The Faintest Dwarf Galaxies

10. SUMMARY AND OUTLOOK

Our understanding of the faintest dwarf galaxies has progressed rapidly since their discovery 14 years ago. As described in Sections 1 and 2, even the basic nature of the first ultra-faint dwarfs was unclear for several years. Now, thanks to dedicated follow-up efforts across a wide range of facilities, the velocity dispersions, masses, densities, metallicities, metallicity dispersions, ages, IMFs, proper motions, and orbits of subsets of the known UFDs have been measured. These observations have shown that UFDs are the most dark matter-dominated, oldest, most metal-poor, and most chemically primitive stellar systems known. Concordant theoretical efforts devoted to simulating galaxy formation in low-mass dark matter halos at increasingly high resolution indicate that the faintest dwarfs appear to naturally correspond to the luminous counterparts of the smallest halos capable of sustaining star formation.

Much work remains to be done, of course. No spectroscopy has been obtained for ∼ 1/3 of the current ultra-faint satellite population, leaving the status of some objects in question, and the highest-quality star formation histories are available for only 6 galaxies. On the theoretical side, simulating the formation and evolution of low-mass dwarfs around a Milky Way-like host to z = 0 remains a computational challenge. Analogs to the lowest-luminosity galaxies (M_* ≲ 10³ M_⊙) have not yet been reliably simulated. Importantly, the census of Milky Way satellites remains significantly incomplete. Even in the most pessimistic predictions, the Milky Way has approximately twice as many dwarf satellites as have been found so far (e.g., Newton et al., 2018). In optimistic scenarios, the total population could be nearly an order of magnitude larger. The missing nearby satellites may be revealed in the next few years by ongoing surveys such as MagLiteS (Drlica-Wagner et al., 2016) and the DESI Legacy Imaging Surveys (Dey et al., 2018), but the more distant ones will require deeper imaging (e.g., LSST). Discovering, confirming, and characterizing possibly hundreds of dwarf galaxy candidates will be a very large undertaking for the worldwide community. As an illustration of this challenge, Figure 8 shows color-magnitude diagrams of the very low-luminosity dwarfs Segue 1 (d = 23 kpc; M_V = −1.3) and Ret II (d = 32 kpc; M_V = −4.0), along with the approximate spectroscopic limits that can be achieved at medium resolution (for velocities) and high resolution (for chemical abundances) with current facilities. Spectroscopy for comparable systems at much greater distances can only be obtained with 30 m-class telescopes.

Figure 8. (left panel) Color-magnitude diagram of Segue 1 (photometry from Muñoz et al. (2018)). The shaded blue and purple magnitude regions indicate the approximate depth that can be reached with existing medium-resolution and high-resolution spectrographs, respectively. (left middle panel) Same, but for Ret II (using DES DR1 photometry). (right middle panel) Segue 1 shifted to a distance of 150 kpc. With current telescopes only a handful of its stars would be spectroscopically accessible. The shaded blue and purple regions now indicate the depth that could be reached with 30-m telescopes. (right panel) Ret II shifted to a distance of 250 kpc, again with the magnitude limits for 30-m telescope spectroscopy.

Looking forward, after completing the census of dwarf galaxies surrounding the Milky Way, stellar kinematics measurements can be used to determine their mass function for comparison with theoretical predictions. Continued chemical reconnaissance via high-resolution spectroscopy may provide new clues to the additional site(s) of r-process nucleosynthesis, and with luck could reveal the signatures of Population III SN explosions. Now that the formation environments of UFDs can be traced by their orbits, precision measurements including an expanded sample of ages from space-based photometry will show how dwarfs that formed in the field or in the Magellanic Group differ from those that have always been close to the Milky Way. Mass and density measurements will provide critical sensitivity and targeting information for indirect detection experiments. Although a detection may seem unlikely given current limits, any signal from dark matter would be of such importance that the search must continue. Finally, we may hope that astrometry or other novel techniques make it possible to determine the dark matter density profiles of the least-perturbed dark matter halos yet found. This measurement would strongly constrain the properties of dark matter. Given the tremendous amount we have already learned from studying UFDs, it would be fitting if the humblest galaxies in the Universe provided the answer to one of its biggest questions.

FUTURE ISSUES

Completing the census of Milky Way satellites. LSST will be needed in order to detect the faintest currently known dwarfs throughout the virial volume of the Milky Way. However, even achieving all-sky coverage (outside the Galactic plane) at SDSS depth or deeper, coupled with well-quantified detection limits, will substantially advance our knowledge of the galaxy luminosity function at faint magnitudes and the likely size of the satellite population of our Galaxy.

Obtaining photometric and spectroscopic follow-up observations for as much of the ultra-faint satellite population as possible. These observations are essential for classifying compact, low-luminosity stellar systems and determining their dark matter content. Metallicity and age measurements will enable us to reconstruct their formation and evolution. Expanding the current small sample of galaxies with precise star formation histories is an especially high priority for understanding the effects of reionization and environment on star formation in the faintest dwarfs. Detailed chemical abundance patterns of UFD stars are likely to provide new insight into nucleosynthesis in the early universe.

Improving numerical simulations of the smallest galaxies. At present, the computational strategy is often to adjust the physics in simulations in order to reproduce the observed properties of dwarfs. As resolution increases, it should be possible to move beyond this approach and learn about the earliest stages of formation of these systems, and how they evolve in the gravitational potential of the Milky Way. A key result from future simulations will be determining how galaxies populate dark matter halos at masses below M_halo ∼ 10⁹ M_⊙.

Testing dark matter physics. In addition to placing a lower limit on the mass function of dark matter subhalos, a sample of hundreds of stellar radial velocities and proper motions or other novel ideas could yield tight constraints on the inner density structure of UFD dark matter halos. These measurements would provide a critical test of the cold dark matter prediction that the density profiles of undisturbed dark matter halos should have central cusps.

DISCLOSURE STATEMENT

The author is not aware of any affiliations, memberships, funding, or financial holdings that might be perceived as affecting the objectivity of this review.

Acknowledgments

We thank Mike Boylan-Kolchin, Marla Geha, Joss Bland-Hawthorn, Alex Ji, Ting Li, and Mike Cooper for helpful suggestions. We are also indebted to Alex Ji for providing his compilation of UFD abundances, which is used in Fig. 6, and to Ting Li for supplying the model stellar population upon which Fig. 7 is based.

This publication is based upon work supported by the National Science Foundation under grants AST-1714873 and AST-1412792. JDS also acknowledges a productive stay during the writing of this article at the Kavli Institute for Theoretical Physics, which is supported in part by the National Science Foundation under Grant No. NSF PHY-1748958, for the program The Small-Scale Structure of Cold(?) Dark Matter. This paper would only barely have been possible without NASA's Astrophysics Data System Bibliographic Services.