The ALPINE-ALMA [CII] survey: Double stellar population and active galactic nucleus activity in a galaxy at z ??? 5.5

Context. GDS J033218.92-275302.7 (here GS-14) is a z  & SIM; 5.5 galaxy with unusual UV spectral features that have been interpreted as signatures of either a double stellar population or of an active galactic nucleus (AGN). GS-14 was detected in [C II] 158 & mu;m as part of the ALPINE survey and was found to be the galaxy with the lowest molecular gas fraction (f(mol) = M-molgas/(M-molgas + M-*)& SIM;0.1) of that sample.Aims. We exploited the multiwavelength coverage of GS-14 to investigate the properties and the origin of its emission.Methods. We fit the UV-to-near-IR spectral energy distribution (SED) with a single and double stellar population and/or AGN component. We analyzed the latest release of the VIMOS spectrum, which shows highly ionized emission lines (O VI1032 & ANGS;, N V1240 & ANGS;, and N IV] 1483,1486 & ANGS;). The line equivalent widths and line ratios were compared with those observed in galaxies and AGN, as well as with the predictions from radiation transfer models for star-forming galaxies, AGN, and shocks.Results. The SED-fitting provides a total stellar mass of M-* = (4 & PLUSMN; 1)x10(10) M-& ODOT;, an age of the main stellar population of & SIM; 670 Myr, and a recent (& SIM; 8 Myr) short burst of star formation (SF) of & SIM;90  M-& ODOT; yr(-1). We do not find a significant contribution from an AGN, although we do not have any coverage in the mid-IR, where the dust emission of the AGN would peak. The N V line has a characteristic P Cygni profile. Fitting it with stellar models suggests a & SIM; 3 Myr old population of stars with a mass of & SIM;5 x 10(7) M-& ODOT;, consistent with a second component of young stars, as found in the SED-fitting analysis. The N V profile also shows evidence for an additional component of nebular emission. The comparison of the line ratios (N IV]/N V and O VI/N V) with theoretical models allows us to associate the emission with SF or AGN, but the strong radiation field required to ionize the O VI is more commonly related to AGN activity.Conclusions. Studying GS-14, we found evidence for an old and already evolved stellar population at z  & SIM; 5.5, and we show that the galaxy is experiencing a second short burst of SF. In addition, GS-14 carries signatures of obscured AGN activity. The AGN could be responsible for the short depletion time of this galaxy, thus making GS-14 one of the two ALPINE sources with indications of an active nucleus and an interesting target for future follow-ups to understand the connection between SF and AGN activity.