${\rm C\, \small {II}}$ line intensity mapping (LIM) is a potential technique to probe the early galaxies from the Epoch of Reionization (EoR). Several experiments e.g. CONCERTO, TIME, CCAT-p are underway to map the ${\rm C\, \small {II}}$ LIM signal fluctuations from the EoR, enabling us to estimate the ${\rm C\, \small {II}}$ power-spectrum and ${\rm C\, \small {II}}$×21-cm cross-power spectrum. Observed LIM signal will have its time evolution embedded in it along the Line of Sight (LoS) due to the finite traveltime of the signal from its origin to the observer. We have investigated this so-called light-cone effect on the observed statistics of our semi-numerically simulated ${\rm C\, \small {II}}$ signal from the EoR. Using a suit of simulated ${\rm C\, \small {II}}$ and neutral hydrogen 21-cm maps and corresponding light-cone boxes, we have shown that the light-cone effect can impact the ${\rm C\, \small {II}}$ power spectrum by more than 15 per cent at large scales (k ∼ 0.1 Mpc−1, at z = 6.8). We have also observed that the impact of light-cone effect on the ${\rm C\, \small {II}}$ power spectrum drops with decreasing redshift within the redshift range considered here (7.2 ≲ z ≲ 6). The ${\rm C\, \small {II}}$×21-cm cross-power spectrum is also affected by light-cone, and in our models where reionization ends before z = 6, we find that the maximum impact on cross-power can reach up to 20 per cent. At z = 6.4, we find comparatively pronounced variation in the light-cone effect with reionization history on the cross power. Faster reionization histories have a more drastic light-cone effect on cross-power. We conclude that we need to incorporate the light-cone in order to properly model the signal, constrain the EoR-related astrophysical parameters and reionization history using the ${\rm C\, \small {II}}$×21-cm cross-power spectrum.

Detecting the line-intensity mapping (LIM) signal from the galaxies of the Epoch of Reionization is an emerging tool to constrain their role in reionization. Ongoing and upcoming experiments target the signal fluctuations across the sky to reveal statistical and astrophysical properties of these galaxies via signal statistics, e.g., the power spectrum. Here, we revisit the [C ii]$_{158 \mu \text{m}}$ LIM power spectrum under non-uniform line-luminosity scatter, which has a halo-mass variation of statistical properties. Line-luminosity scatter from a cosmological hydrodynamic and radiative transfer simulation of galaxies at z = 6 is considered in this study. We test the robustness of different model frameworks that interpret the impact of the line-luminosity scatter on the signal statistics. We use a simple power-law model to fit the scatter and demonstrate that the mean luminosity-halo mass correlation fit cannot preserve the mean intensity of the LIM signal (hence the clustering power spectrum) under non-uniform scatter. In our case, the mean intensity changes by ∼48 per cent compared to the mean correlation fit in contrast to the general case with semi-analytic scatter. However, we find that the prediction for the mean intensity from the most-probable fit can be modelled robustly, considering the generalized and more realistic non-uniform scatter. We also explore the possibility of diminishing luminosity bias under non-uniform scatter, affecting the clustering power spectrum, although this phenomenon might not be statistically significant. Therefore, we should adopt appropriate approaches that can consistently interpret the LIM power spectrum from observations.

Redshifted [C II] 158µm line-intensity mapping (LIM) of the Epoch of Reionization (EoR) with ongoing and upcoming experiments like the CONCERTO, TIME and FYST, is a new tool to constrain the role of the early galaxies in reionization. We expect statistics, e.g., the power spectrum of the LIM signal to be detectable by these experiments which will help us understand the clustering and astrophysical properties of the [C II] line emitters. Although the fluctuations of the LIM signal are generally modeled by assuming a one-to-one L [CII] -M halo relationship, the scatter in the [C II] luminosity, arising due to various ongoing astrophysical processes under varied environment inside individual galaxies, will also impact the power spectrum. In this work, using the results from a hydrodynamic and radiative transfer simulation of early galaxies, we find that this scatter can enhance the LIM power spectrum up to a factor of ∼ 2.7 − 2.9 (at z = 6). It is, therefore, crucial to take the effect of [C II] luminosity scatter into account while interpreting the [C II] LIM power spectrum from future observations.

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