Globular clusters (GC) are found around galaxies spanning an extensive range of masses, from dwarfs to giants. They are discrete bright beacons that help shed light on the evolution of their host galaxies up to distances of hundreds of Mpc (e.g. Beasley 2020, Alamo-Martinez 2020). However, few studies exist targeting GC systems of spiral galaxies up to several effective radii. In Chies-Santos, de Souza, Caso et al. 2022, we search for GC candidates around the M81/M82/NGC3077 triplet with imaging from J-PLUS DR2. We first perform a tailored galaxy subtraction, photometry and cross-match with confirmed GCs from literature to build an initial catalogue with the 12-bands J-PLUS photometry. Then we employ a customised statistical model to search for new GC candidates based on their proximity, via propensity score matching, to the list of known GCs in a low-dimensional PCA projection of the multi-dimensional J-PLUS photometric parameter space. The model properly accounts for missing data and photometric errors. We report 642 new GC candidates in a region of 3.5 deg² around the triplet, ranked according to their Gaia astrometric proper motions when available. We find tantalising evidence for an overdensity of GC candidate sources forming a bridge connecting M81 and M82. The spatial distribution of the GC candidates (g-i) colours is consistent with halo/intra-cluster GCs, i.e. it gets bluer as they get further from the closest galaxy in the field. We map the metallicity distribution of the spectroscopically confirmed GCs into the metallicity distribution of the sample candidates. The metallicity distribution of the sample candidates is broad and displays a bump towards the metal-rich end. Our list increases the population of GC candidates around the triplet by 3-fold, stresses the usefulness of multi-band surveys in finding these objects, and provides a testbed for further studies analysing their spatial distribution around nearby (spirals) galaxies.

Figure 1. Left panel: Spatial distribution in Aitoff projection of spectroscopically confirmed GCs (violet filled circles) and GC candidates according to GAIA proper motion (pm) information. Orange filled squares are GC candidates with pm <3.5 mas/yr, orange open squares with pm >3.6 mas/yr, and cyan squares are GC candidates with unknown pm. Right Panel: Count distribution of GC candidates from low (light grey) to high (dark grey) counts with over-plotted density contours. A GC count excess between M81 and M82 provides a tantalising indication of a potential GC bridge.

All the details of the candidate selection are extensively explained in the paper:

The R package used in this work is available here, and the python version is here.

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How to cite

If you have used this data in your paper, please cite the following paper:

• Chies-Santos, de Souza, Caso et al. 2022

If you have any issue or doubt write to ana.chies@ufrgs.br.