2025ApJ...991....3A

Through studying rotation curves, which depict how the velocity of the stars and gas changes with distance from the center of a galaxy, it has been confirmed that dark matter dominates galaxies' outer regions, as their rotation curve remains flat. However, recent studies of star-forming galaxies at cosmic noon have shown a decline in their rotation curve beyond a certain point, suggesting a decrease of the abundance of dark matter in galactic halos during earlier times. In this work, we investigate the influence of cosmological surface brightness dimming and loss of resolution on observations of rotation curves at cosmic noon. We used a sample of 19 Lyman break analogs at z ∼ 0.2 and artificially redshifted them as if they were at z ∼ 2.2. By comparing both rotation curves of the observed and mocked objects, we find that the asymmetry of the cosmic noon galaxies is smaller than that of the low-z galaxies. In low-z galaxies, asymmetry increases with radius and becomes relevant at the external parts, where mergers and interactions cause more disturbance in the galaxy's gravitational field. In contrast, cosmic-noon galaxies appear smoother, smaller, and suitable for dynamical modeling-when in reality they are not. The combined effects of the cosmological bias and loss of resolution lead us to the conclusion that caution should be exercised when using cosmic-noon rotation curves, as they might not accurately trace the gravitational potential of the galaxy.