1.1 Astrometry and photometry for omega Centauri

Author(s): K. Janßen, A. Mints, J. Castañeda, Z. Kostrzewa-Rutkowska, M. Davidson

This FPR features astrometry and $G$-band photometry of half a million sources located on the sky in the $\omega$ Centauri globular cluster. The source density in this region is above the astrometric crowding limit for Gaia ($\sim \mathrm{1\hspace{0.17em}050\hspace{0.17em}000}$ objects deg${}^{-2}$ Gaia Collaboration et al. 2016, Section 6.6) which leads to severe incompleteness of the Gaia catalogue in this region. In order to mitigate this issue special observations of $\omega$ Cen are taken using the so-called Service Interface Function (SIF) from Gaia. This functionality enables the collection of full-frame images from the Sky Mapper CCDs which can be processed iteratively to obtain astrometry and $G$-band photometry for sources otherwise not found in the standard Gaia survey. These two dimensional images thus overcome the Gaia crowding limitations.

The resulting catalogue of the $\omega$ Cen region contains the astrometry (positions, parallaxes, and proper motions) and photometry (mean $G$-band flux) for sources that were detected in the full frame images and did not exist as sources in the main Gaia DR3 catalogue already. Hence the data contained in this FPR is fully complementary to Gaia DR3.

In the very dense core of the $\omega$ Centauri cluster this FPR catalogue contains ten times as many sources as Gaia DR3, and the combined catalogue goes more than three magnitudes deeper. Furthermore, comparing the newly found sources to a dedicated HST sample introduced by Bellini et al. (2017), it could be shown that 90% of the FPR sources match HST and can thus be regarded as reliable.

Note: The crowded_field_source data model description detailed in Section 2.1.1 was copied from the nominal gaia_source descriptions for Gaia DR3 and adapted to peculiarities of the SIF CF data. That adaptation led to the following inaccuracies in the descriptions of some of the data fields, which were discovered when it was technically too late to change them:

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  The nominal limitations regarding the magnitude (G ) and the AC observations (astrometric_n_obs_ac $>0$) mentioned in the fields astrometric_n_obs_ac, ipd_gof_harmonic_phase and scan_direction_* don’t hold for the SIF CF FPR data. For this FPR, these fields are always filled.

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  The items referring to the quantities nu_eff_used_in_astrometry and ipd_frac_multi_peak in the fields pseudocolour and ipd_gof_harmonic_phase are irrelevant for this FPR dataset, because these quantities do not exist for crowded_field_source data.

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  pseudocolour and related fields are empty for a few sources for which a position-only solution is reported.

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  In a few places the descriptions refer to “Gaia DR3” where “this FPR” would be more appropriate.

The data processing, validation and data model for this FPR are described in Gaia Collaboration, Weingrill, K., et al. (2023e).