Investigating the nature of the z ≃ 2.8 submillimeter selected galaxy SMM J02399-0136 with VLT spectropolarimetry

Abstract

We present deep optical spectropolarimetry of SMM J02399-0136 (z = 2.8) done with the VLT Antu 8.2 m telescope equipped with FORS1. Moderate continuum and emission line polarization are measured (P ∼ 5%). We do not detect broad lines in scattered flux as would be expected for a type-2 object but rather a polarization behaviour similar to BAL quasars. This classification is confirmed by the detection of both high and low ionization broad absorption troughs and a very red continuum. We argue that this object shares several properties with local ULIGs such as Mrk 231 and other ultraluminous infrared Lo-BAL quasars. However, the fact that the ultraviolet spectrum is dominated by non-stellar radiation does not prove that the dust that is thermally radiating in the far infrared is predominantly heated by the AGN. Since the energy that we get in the far-infrared is precisely that which is removed from the ultraviolet spectrum, this could mean that the starburst is more dust-enshrouded than the AGN due to a peculiar dust distribution. The limits we place on the putative starburst contribution to the restframe ultraviolet continuum together with constraints on the amount of extinction provide an upper limit to the star formation rate of about 2000 M⊙ yr-1, consistent with previously claimed high star formation rates level in this object.