Ursa Major III/UNIONS 1: the darkest galaxy ever discovered? (2311.10134v2)

Abstract: The recently discovered stellar system Ursa Major III/UNIONS 1 (UMa3/U1) is the faintest known Milky Way satellite to date. With a stellar mass of $16^{+6}_{-5}\,\rm M_\odot$ and a half-light radius of $3\pm1$pc, it is either the darkest galaxy ever discovered or the faintest self-gravitating star cluster known to orbit the Galaxy. Its line-of-sight velocity dispersion suggests the presence of dark matter, although current measurements are inconclusive because of the unknown contribution to the dispersion of potential binary stars. We use $N$-body simulations to show that, if self-gravitating, the system could not survive in the Milky Way tidal field for much longer than a single orbit (roughly 0.4Gyr), which strongly suggests that the system is stabilized by the presence of large amounts of dark matter. If UMa3/U1 formed at the center of a ~$10^9\rm M_\odot$ cuspy LCDM halo, its velocity dispersion would be predicted to be of order ~1km/s. This is roughly consistent with the current estimate, which, neglecting binaries, places $\sigma_{\rm los}$ in the range 1 to 4km/s. Because of its dense cusp, such a halo should be able to survive the Milky Way tidal field, keeping UMa3/U1 relatively unscathed until the present time. This implies that UMa3/U1 is plausibly the faintest and densest dwarf galaxy satellite of the Milky Way, with important implications for alternative dark matter models and for the minimum halo mass threshold for luminous galaxy formation in the LCDM cosmology. Our results call for multi-epoch high-resolution spectroscopic follow-up to confirm the dark matter content of this extraordinary system.

• The paper identifies UMa3/U1 as an ultra-faint Milky Way satellite with the lowest stellar mass and a very small half-light radius.
• N-body simulations indicate that without dark matter, UMa3/U1’s survival is limited to just 0.4 Gyr, underscoring the importance of dark matter for stability.
• The study challenges standard LCDM models, prompting calls for deeper spectroscopic analyses to confirm the system's dark matter content.

Ursa Major III/UNIONS 1: The Darkest Galaxy Ever Discovered?

The paper entitled "Ursa Major III/UNIONS 1: The Darkest Galaxy Ever Discovered?" by Errani et al. investigates the nature of a newly discovered Milky Way satellite, Ursa Major III/UNIONS 1 (UMa3/U1). The paper explores whether UMa3/U1 can be characterized as the darkest known galaxy due to its unique properties or the faintest self-gravitating star cluster orbiting the Galaxy. This duality arises primarily from its extreme faintness and the undetermined contribution of dark matter to its dynamic properties.

The authors provide a comprehensive analysis of UMa3/U1, reporting that it has the lowest stellar mass among known Milky Way satellites, approximately 16 solar masses, with an equally minuscule half-light radius of about 3 parsecs. Such characteristics provoke questions about its survival in the tidal field of the Milky Way, which is addressed using their $N$-body simulations. The simulations suggest that the system, if self-gravitating, could only survive for approximately 0.4 Gyr—equivalent to one orbital period—implying that the presence of dark matter may be necessary for its stabilization.

Further, the paper examines the implications of UMa3/U1 hypothetically residing within a cuspy LCDM halo with a mass on the order of %%%%1%%%% solar masses, which would justify the medial velocity dispersion of 1 to 4 km/s observed. This suspicion is partly due to the stability supposedly requisite to withstand the substantial tidal forces of the Milky Way. The evidence compels the authors to call for enhanced spectroscopic studies to confirm these preliminary findings regarding UMa3/U1's dark matter content.

The detection of such a dwarf galaxy, possessing both the faintest known luminosity and an extremely high density, carries noteworthy implications for theoretical models concerning dark matter, potentially challenging existing paradigms. Particularly, UMa3/U1 may impose new constraints on the lower mass thresholds necessary for galaxy formation within the framework of the LCDM model. It may also call into question alternative models of dark matter such as Warm Dark Matter (WDM) and Fuzzy Dark Matter (FDM), which speculate about smaller dark matter halos' inability to form luminous galaxies.

The discovery of UMa3/U1 as potentially the faintest galaxy provides a rare observational perspective on the characteristics of the smallest dark matter halos that enable the formation of luminous clusters. Such astoundingly dense objects have prospectively numerous implications for cosmology and galaxy formation theories. Moreover, UMa3/U1 prompts a re-examination of the cosmic web at small scales, and hence, suggests prospering future investigations into the interactions between dark matter and baryonic matter in environments previously thought unreachable.

In conclusion, Errani et al.'s paper on UMa3/U1 identifies it as a critical subject for the intersection of cosmology and astrophysics. It suggests UMa3/U1 either represents a scenario of extreme stellar cluster stability or an exceptional case of a dark matter-heavy micro galaxy. Continued observation and rigorous analysis can elucidate UMa3/U1's nature, offering substantial advancements in understanding dark matter's role within the galactic hierarchy. This work also paves the way for discovering more ultrafaint dwarf galaxies and enriching our comprehension of the low-luminosity end of the galaxy luminosity function.