A $\mathbf{1.4}$ deg${}^{\mathbf{2}}$ blind survey for CII], CIII] and CIV at $\mathbf{z}\sim \mathbf{0.7}-\mathbf{1.5}$. I: nature, morphologies and equivalent widths

While traditionally associated with active galactic nuclei (AGN), the properties of the CII] ($\lambda =2326$,{\AA}), CIII] ($\lambda ,\lambda =1907,1909$,{\AA}) and CIV ($\lambda,\lambda=1549, 1551$,{\AA}) emission lines are still uncertain as large, unbiased samples of sources are scarce. We present the first blind, statistical study of CII], CIII] and CIV emitters at $z\sim 0.68,1.05,1.53$, respectively, uniformly selected down to a flux limit of $\sim 4\times {10}^{-17}$ erg s${}^{-1}$ cm${}^{-1}$ through a narrow band survey covering an area of $\sim 1.4$ deg${}^2$ over COSMOS and UDS. We detect 16 CII], 35 CIII] and 17 CIV emitters, whose nature we investigate using optical colours as well as HST, X-ray, radio and far infra-red data. We find that $z\sim 0.7$ CII] emitters are consistent with a mixture of blue (UV slope $\beta =-2.0\pm 0.4$) star forming galaxies with disky HST structure and AGN with Seyfert-like morphologies. Bright CII] emitters have individual X-ray detections as well as high average black hole accretion rates (BHAR) of $\sim 0.1$ $M_{\odot }$ yr${}^{-1}$. CIII] emitters at $z\sim 1.05$ trace a general population of SF galaxies, with $\beta =-0.8\pm 1.1$, a variety of optical morphologies, including isolated and interacting galaxies and low BHAR ($<0.02$ $M_{\odot }$ yr${}^{-1}$). Our CIV emitters at $z\sim 1.5$ are consistent with young, blue quasars ($\beta \sim -1.9$) with point-like optical morphologies, bright X-ray counterparts and large BHAR ($0.8$ $M_{\odot }$ yr${}^{-1}$). We also find some surprising CII], CIII] and CIV emitters with rest-frame equivalent widths which could be as large as $50-100$ {\AA}. AGN or spatial offsets between the UV continuum stellar disk and the line emitting regions may explain the large EW. These bright CII], CIII] and CIV emitters are ideal candidates for spectroscopic follow up to fully unveil their nature.