Direct observation of cosmic strings via their strong gravitational lensing effect - II. Results from the HST/ACS image archive

We have searched 4.5 deg² of archival Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) images for cosmic strings, identifying close pairs of similar, faint galaxies and selecting groups whose alignment is consistent with gravitational lensing by a long, straight string. We find no evidence for cosmic strings in five large-area HST treasury surveys (covering a total of 2.22 deg²) or in any of 346 multifilter guest observer images (1.18 deg²). Assuming that simulations accurately predict the number of cosmic strings in the Universe, this non-detection allows us to place upper limits on the dimensionless Universal cosmic string tension of Gμ/c² < 2.3 × 10^-6 and cosmic string density of Ω_s < 2.1 × 10^-5 at the 95 per cent confidence level (marginalizing over the other parameter in each case). We find four dubious cosmic string candidates in 318 single-filter guest observer images (1.08 deg²), which we are unable to conclusively eliminate with existing data. The confirmation of any of these candidates as cosmic strings would imply Gμ/c² ≈ 10^-6 and Ω_s ≈ 10^-5. However, we estimate that there is at least a 92 per cent chance that these string candidates are random alignments of galaxies. If we assume that these candidates are indeed false detections, our final limits on Gμ/c² and Ω_s fall to 6.5 × 10^-7 and 7.3 × 10^-6, respectively. Due to the extensive sky coverage of the HST/ACS image archive, the above limits are universal. They are quite sensitive to the number of fields being searched and could be further reduced by more than a factor of 2 using forthcoming HST data.