We present results from a fully cosmological , very high-resolution , \Lambda CDM “ zoom-in ” simulation of a group of seven field dwarf galaxies with present-day virial masses in the range M _ { vir } = 4.4 \times 10 ^ { 8 } -3.6 \times 10 ^ { 10 } { M _ { \odot } } .
The simulation includes a blastwave scheme for supernova feedback , a star formation recipe based on a high gas density threshold , metal-dependent radiative cooling , a scheme for the turbulent diffusion of metals and thermal energy , and a uniform UV background that modifies the ionization and excitation state of the gas .
The properties of the simulated dwarfs are strongly modulated by the depth of the gravitational potential well .
All three halos with M _ { vir } < 10 ^ { 9 } { M _ { \odot } } are devoid of stars , as they never reach the density threshold for star formation of 100 atoms cm ^ { -3 } .
The other four , M _ { vir } > 10 ^ { 9 } { M _ { \odot } } dwarfs have blue colors , low star formation efficiencies , -4.5 \leq \log M _ { * } / M _ { vir } \leq - 2.5 , high cold gas to stellar mass ratios , 0.2 \leq M _ { HI } / M _ { * } \leq 20 , and low stellar metallicities , -2 \leq \langle { [ Fe / H ] } \rangle \leq - 1 .
Their bursty star formation histories are characterized by peak specific star formation rates in excess of 50 - 100 Gyr ^ { -1 } , far outside the realm of normal , more massive galaxies , and in agreement with observations of extreme emission-line starbursting dwarfs by the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey .
The median stellar age of the simulated galaxies decreases with decreasing halo mass , with the two M _ { vir } \simeq 2 - 3 \times 10 ^ { 9 } { M _ { \odot } } dwarfs being predominantly young , and the two more massive systems hosting intermediate and older populations .
The two cosmologically young dwarfs are lit up by tidal interactions , have compact morphologies , and have metallicities and cold gas fractions similar to the relatively quiescent , extremely metal-deficient dwarf population that includes the recently-discovered Leo P. Metal-enriched galactic outflows produce sub-solar effective yields and pollute with heavy elements a Mpc-size region of the intergalactic medium , but are not sufficient to completely quench star formation activity and are absent in the faintest dwarfs .
Within the limited size of the sample , our simulations appear to simultaneously reproduce the observed stellar mass and cold gas content , resolved star formation histories , and metallicities of field dwarfs in the Local Volume .