Blobs and Blurs: A Citizen Science-Identified Catalog of Diffuse Galaxies in the Fornax Cluster

TL;DR

This study presents Blobs and Blurs, the first cluster-wide catalog of diffuse galaxies (DGs) identified through citizen science in the Fornax cluster, leveraging deep optical imaging from the Fornax Deep Survey (FDS) and the Dark Energy Camera Legacy Survey (DECaLS). Across 26 deg$^{2}$, more than 1,400 volunteers contributed to visually identifying 643 DG candidates, achieving high completeness relative to existing dwarf catalogs for $r_{ m eff} \,\ge\, 5\arcsec$ and $\\\ ilde{\\mu}_r \lesssim 26$ mag arcsec$^{-2}$, and uncovering DGs missed by automated searches (97 candidates) plus 3 new candidates. The authors perform detailed photometric modeling with GALFIT (including potential nuclear star clusters) to derive structural parameters, assess nucleation reliability (with an 84% volunteer consensus yielding perfect expert agreement), and analyze the nucleated population, radial distribution, and NSC mass properties. The results demonstrate the viability and efficiency of citizen science for mapping diffuse galaxy populations in large surveys and provide insights into DG formation and NSC growth in Fornax, with implications for future LSST-era studies. The Blurs catalog thus offers a valuable, complementary dataset to automated searches and visual catalogs, informing models of DG assembly and NSC formation in cluster environments.

Abstract

We present a catalog of 643 diffuse galaxies identified through a citizen science search of the Fornax cluster, of which we estimate 21.8% are nucleated (139/637; 6 inconclusive). This marks the first crowd-sourced effort to construct a cluster-scale census of diffuse galaxies. These objects were visually identified using a combination of the Fornax Deep Survey and Dark Energy Camera Legacy Survey imaging across 26 deg$^2$. Over 1,400 volunteers cataloged the candidates within this sky area at a rate of 1.15 days/deg$^2$. Our catalog is highly complete relative to existing dwarf catalogs of Fornax ($> 80\%$ of objects recovered) down to an effective radius $r_{\mathrm{eff}} = 5^{\prime \prime}$, the minimum size we suggested volunteers classify, and to an effective r-band surface brightness as faint as $\langle μ_r \rangle \simeq26$ mag arcsec$^{-2}$. We detect 97 candidates that existing automated searches of Fornax did not find and three candidates not found by any prior search, automated or visual. The stellar mass distribution of our sample is consistent with similar dwarf studies of Fornax, with the nucleated fraction peaking at 80% for a host galaxy mass of $\sim$10$^{8.5}M_{\odot}$. The efficiency and completeness of our catalog thus establishes citizen science as a valuable tool for mapping diffuse galaxy populations in future sky surveys, such as the Legacy Survey of Space and Time.

Figures (19)

• Figure 1: The Fornax Deep Survey (FDS; Peletier_2020) sky coverage plotted over an optical image of the Fornax cluster from the Legacy Survey. Each observing field is numbered and covers an area of approximately 1.17$^{\circ} \times$ 1.17$^{\circ}$. The white tinted fields have ugri coverage while the blue tinted fields are missing the u-band. The F3 and F33 tiles (to the left of F7 and F8) were not used in this project due to incomplete sky and filter coverage.
• Figure 2: Zooniverse interface used by volunteers to classify objects on our project. The top screenshot shows the user interface when the VST image is selected and the bottom screenshot is the same interface with the DECaLS image selected. The three circles below the image cutout allow the volunteer to select from the three image types we provide: VST (left circle), DECaLS (middle circle), and GALEX (right circle). In this cutout, a nucleated diffuse galaxy is present in the field of view, therefore a volunteer would be expected to draw a circle around it with the Zooniverse shape tool. After drawing the circle, a pop-up window asking if this object is nucleated will appear, which in this case they should select "Yes".
• Figure 3: Visual aid demonstrating how unique and duplicate diffuse galaxy selections are distinguished by our reduction process. The green circles indicate hypothetical selections of DGs that could realistically be drawn by Zooniverse volunteers on DECaLS cutouts when a DG is present (intentionally centered on object). Two (or more) circular selections are considered unique objects if their respective centers fall outside of the area in which the circles intersect. A special case occurs (top right panel) when volunteers do not center their selection on a galaxy, possibly because the object is quite large or it is split across multiple cutouts. In this case, the selections must be manually merged together.
• Figure 4: Flowchart demonstrating the successive cuts made to reduce the initial list of 27,979 circular selections drawn by volunteers to a final catalog of 643 DG candidates. Information on each stage of the catalog preparation process can be found in the corresponding sections indicated by the curly brackets.
• Figure 5: Three potential diffuse galaxies (indicated by dashed circle) detected by Blurs that are not present in any existing catalogs of Fornax, automated or visual. The green bar at the top of each image represents 10 in the image pixel scale. All images are DECaLS cutouts taken from the Legacy Survey Sky Viewer.