tokens: the LG (e.g. see updates to McConnachie 2012 ). While environmental effects (e.g. efficient destruction of satellites, dwarfdwarf mergers, stellar stripping) could alleviate some tension, these mechanisms do not appear efficient enough to resolve the discrepancy (e.g. Kirby etal. 2013 ; Deason etal. 2014 ; Garrison-Kimmel etal. 2017 ). Furthermore, the overabundance of LG dwarfs is not limited to the faintest UFDs, as it is known to persist in the classical dwarf regime (Boylan-Kolchin etal. 2015 ), which is thought to be observationally complete (e.g. Koposov etal. 2008 ). Even more challenges arise if (i) the faint end of the UVLF approaches 2.4 (i.e. many more faint galaxies); and/or (ii) such low-mass galaxies have a low star formation duty cycle (e.g. 10percent; Wyithe etal. 2014 ). In the latter case, 90percent of the haloes would be in the off state at any given time, and thus have reduced UV luminosities. Effectively, this results in higher luminosity galaxies hosted in even lower mass haloes, assuming a canonical SHM. 3.2 Minimizing the flux contribution of ultra-faint dwarfs Up to this point, we have considered the consequences of extrapolating a Schechter UVLF into the regime of UFDs. However, several numerical simulations of galaxy formation predict a turnover in the UVLF, decreasing the predicted number of very faint galaxies in the early Universe (e.g. Jaacks etal. 2013 ; OShea etal. 2015 ; Gnedin 2016 ; Liu etal. 2016 ; Ocvirk etal. 2016 ; Yue etal. 2016 ; Finlator etal. 2017 ). Although details vary, the common threads among these simulations are that not all low-mass haloes necessarily host a galaxy and that baryonic effects internal to the galaxies (e.g. supernova feedback, radiation pressure) can significantly affect galaxy formation in low-mass haloes. Alternately, a turnover in the high- z UVLF is consistent with models that suppress or eliminate small-scale structure (e.g. warm dark matter; Schultz etal. 2014 ; Dayal etal. 2015 ). A broken luminosity function at high z is also necessary in order to avoid dramatically overproducing UFDs and even classical dwarfs in the LG (Boylan-Kolchin etal. 2014 , 2015 ). Fig. 4 illustrates the effects of two plausible UVLF turnovers. For simplicity, we focus only on the z 7 case and adopt analytic UVLFs that turn over from Jaacks etal. ( 2013 ) and Boylan-Kolchin etal. ( 2015 ). The UVLF from the latter study has 2 for M UV 13 and 1.2 for M UV 13 and is designed to reproduce z 0 LG dwarf galaxy counts. We have tuned the UVLF from Jaacks etal. ( 2013 ) to match that of Boylan-Kolchin etal. ( 2015 ). Figure 4. Top panel: a comparison between the UVLFs at z 7 with and without turnovers. The steep UVLF (black; 2) is from Finkelstein etal. ( 2015 ), while the other curves show models from Boylan-Kolchin etal. ( 2015 , orange) and Jaacks etal. ( 2013 , magenta). Bottom panel: the fraction of UV flux at z 7 for each of the UVLFs shown in the top panel, when integrated to M min 3.1. A break or
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