A homogeneous analysis of the structural properties of the early SDSS UFDs was provided by Martin, de Jong & Rix (2008). Muñoz et al. (2018) recently updated this work, presenting uniform processing of deep photometry for all UFDs known as of mid-2015 (of course, a number of new dwarfs have been discovered since that date). Many previous studies have shown that the radial profiles of ultra-faint dwarfs can be accurately described by either exponential or Plummer (1911) profiles 9 (e.g., Belokurov et al., 2006, Martin, de Jong & Rix, 2008, Sand et al., 2010). Muñoz et al. (2018) advocated instead for Sérsic profiles, which match the observed radial profiles more closely (and are widely used for brighter galaxies), at the cost of one additional degree of freedom in the fit. Confirmed UFDs have half-light radii ranging from 24 pc (Segue 1) to 295 pc (UMa I), with candidate UFDs extending down to 15−20 pc in a few cases. In comparison, the classical dSphs have half-light radii between 170 pc (Leo II) and 2660 pc (Sagittarius).
Apart from simply being smaller on average, it has also been suggested that the faintest galaxies have significantly larger ellipticities than larger systems (Martin, de Jong & Rix, 2008). Updating the samples from what was available ten years ago, we calculate a weighted average ellipticity for the UFDs of 0.50 ± 0.01, while the weighted average ellipticity of the classical dSphs is 0.350 ± 0.003, in good agreement with the statistics determined by Martin, de Jong & Rix (2008). However, using a two-sided Kolmogorov-Smirnov test we find that there is a 19% probability that the two samples are drawn from a common distribution (as previously indicated by Sand et al. (2012)). We therefore conclude that there is no significant evidence at present that UFDs have more elongated shapes than more luminous dwarfs.
A recurring question regarding the structure of the faintest dwarfs is whether their isophotes are irregular or distorted in any way, which could suggest recent tidal stripping (e.g., Zucker et al., 2006a, Belokurov et al., 2006, Okamoto et al., 2012). Several analyses of simulated photometric data sets of faint dwarfs have shown that these apparently irregular shapes are the result of Poisson fluctuations in the distribution of stars in the lowest surface brightness regions of these systems rather than evidence for disturbed morphology (Walsh et al., 2008, Martin, de Jong & Rix, 2008, Muñoz, Geha & Willman, 2010).
The recent discovery of the relatively luminous (MV = −8.2), but extremely diffuse, Crater II dwarf (Torrealba et al., 2016a) highlights the possibility that the currently known population of dwarf galaxies may be limited in surface brightness by the sensitivity of existing photometric surveys. Indeed, Muñoz et al. (2018) clearly illustrate how the discovery of new Milky Way satellites has pushed to lower and lower surface brightnesses as the available data have improved. There are also theoretical reasons to suspect that significant numbers of even lower surface brightness dwarfs could exist (e.g., Bullock et al., 2010). In the next decade, LSST observations will reveal whether there is a large population of even feebler dwarf galaxies, or if we have already reached the lowest surface brightness at which galaxies are able to form.
9 As discussed in Section 2.1.3, the only UFD for which these profiles do not fit is UMa II (Muñoz, Geha & Willman, 2010, Muñoz et al., 2018). Back.