects, but there are biases between surveys and each survey will have its own unique passbands, though they may be similar. To minimize these concerns, we chose a match tolerance close enough that the difference in position between associations does not indicate the presence of different associations being assigned to the same object (e.g., an LQAC6 object having magnitudes from both an SDSS source and a different DES2 source). This match tolerance was 1″ for all optical catalogs. LQAC-6 also includes a code indicating the source of photometry so that users wishing to do spectral energy distribution fitting or synthetic photometry may select the correct passband for each magnitude of an object. For objects in which measurements from two or more surveys have the same error (e.g., due to rounding), photometry was preferentially selected from SDSS, DES2, PS1, and, finally, SMSS. By selecting photometry with the smallest photometric errors, 90% of sources have ugrizy errors less than 0.33 mag, 0.07 mag, 0.07 mag, 0.07 mag, 0.16 mag, and 0.23 mag, respectively. We note, however that at least two additional factors contribute systematic photometric errors significantly larger than the typical formal errors. First, photometry taken at different epochs will exhibit scatter due to AGN variability, which is typically of order 10% (~0.1 mag). Second, residual systematic errors will occur due to differences in the effective aperture used for each source in each catalog. For example, after adding 0.1 mag quadratically to the SDSS and DES i magnitude errors to account for variability, we find a residual dispersion of ~0.2 mag between these two bands. Therefore, for the vast majority of objects the systematic errors should dominate over the formal errors, which are typically well under 0.1 mag, so we do not quote formal errors in LQAC-6.

Finally, there is certainly Gaia G, G_BP, and G_RP photometry available for the majority of LQAC-6 objects. Being a space-based observatory, Gaia’s photometry is generally exquisite, with uncertainties of a few mmag. However, Gaia is designed for optimal astrometry and photometry of unresolved objects, namely stars. Consequently, objects with significant source extent are under-photometered by Gaia. Fortunately, due to differences in the way the BP and RP photometry is handled compared to the G photometry, extended objects will have a flux excess in G_BP + G_RP when compared to G beyond that expected from differences in passband. This is captured in the Gaia catalog as the BP/RP flux excess factor, and we require that it be less than 2 to ensure that only compact objects are assigned Gaia photometry (e.g., Secrest 2022). The G, G_BP, and G_RP magnitudes are on the Vega system.

4.3 NIR photometry

We used JHK photometry (1.2–42.2 µm) from a compilation of LQAC-6 matches to the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006), which includes both the Point Source Catalog (PSF; Cutri et al. 2003) and the Extended Source Catalog (Jarrett et al. 2000), as well as matches to the UKIDSS Large Area Survey for DR9 (Lawrence et al. 2007) and the DENIS database (Denis 2005). To ensure consistency in the meaning of photometry across the wide range of redshifts where quasars and AGNs are present, again, we fixed our priority on extended photometry (where available), and PSF/aperture photometry where not. LQAC-6 objects with matches to the 2MASS XSC were therefore assigned the corresponding JHK magnitudes irrespective of whether photometry existed in other catalogs with smaller formal photometric uncertainties. As 2MASS XSC is essentially complete outside of low Galactic latitudes out to K ≲ 14, which corresponds approximately to D ≲ 100 Mpc (z ≲ 0.02 for h = 0.7) for galaxies larger than the Milky Way (M ~ −21), AGN without XSC counterparts are assumed to be compact enough that PSF or aperture photometry is sufficient. For these objects, priority was placed on photometry with the lowest formal error, as with the optical photometry (Sect. 4.2).

4.4 MIR

The previous LQAC release, LQAC-5 (Souchay et al. 2019), was the first to incorporate mid-IR photometry, which has come from the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) AllWISE release. WISE is space-based, all-sky surveyor that has surveyed the sky multiple times in the 3.4 µm (W1), 4.6 µm (W2), 12 µm (W3), and 22 µm (W4) bands between the end of 2009 and the end of 2010 when its cryogen was depleted. The post-cryo NEOWISE mission (Mainzer et al. 2011) continued surveying the sky in W1 and W2 into 2011 before going into hibernation. In 2013, WISE was reactivated for the NEOWISER program (Mainzer et al. 2014), which continues to this day. Using data from NEOWISE-R through the end of 2018, the new CatWISE2020 catalog (Marocco et al. 2021) is significantly deeper than AllWISE, especially in the AGN-sensitive W2 band, and benefits from increased source completeness in crowded regions due to using the sharper unWISE co-adds and detection list (Schlafly et al. 2019).

For LQAC-6, we use photometry from both AllWISE and CatWISE2020. As the latter does not include the 2MASS XSC-based elliptical aperture magnitudes available for extended objects, we used AllWISE for all four bands where these magnitudes exist. For compact or unresolved objects, we chose the smallest uncertainty W1 and W2 magnitudes measured between AllWISE and CatWISE2020, with the W3 and W4 magnitudes being available only from AllWISE.

4.5 Radio

To get as complete a sample of LQAC-6 objects with measured radio fluxes as possible, we used, in order of central frequency, the 0.15 GHz TIFR GMRT Sky Survey (TIFR) First Alternate Data Release (Intema et al. 2017), 0.84 GHz Sydney University Molonglo Sky Survey (SUMSS; Mauch et al. 2003) v. 2.1, 0.90 GHz Rapid ASKAP Continuum Survey (RACS; McConnell et al. 2020), 1.4 GHz NRAO VLA Sky Survey (NVSS; Condon et al. 1998), Faint Images of the Radio Sky at Twenty-centimeters (FIRST; Helfand et al. 2015) v. 2014Dec7, and the recent 3.0 GHz Very Large Array Sky Survey (VLASS; Lacy et al. 2020). We note that the TGSS currently has position-dependent flux calibration problems (Hurley-Walker 2017) and RMS noise systematics (Intema et al. 2017). Hurley-Walker (2017) derived a re-scaled version of the TGSS catalog using the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey (Hurley-Walker et al. 2017), yielding a distribution of flux scaling factors with a dispersion of ~10%, and Secrest et al. (2022) derived position-dependent scaling factors using a comparison with the NVSS. However, as neither GLEAM nor the NVSS cover the entirety of the TGSS footprint, precluding a correction to all sources, and the distribution of flux scaling errors is reasonable, we did not apply any corrections to the TGSS flux densities. However, we enforce a noise floor of 10% to all sources, or a minimum logarithmic flux error of 0.043 dex. We also did not use the 0.17 MHz GLEAM survey catalog, as with its ~2′ beam the LQAC-6 will likely suffer significant confusion issues with its typical ~0.05 source arcmin² density outside of the Galactic plane. The angular resolutions of the TGSS, SUMSS, RACS, NVSS, FIRST, and VLASS are approximately 25″, 45″, 15″, 45″, 5.4″, and 2.5″, respectively. We additionally note that the VLASS catalog also has flux calibration issues, with Gordon et al. (2021) finding that $S_3{}_{\text{\hspace{0.17em}GHz}}=S_{\text{\hspace{0.17em}VLASS}}/0.87$. We applied this scaling factor to the VLASS flux densities.

We handled matching the radio data differently than the data at shorter wavelengths. While we performed symmetric, closest matches between the optimized LQAC-6 positions and the UV through mid-IR associations, radio catalogs have two complications that make this method problematic. First, lower resolution radio catalogs such as the NVSS have correspondingly poorer astrometric precision (e.g., typical ~5″ positional errors for NVSS). This creates a significant confusion problem for counterpart identification at other wavelengths, as multiple associations may exist within the position error ellipse. As LQAC-6 is designed to be reliable, but not necessarily complete, the true counterpart of a radio source may be a projected neighbor of an LQAC-6 object, such as an uncatalogued radio AGN or Galactic contaminant. Second, the radio source may be the distant synchrotron lobe of a jet originating from an AGN well outside even a generous match tolerance, especially at lower frequencies (~ 1 GHz) and the”core” (which might pull the radio centroid closer to the AGN) can be entirely synchrotron self-absorbed. For example, a 100 kpc-scale radio lobe originating from a z = 1 radio galaxy will have a projected offset of ~13″ (h = 0.7). As radio-loud AGNs are often bolometrically faint, this problem is further compounded by the faintness of host galaxy counterpart. A radio AGN-hosting elliptical galaxy with an absolute magnitude of M_V = −22, for example, will have an apparent magnitude of m_V ~ 21 at z = 1 (h = 0.7), at the limit of Gaia’s sensitivity.

To circumvent these complications, we first match all of the radio catalogs to themselves. This is motivated by the assumption that a non-thermal emission source at one frequency (i.e. a synchrotron source) is more likely to spatially coincide with the same non-thermal emission source at a similar frequency than it is to coincide with a thermal source (i.e., one bright at UV–IR wavelengths). This helps mitigate genuine physical offsets, so that the remaining offsets are statistical, assuming systematic astrometric errors are negligible by comparison. The error-weighted mean position of a true counterpart to LQAC-6 will therefore be closer, reducing ambiguity and allowing more reliable rejection of false matches. A complication to this scheme is that the FIRST catalog does not have formal position errors. However, with its 5.4″ resolution we find that a 10″ match to the even sharper VLASS catalog yields a set of matches with no significant evidence of false positives. With this result, we match the FIRST and VLASS catalogs separately. For the TGSS, SUMSS, RACS, and NVSS catalogs, we use a quadruple match in TOPCAT with the”Sky Ellipses” algorithm, setting the semimajor and semi-minor axes to three times the formal position errors in α and δ, respectively. The weighted arithmetic mean of α and δ is taken using the inverse square of the formal errors as weight (reiterating that correlation information is not provided). Matching to the FIRST catalog, we find that the typical offset is 2.3″, compared to 4.5″ using NVSS coordinates, confirming that taking the mean position across multiple low resolution catalogs significantly improves coordinate accuracy, by the factor of $\sqrt{2}$ expected.

We matched the FIRST and VLASS catalogs with a 10″ tolerance, using only FIRST entries with a sidelobe probability of 0.014 (the lowest value) and VLASS sources with Duplicate_flag < 2 and Quality_flag == 0 for VLASS sources, as is recommended in the documentation⁵. For sources with both FIRST and VLASS counterparts, we take the median value of their α, δ as the new source coordinates.

For objects with radio fluxes measured at two or more frequencies v, we calculated the spectral index, α, such that the flux density S_v ∝ v −α. For objects with more than three radio flux measurements, α is calculated using least squares, weighting by the inverse flux errors, which include the 10% error floor to account for inter-survey calibration offsets. The typical value of a for optically thin synchrotron emission is ~0.75; we find that the mean and median value of α for the full match between the aforementioned catalogs is ~0.7.

Cross-matching with LQAC-6, we again find that a 10” match tolerance is optimal. This results in 104 539 objects with at least one radio flux measurement, or 5% of LQAC-6. We again note that large-scale radio lobes can be expected to exist on scales larger than this, so this match necessarily excludes these objects in the interest of maintaining a good balance of reliability and completeness. Consequently, these matches should be more core-dominated objects, and indeed their median spectral index shows a dependence of position offset: sources with a radio association larger than ~4″ have a median spectral index of 0.75, while sources with a radio association closer than ~4″ (< 30 kpc at z = 1 for h = 0.7) have progressively flatter spectral indices, approaching α ~ 0.25 as the offset goes to zero (Fig. 1).

Fig. 1 Blue: median radio spectral index (S ∝ v−α), using a Gaussian kernel density estimator, as a function of position offset of the radio counterpart from LQAC-6. Gold: α = 0.75, typical for optically thin synchrotron emission.

5 Redshifts

As is clearly shown in Table 1, for a majority of objects of the various catalogues of the compilation, a redshift value is given. Values of the redshifts can be obtained either from the ground-based surveys taken into account in the compilation or the Gaia DR3 release. In this last case, among the 2 073 099 objects of the LQAC-6, 452 869 objects (21.84 %) have a redshift value both in a ground-based survey and in Gaia DR3.455 260 objects (21.96 %) have a redshift value in a ground-based survey but not in DR3. And 1 031 432 objects (49.75%) have a redshift in Gaia DR3, but not in a ground-based survey. By adding the numbers of objects of these three sub-groups together, we conclude that 1 939 561 objects representing 93.56% of the whole LQAC-6 have at least one redshift value, whereas the complementary sample, that is to say 133 538 objects (6.44%) have no redshift value.

In fact the Gaia DR3 catalogue gives three redshift values when available: a nominal one (z_nom) together with a minimal one (z_min) and a maximal one (z_max)· Therefore, it looks interesting, in the case of the existence of a redshift value, z, given by a ground-based catalogue of the LQAC-6 compilation, to look at the extent of which z lies inside the interval of uncertainty [z_min, z_max]. Among the 452 869 objects which have a redshift both in a ground-based survey z and in the Gaia DR3, 185 058 objects verify z_min < z < z_max, that is to say a mere 40.86%. Thus 267 811 objects (59.14%) are outliers, with z < z_min or z < z_max. However, among the 267 811 outliers, 102 633 (38.33%) verify: −0.1 < z − z_nom < 0.1, which means that although they are considered as outliers, the difference remains reasonably small. The distribution of number of outliers as a function of the redshift difference is shown in Fig. 2.

Fig. 2 Histogram of the difference between the redshift value z given by a ground-based survey of the LQAC-6 and the redshift value given by Gaia DR3 concerning the 267 811 outliers verifying: z < zmin or z > zmax, where zmin and zmax are respectively the minimum and maximum values given by Gaia DR3.

6 Derived properties

In addition to astrometric and photometric data for LQAC-6 objects, we aim to provide the derived properties, such as the absolute magnitudes and morphological indices.

6.1 Absolute magnitudes

As in previous LQAC releases, we calculated the absolute magnitudes as:

$$M=m+5-5\text{\hspace{0.17em}\hspace{0.17em}log\hspace{0.17em}}\left(D_{\text{L}}\right)-A-A_{\text{IG}}-A_{\text{host}}-K$$(3)

where m is the apparent magnitude in the observer frame, D_L is the luminosity distance, A is the Galactic reddening, A_IG is the intergalactic reddening, A_host is the reddening internal to the AGN host galaxy, and K is the passband correction due to red-shift. To calculate D_L, we assumed a flat ACDM cosmology with h = 0.7 and Ω_M = 0.3. For the Galactic reddening, we determine E(B – V) using the Galactic extinction map from Planck Collaboration XI (2014), assume R_V = 3.1, and use the extinction coefficients from Wang & Chen (2019). We did not calculate absolute magnitudes for the new objects which were not included in the LQAC-5, which means that the contents for this item are exactly the same as in the LQAC-5. To investigate the procedure to calculate these absolute magnitudes, we refer to Souchay et al. (2019).

6.2 Morphological indices

In the LQAC-5, the point-spread functions of the quasars’ image was used to establish a morphological index. Images were extracted from the Digitized Sky Survey (DSS plates in B, R and IR bands. As it is the case of the absolute magnitudes, no new calculations were done with respect to the LQAC-5, so that for this item the contents are the same. And we can refer to Souchay et al. (2019) to understand the procedure.

7 Summary

In this paper we have constructed the sixth Large Quasar Astrometric Catalogue, namely LQAC-6, by adding a large number of new objects coming from the last up-date DR16Q of the SDSS (Lyke et al. 2020) as well as the objects belonging to the GCRF3 (Gaia Collaboration 2022). Thus the total number of recorded quasars in our compilation reaches 2073 099 objects, which represents 3.5 times the number of objects in the LQAC-5 (Souchay et al. 2019).

In addition to the 1 480 290 new entries, an important advantage with respect to the LQAC-5 is the cross-matching of the objects with the Gaia DR3 release instead of the DR2 release. Yet, the difference is both qualitative and quantitative, as explained with details by Souchay et al. (2022a). In particular the uncertainty of the determination of positions, parallaxes and proper motions is generally much smaller when using the DR3 with respect to the DR2 measurements.

Another drastic improvement was made in terms of compilation : 20 new catalogues of quasars were included, which were generally a sub-sample of extragalactic surveys concerning a restricted area of the celestial sphere. Following the GCRF3, the two main contributions were coming from LAMOST and ZCAT with roughly 57 000 and 82000 objects. Among small contributors, some of them have an important place due to their specificities, as the MQS, with 750 objects located behind the Magellanic clouds.

Because of time constraints we have not included in our compilation the very recent Dark Energy Spectroscopic Instrument Early Data Release (DESIEDR; DESI Collaboration 2023) which was recently published containing concatenated redshift and source classifications for 2 362 704 objects primarily in the northern sky. Two versions of the catalogue a re found, a HEALPix-based version combining redshifts across all programs, and a cumulative, tile-based version. While both have the same number of “primary” objects, we find that the cumulative version has more objects without flagged redshift warnings (ZWARN==0) and somewhat larger DELTACHI2 values, indicating higher confidence in the best-fit redshifts. There are 89 402 objects with ZWARN==0 and SPECTYPE==“QSO”, which we have scheduled to include in a future version of the LQAC.

References

All Tables

All Figures

	Fig. 1 Blue: median radio spectral index (S ∝ v−α), using a Gaussian kernel density estimator, as a function of position offset of the radio counterpart from LQAC-6. Gold: α = 0.75, typical for optically thin synchrotron emission.
In the text

	Fig. 2 Histogram of the difference between the redshift value z given by a ground-based survey of the LQAC-6 and the redshift value given by Gaia DR3 concerning the 267 811 outliers verifying: z < zmin or z > zmax, where zmin and zmax are respectively the minimum and maximum values given by Gaia DR3.
In the text