Overlap Zoo Beta: A Catalogue of ~800 Occulting Pairs in the DESI Legacy Survey using Citizen Science

Abstract

Overlapping galaxies, in which a foreground galaxy partially overlaps a background galaxy, offer a unique opportunity to measure dust attenuation, a key nuisance parameter in galaxy studies, empirically and in great detail by modelling the light of both the foreground and background galaxy and inferring the missing light in the overlapping region. However, the current catalogue of overlapping pairs is relatively limited in number compared to catalogues dedicated to individual galaxies. Expanding this catalogue is not only a necessity to facilitate further detailed dust studies beyond the few limited studies conducted thus far, but also to improve pair-to-pair variance and support automated identification through machine learning techniques. To achieve this, we utilise galaxies classified as "overlapping" from Galaxy Zoo DECaLS (GZD-1, -2, and -5), along with images from Data Release 10 (DR10) of the DESI Legacy Survey, in our individual citizen science project to classify these pairs directly using volunteers. This new catalogue will not only provide a wealth of targets for future dust studies but will also contribute to a deeper understanding of these pairs and dust as a whole.

Figures (8)

• Figure 1: The distribution of galaxies classified as 'overlapping' in GZD-1, -2, and -5. We plot the GZD-1 and GZD-2 data as black dots, the GZD-5 data as red dots, and our selections of the combined data as blue dots. Our selection criteria include only galaxies classified as overlapping with votes >13 and voting fractions >0.28.
• Figure 2: Flowchart of the workflow presented to our internal classifiers. We labelled each question with its corresponding task number. Tasks are also colour-coded based on their tier level. Each tier, coloured grey, green, blue, and purple, represents zero, one, two, three, or four branching points in the decision tree, respectively.
• Figure 3: The distribution of user consistency $\kappa$ based on the methods outlined in Willett13Willett17. Bottom: the cumulative distribution of consistency for all users. Top: the average number of galaxy pairs, $\langle{n}\rangle$, classified by users as a function of their consistency.
• Figure 4: Sample pairs based on the pair types described in Keel13. PNG images were obtained from the Legacy Survey Viewer using the legacystamps Python module Sweijen22 and independently verified for use in this figure. We identify pairs by their common name (e.g., NGC 450) or truncated coordinates (e.g., 1226+00).
• Figure 5: The remaining sample pairs following Keel13 mnemonics.