Studying high-z galaxies with [CII] intensity mapping

Abstract: We investigate the [CII] line intensity mapping (IM) signal from galaxies in the Epoch of Reionization (EoR) to assess its detectability, the possibility to constrain the $L_{\rm CII}-{\rm SFR}$ relation, and to recover the [CII] luminosity function (LF) from future experiments. By empirically assuming that ${\rm log}L_{\rm CII}={\rm log}A+\gamma {\rm SFR}\pm\sigma_L$, we derive the [CII] LF from the observed UV LF, and the [CII] IM power spectrum. We study the shot-noise and the full power spectrum separately. Although, in general, the shot-noise component has a much higher signal-to-noise ratio than the clustering one, it cannot be used to put independent constraints on log$A$ and $\gamma$. Full power spectrum measurements are crucial to break such degeneracy, and reconstruct the [CII] LF. In our fiducial survey S1 (inspired by CCAT-p/1000 hr) at $z \sim 6$, the shot-noise (clustering) signal is detectable for 2 (1) of the 5 considered $L_{\rm CII}-{\rm SFR}$ relations. The shot-noise is generally dominated by galaxies with $L_{\rm CII}\gtrsim 10^{8-9}~L_\odot$ ($M_{\rm UV}\sim-20$ to $-22$), already at reach of ALMA pointed observations. However, given the small field of view of such telescope, an IM experiment would provide unique information on the bright-end of the LF. The detection depth of an IM experiment crucially depends on the (poorly constrained) $L_{\rm CII}-{\rm SFR}$ relation in the EoR. If the $L_{\rm CII}-{\rm SFR}$ relation varies in a wide log$A - \gamma$ range, but still consistent with ALMA [CII] LF upper limits, even the signal from galaxies with $L_{\rm CII}$ as faint as $\sim 10^7~L_\odot$ could be detectable. Finally, we consider the contamination by continuum foregrounds (CIB, dust, CMB) and CO interloping lines, and derived the requirements on the residual contamination level to reliably extract the [CII] signal.