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 Wolf–Lundmark–Melotte (WLM) measured from colour–magnitude diagrams (CMDs) constructed from deep Hubble Space Telescope imaging. Our observations include a central (⁠0.5𝑟_ℎ) and outer field (⁠0.7𝑟_ℎ⁠) that reach below the oldest main-sequence turn-off. WLM has no early dominant episode of star formation: 20 per cent of its stellar mass formed by ∼ 12.5 Gyr ago (⁠⁠𝑧 ∼ 5). It also has an SFR that rises to the present with 50 per cent 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 Feedback in Realistic Environment suite are in good agreement for M_⋆(⁠𝑧 = 0) ∼ 10⁷–10⁹M⊙, but in worse agreement at lower masses (⁠M_*(𝑧 = 0) ∼ 10⁵−10⁷M⊙⁠). 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.⁠