The ALPINE-ALMA [C II] Survey: on the nature of an extremely obscured serendipitous galaxy

We report the serendipitous discovery of a dust-obscured galaxy observed as part of the Atacama Large Millimeter Array (ALMA) Large Program to INvestigate [C II] at Early times (ALPINE). While this galaxy is detected both in line and continuum emissions in ALMA Band 7, it is completely dark in the observed optical/near-infrared bands and only shows a significant detection in the UltraVISTA K_s band. We discuss the nature of the observed ALMA line, that is [C II] at $z$ ∼ 4.6 or high-J CO transitions at $z$ ∼ 2.2. In the first case, we find a [C II]/FIR luminosity ratio of $\mathrm{log}{(L_{[\mathrm{ C}\, \rm {\small {II}}]}/L_{\mathrm{ FIR}})} \sim -2.5$ , consistent with the average value for local star-forming galaxies (SFGs). In the second case instead, the source would lie at larger CO luminosities than those expected for local SFGs and high-z submillimetre galaxies. At both redshifts, we derive the star formation rate (SFR) from the ALMA continuum and the physical parameters of the galaxy, such as the stellar mass (M_*), by fitting its spectral energy distribution. Exploiting the results of this work, we believe that our source is a 'main-sequence', dusty SFG at $z$ = 4.6 (I.e. [C II] emitter) with $\mathrm{log(SFR/M_{\odot }\, yr^{-1})}\sim 1.4$ and log(M_*/M_⊙) ∼ 9.9. As a support to this scenario our galaxy, if at this redshift, lies in a massive protocluster recently discovered at $z$ ∼ 4.57, at only ∼1 proper Mpc from its centre. This work underlines the crucial role of the ALPINE survey in making a census of this class of objects, in order to unveil their contribution to the global SFR density at the end of the Reionization epoch.