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Tuesday 21 Jul 2020[Journal Club] Title: : The Effects of Unresolved Contaminant Stars on the Cross-Matching of Photometric Catalogues

Tom J Wilson -

Zoom 11:15-11:50

Recent years have seen the advent of high resolution datasets, such as Gaia, leading to the ability to constrain astrometry with a degree of precision never before possible. However, this throws into stark relief lower angular resolution datasets, such as TESS and WISE. In this presentation I will discuss the effects of crowding, both astrometric and photometric, in photometric surveys. Blended objects exhibit large, non-Gaussian separation tails when cross-matched with Gaia. These perturbations can be several arcseconds, in cases of large point spread functions, with WISE suffering from a 6" FWHM PSF. I will use WISE to highlight the issue of crowding, with approximately 50% of Gaia-WISE cross-matches have separations larger than their 3-sigma uncertainty matching radius. This would result in half of the potential matches being rejected by a Gaussian-based probability matching scheme, with some objects perturbed even beyond a 3" nearest neighbour matching radius.

I will present more general matching techniques, combining flexibility of modelling non-Gaussian distributions with rejection of false matches through the comparison of ensemble photometric magnitudes. I will also consider the potential for unphysical excesses in stellar fluxes through photometric contamination, again using WISE as an example. Lost cross-matches suffer, on average, 25% flux contamination, potentially leading to the misidentification of e.g. stellar disk excesses. These effects are critical for all photometric catalogues, being important for the next generation of deep photometric surveys such as LSST, and I will discuss the implication my work with the LSST:UK consortium has on the survey's cross-matches.

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