Probing the dynamics of extreme galaxies. I. Dark matter content in ultra-diffuse galaxies

Abstract

We investigate the internal structure of two galaxies from the LEWIS sample: the ultra-diffuse galaxy UDG-1 and the extended dwarf galaxy LSB-6. Both systems exhibit coherent stellar rotation in combination with a non-negligible fraction of random motion, with no signs of ongoing disturbance. This work represents the first attempt to place constraints on the galaxy dynamics and on the dark matter physics of rotation-supported UDGs through the modeling of integral-field stellar kinematics. The aim of this work is to constrain the parameters that describe the internal structure of the galaxies and to assess which dark matter model and UDG formation scenario are favored by the observed stellar kinematics. We consider the standard cold dark matter paradigm as well as alternative frameworks, including fuzzy, self-interacting, and non-minimally coupled dark matter. We modeled the galaxies as stellar spheroidal systems with typical dwarf-like thickness, embedded in spherical dark matter halos whose density profiles encode the properties of the dark matter models under consideration. We derived two-dimensional velocity fields and compared them with the observations within a Bayesian framework. The kinematic data suggest a cuspy dark matter halo in UDG-1 and a cuspless halo in LSB-6. Beyond the standard cold dark matter scenario, all the alternative dark matter models emerge as viable candidates, although the limitations of current data do not enable us to conclusively discriminate between them. Within the fuzzy dark matter framework, we obtain mutually consistent constraints on the boson mass: m_α = 4.7^ +1.8 _ -1.1 10^ -22 eV for UDG-1 and m_α = 6.3^ +0.4 _ -0.4 10^ -22 eV for LSB-6. Within the self-interacting dark matter scenario, LSB-6 yields a robust constraint on the velocity-weighted cross section, łangle σ v̊angle/m = 14.3^ +2.0 _ -1.9 , cm ^2 , km , g ^ -1 , s ^ -1 . For non-minimally coupled dark matter, we derive upper bounds on the coupling length, which implies marginal deviations from standard ΛCDM behavior. Regardless of the underlying dark matter model, both UDG-1 and LSB-6 exhibit a dark matter content comparable with the halos of typical dwarf galaxies with similar stellar masses. Together with information from stellar kinematics, stellar population analyses, and globular clusters, these results support a scenario in which UDG-1 and LSB-6 originate from puffed-up dwarfs. By probing the kinematics of extremely low-surface-brightness systems, we infer their internal structure and translate these results into constraints on dark matter particle properties. Our analysis also provides new insights into the formation mechanisms of UDGs. This work is intended as the first in a series exploiting kinematic data of extremely low-surface-brightness galaxies to test fundamental physics, from dark matter to modified theories of gravity.