Star formation at the edge of the Local Group: a rising star formation
  history in the isolated galaxy WLM
  We present the star formation history (SFH) of the isolated (D~970 kpc) Local
Group dwarf galaxy WLM measured from color-magnitude diagrams constructed from
deep Hubble Space Telescope imaging. Our observations include a central (0.5
$r_h$) and outer field (0.7 $ r_h$) that reach below the oldest main sequence
turnoff. WLM has no early dominant episode of star formation: 20% of its
stellar mass formed by ~12.5 Gyr ago (z~5). It also has an SFR that rises to
the present with 50% of the stellar mass within the most recent 5 Gyr (z<0.7).
There is evidence of a strong age gradient: the mean age of the outer field is
5 Gyr older than the inner field despite being only 0.4 kpc apart. Some models
suggest such steep gradients are associated with strong stellar feedback and
dark matter core creation. The SFHs of real isolated dwarf galaxies and those
from the the Feedback In Realistic Environment suite are in good agreement for
$M_{\star}(z=0) \sim 10^7-10^9 M_{\odot}$, but in worse agreement at lower
masses ($M_{\star}(z=0) \sim 10^5-10^7 M_{\odot}$). These differences may be
explainable by systematics in the models (e.g., reionization model) and/or
observations (HST field placement). We suggest that a coordinated effort to get
deep CMDs between HST/JWST (crowded central fields) and WFIRST (wide-area halo
coverage) is the optimal path for measuring global SFHs of isolated dwarf
galaxies.
