8. ULTRA-FAINT DWARFS AS DARK MATTER LABORATORIES

The nature of dark matter is one of the most significant outstanding questions in astrophysics, and the smallest dwarfs may play an outsized role in helping to answer it. In this section we mention some of the ways in which UFDs can constrain dark matter properties and dark matter models. For broader discussions of dwarf galaxies from a dark matter perspective, see, e.g., Porter, Johnson & Graham (2011), Weinberg et al. (2015), Bullock & Boylan-Kolchin (2017), Buckley & Peter (2017), or Strigari (2018).

UFDs can potentially provide insight into dark matter for several reasons:

• They are the most dark matter-dominated systems known. Unlike in larger and more luminous dwarfs (e.g., Brooks & Zolotov, 2014, Di Cintio et al., 2014), their baryonic components are likely to have been dynamically negligible at all times. Their inefficient star formation means that feedback should not be powerful enough to alter their internal density structure (e.g., Oñorbe et al., 2015).
• Because of their small sizes, they offer probes of dark matter on smaller scales (∼ 20−30 pc for the most compact ultra-faint dwarfs) than is possible anywhere else.
• The number of dwarf galaxies orbiting the Milky Way sets a lower bound on the abundance of low-mass dark matter subhalos, which translates to a limit on the allowed mass of warm dark matter particles (e.g., Kennedy et al., 2014).
• With the exception of the Galactic Center and Sagittarius (which they greatly outnumber), they are the closest dark matter halos to us. The combination of their proximity, their high measured densities (e.g., Simon & Geha, 2007), and their low astrophysical backgrounds makes them promising targets for indirect detection experiments.
• Their internal dynamics are so gentle that heating by very weak effects is potentially measurable. For example, Brandt (2016) used the presence of a star cluster in Eridanus II to place tight constraints on MACHO dark matter, and Peñarrubia et al. (2016) proposed that wide binary stars may be disrupted by the dark matter potential of a UFD, allowing a measurement of the dark matter density profile.

Because of the arguments listed above, UFDs have attracted a great deal of attention from a broad cross-section of astrophysicists. Their potential to facilitate indirect detection of dark matter has been a particular focus of attention. The majority of indirect detection experiments search for gamma-rays resulting from annihilation of dark matter particles, using either the Fermi Gamma-Ray Space Telescope or ground-based atmospheric Cherenkov telescopes. UFDs are prime targets for both types of facilities (e.g., MAGIC Collaboration et al., 2016, Albert et al., 2017, Archambault et al., 2017). The sensitivity of these searches will continue to improve as integration times increase and new observatories such as the Cherenkov Telescope Array begin operation. Indirect detection searches in UFDs will offer a critical testing ground for possible dark matter signals seen in other parts of the sky (e.g., Abazajian & Keeley, 2016). Dark matter annihilation or decay signals could also manifest in dwarf galaxies as synchrotron emission at radio wavelengths (Spekkens et al., 2013, Regis, Richter & Colafrancesco, 2017) or as X-ray emission lines (e.g., Jeltema & Profumo, 2016).

The holy grail for dark matter research in dwarf galaxies is the conclusive measurement of the inner density profile of a highly dark matter-dominated system. As mentioned above, UFDs are ideal in the sense that they have the highest known dark matter fractions of any galaxies, and their density structure is unlikely to have been affected by stellar feedback. Their disadvantage is that they contain so few stars that there may not be enough dynamical tracers for a robust measurement of the mass distribution. Given the difficulties encountered in analyzing radial velocity data sets containing hundreds to thousands of stars in the classical dSphs, the maximum achievable sample of ∼ 100 stars in the most accessible UFDs will not be sufficient to separate a central dark matter cusp from a core. However, the combination of radial velocities and proper motions can provide much more accurate measurements (e.g., Strigari, Bullock & Kaplinghat, 2007, Kallivayalil et al., 2015). Measuring proper motions with an accuracy of ∼ 35 µas yr⁻¹ (5 km s⁻¹ at a distance of 30 kpc) for stars as faint as r ∼ 22 is a daunting task, but may be feasible with extremely large ground-based telescopes or by combining data from space-based facilities such as HST, Gaia, JWST, and WFIRST.