On the dearth of ultra-faint extremely metal poor galaxies
  Local extremely metal-poor (XMP) galaxies are of particular astrophysical
interest since they allow us to look into physical processes characteristic of
the early Universe, from the assembly of galaxy disks to the formation of stars
in conditions of low metallicity. Given the luminosity-metallicity
relationship, all galaxies fainter than Mr < -13 are expected to be XMPs.
Therefore, XMPs should be common in galaxy surveys. However, they are not,
because several observational biases hamper their detection. This work compares
the number of faint XMPs in the SDSS-DR7 spectroscopic survey with the expected
number, given the known biases and the observed galaxy luminosity function. The
faint end of the luminosity function is poorly constrained observationally, but
it determines the expected number of XMPs. Surprisingly, the number of observed
faint XMPs (around 10) is over-predicted by our calculation, unless the upturn
in the faint end of the luminosity function is not present in the model. The
lack of an upturn can be naturally understood if most XMPs are central galaxies
in their low-mass dark matter halos, which are highly depleted in baryons due
to interaction with the cosmic ultraviolet background and to other physical
processes. Our result also suggests that the upturn towards low luminosity of
the observed galaxy luminosity function is due to satellite galaxies.
