2024A&A...692A.105C

Context. UGC 2885 (z = 0.01935) is one of the largest and most massive galaxies in the local Universe, yet it has an undisturbed spiral structure, which is unexpected for such an object and is not predicted by cosmological simulations. Understanding the detailed properties of extreme systems such as UGC 2885 can provide insight into the limits of scaling relations and the physical processes driving galaxy evolution.
Aims. Our goal is to understand whether UGC 2885 has followed a similar evolutionary path as other high-mass galaxies by examining its place in the fundamental metallicity relation and on the star-forming main sequence.
Methods. We present new observations of UGC 2885 with the Canada-France-Hawaii Telescope and the Institut de radioastronomie millimétrique 30 m telescope. We used these novel data to calculate metallicity and molecular hydrogen mass values, respectively. We estimated the stellar mass (M_⋆) and star formation rate (SFR) based on mid-infrared observations with the Wide-field Infrared Survey Explorer.
Results. We find global metallicities Z = 9.28, 9.08, and 8.74 at the 25 kpc ellipsoid from the N2O2, R23, and O3N2 indices, respectively. This puts UGC 2885 at the high end of the galaxy metallicity distribution. We find a molecular hydrogen mass of M_H2 = 1.89 ± 0.24 × 10¹¹ M_⊙, a SFR of 1.63 ± 0.72 M_⊙ yr^–1, and a stellar mass of 4.83 ± 1.52 × 10¹¹ M_⊙, which gives a star formation efficiency (SFR/M_H2) of 8.67 ± 4.20 × 10¹² yr^–1. This indicates that UGC 2885 has an extremely high molecular gas content compared to known samples of star-forming galaxies (∼100 times more) and a relatively low SFR for its current gas content.
Conclusions. We conclude that UGC 2885 has gone through cycles of star formation periods, which increased its stellar mass and metallicity to its current state. The mechanisms that are fuelling the current molecular gas reservoir and keeping the galaxy from producing stars remain uncertain. We discuss the possibility that a molecular bar is quenching star-forming activity.