New observations suggest that ultrafaint dwarf galaxies ( UFD ) – the least luminous systems bound by dark matter halos ( \lesssim 10 ^ { 5 } { L } _ { \odot } ) – may have formed before reionization .
The extrapolated virial masses today are uncertain with estimates ranging from 10 ^ { 8 } { M } _ { \odot } to as high as 10 ^ { 9 } { M } _ { \odot } depending on the assumed form of the underlying potential .
We show that the progenitor halo masses of UFDs can , in principle , be as low as M _ { vir } \approx 10 ^ { 7 } { M } _ { \odot } .
Under the right conditions , such a halo can survive the energy input of a supernova ( SN ) and its radiative progenitor .
A clumpy ( fractal ) medium is much less susceptible to both internal and external injections of energy .
It is less prone to SN sweeping ( particularly if it is off-centred ) because the coupling efficiency of the explosive energy is much lower than for a diffuse interstellar medium .
With the aid of the 3D hydro/ionization code Fyris , we show that sufficient baryons are retained to form stars following a single supernova event in dark matter halos down to M _ { vir } \approx 10 ^ { 7 } { M } _ { \odot } in the presence of radiative cooling .
In these models , the gas survives the SN explosion , is enriched with the specific abundance yields of the discrete events , and reaches surface densities where low mass stars can form .
Our highest resolution simulations reveal why cooling is so effective in retaining gas compared to any other factor .
In the early stages , the super-hot metal-enriched SN ejecta exhibit strong cooling , leading to much of the explosive energy being lost .
Consistent with earlier work , the baryons do not survive in smooth or adiabatic models in the event of a supernova .
The smallest galaxies may not contribute a large fraction of matter to the formation of galaxies , but they carry signatures of the earliest epochs of star formation , as we show .
These signatures may allow us to distinguish a small primordial galaxy from one that was stripped down to its present size through tidal interaction .
We discuss these results in the context of local ultra-faint dwarfs and damped Ly \alpha systems ( z \sim 2 ) at very low metallicity ( [ Fe/H ] \sim -3 ) .
We show that both classes of objects are consistent with primordial low-mass systems that have experienced only a few enrichment events .