Delensing degree-scale $B$-mode polarization with high-redshift line intensity mapping

Cosmic microwave background (CMB) experiments that constrain the tensor-to-scalar ratio $r$ are now approaching the sensitivity at which delensing—removing the $B$ modes induced by the gravitational lensing of large-scale structure—is necessary. We consider the improvement in delensing that maps of large-scale structure from tomographic line intensity mapping (IM) experiments targeting $2<z<10$ could provide. Compared to a nominal baseline of cosmic infrared background and internal delensing at CMB-S4 sensitivity, we find that the addition of high-redshift IM data could improve delensing performance by $\sim 11\%$. Achieving the requisite sensitivity in the IM data is feasible with next-generation experiments that are now being planned. However, these results are contingent on the ability to measure low-$k$ modes along the line of sight. Without these modes, IM datasets are unable to correlate with the lensing kernel and do not aid in delensing.

Figure 1

Lensing kernels $W(z)$ for the CMB (black), CIB (red), and IM surveys considered in this paper (blue, green, cyan, magenta). Since the IM surveys target specific emission lines, redshift is determined by the observing frequency and is limited by the spectrometer resolution (much smaller than the top-hat bins shown here).

Figure 2

Correlation coefficients $\rho$ for the external tracers of large-scale structure considered here: CIB (red), IM (blue, green, cyan, magenta) for $2<z<9$, and in combination with CIB. No noise has been added.

Figure 3

Delensing improvement factor $\alpha$ (colormap) and percent improvement (white contours) relative to the baseline of CMB-S4 internal $\text{delensing}+\text{CIB delensing}$, as the SNR of IM low and IM high surveys are varied. The baseline (lower left corner) corresponds to $\alpha =4.9$. “SNR” is the ratio of the line brightness to the map’s white noise level, and not signal-to-noise per mode. Foreground mode loss is not included.

Figure 4

Delensing improvement factors $\alpha$ (colormap) and percent improvement (white contours) relative to the baselines of $\text{internal}+\mathrm{IM}$ (left), $\mathrm{CIB}+\mathrm{IM}$ (middle), and IM only delensing (right), as the SNR of IM low and IM high surveys are varied. The baselines (lower left corners) correspond to $\alpha =4.2$, 2.1, and 1.0 respectively. “SNR” is the ratio of the line brightness to the map’s white noise level, and not signal-to-noise per mode. Foreground mode loss is not included.