GRB 050223: a dark GRB in a dusty starburst galaxy

Aims.We aim at detecting and determining the properties of the host galaxy of the dark GRB 050223.
Methods: .We use VLT optical/NIR images coupled to Swift X-ray positioning, and optical spectra of the host galaxy to measure its properties.
Results: .We find a single galaxy within the Swift error box of GRB 050223. It is located at z = 0.584 and its luminosity is L ∼ 0.4 L^*. Emission lines in the galaxy spectrum imply an intrinsic {SFR} > 7 M_⊙ yr^-1, and a large extinction A_V > 2 mag within it. We also detect absorption lines, which reveal an underlying stellar population with an age between 40 Myr and 1.5 Gyr.
Conclusions: .The identification of a host galaxy with atypical properties using only the X-ray transient suggests that a bias may be present in the former sample of host galaxies. Dust obscuration together with intrinsic faintness are the most probable causes for the darkness of this burst.