Rotation of Galaxies as a Signature of Cosmic Strings in Weak Lensing Surveys

Vector perturbations sourced by topological defects can generate rotations in the lensing of background galaxies. This is a potential smoking gun for the existence of defects since rotation generates a curl-like component in the weak lensing signal which is not generated by standard density perturbations at linear order. This rotation signal is calculated as generated by cosmic strings. Future large scale weak lensing surveys should be able to detect this signal even for string tensions an order of magnitude lower than current constraints.

Figure 1

The geometrical setup for the single string lensing calculation. The moving string is aligned with the $y$ axis, perpendicular to the line of sight. A light ray sourced at position $\chi {\overrightarrow{\theta }}^{\prime }$ in the source plane ($\chi ={\chi }_g$) is imaged onto $\chi \overrightarrow{\theta }$. The vector source due to the moving string can rotate an image in addition to the usual shearing for the general case where the string is not aligned with the $y$ axis.

Figure 2

The solution $\log |\rho |$, in arbitrary units, as a function of angular distance ${\theta }_1$ from the string and distance of the string with respect to the distance of the source object $\zeta \equiv {\chi }_{cs}/{\chi }_g$. The solution is only valid for ${\theta }_1\ll 1$. $\rho$ is antisymmetric about ${\theta }_1=0$.

Figure 3

The angular power spectrum of rotation for a network of strings with $G\mu =1\times {10}^{-7}$. The boxes show the forecasted error for two surveys with $f_{\mathrm{sky}}=0.1$ and $f_{\mathrm{sky}}=0.5$, respectively. The errors include both sample and intrinsic ellipticity noise contributions. The intrinsic ellipticity term dominates at the relatively small scales being considered.