Tuesday 26 Sep 2017The Effects of Unresolved Contaminant Stars on the Cross-Matching of Photometric Catalogues

Tom Wilson - University of Exeter

4th Floor Interaction Area 11:15-11:45

Understanding the interplay between the molecular clouds as the birthplace of stars and the effects those stars subsequently have on the interstellar medium requires an understanding of the stars themselves. In this talk I will discuss the effects of crowding in photometric surveys on both the astrometric and photometric properties measured. Blended objects exhibit large, non-Gaussian separation tails when cross-matched with Gaia, for catalogue densities of stars per point spread function area as low as 0.5%. These perturbations can be on the order of several arcseconds, in cases of large point spread functions. I will highlight some ways to minimise the impact this effect has, either through strict selection criteria or a more general probability-based matching scheme, used in lieu of a nearest neighbour match. The more general matches have the flexibility to model the non-Gaussian distributions, combined with rejection of false matches through magnitude comparison. This allows for the recovery of objects of high astrometric perturbation and the avoidance of matches below the sensitivity limit of one survey. Finally, I will also consider the potential for unphysical excesses in the stellar fluxes through photometric contamination, using WISE as an example. At low Galactic latitudes, stars as bright as W1=13 will, on average, have at least one additional source of flux in their point spread function. This leads to approximately 50% of Gaia-WISE cross-matches having separations larger than their 3-sigma uncertainty matching radius, assuming traditional Gaussian statistics. This means that half of the potential matches would be rejected by a Gaussian-based probability-matching scheme, with some objects perturbed even beyond a 3" nearest neighbour matching radius. These lost matches suffer, on average, 20% flux contamination, potentially leading to misinterpretation of completeness and/or the misidentification of stellar mid-IR disk excesses.

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