Strong Lensing on High‐Redshift Galaxies

The observed far greater number of giant arcs than that predicted by the ΛCDM cosmology has been a long-standing puzzle. We adopt high-resolution cosmological simulations to assess this problem and quantitatively show that this issue can be resolved by inclusion of high-redshift source galaxies. Our ΛCDM results reveal a much higher giant-arc probability (>10-5) for source galaxies of z > 2 than that for z < 2. Together with the source redshift distribution derived from the NTT catalogs, we obtain the predicted giant-arc number counts, which turn out to be consistent with most observations. The fact that the high-redshift (z ≥ 1.5) galaxies are preferentially magnified by the foreground lenses with a surprisingly high efficiency explains why previous theoretical work substantially underestimated the giant-arc number density. In previous work, the source galaxies were assumed to be mostly around z = 1, an assumption commonly adopted in weak-lensing studies. In addition, although the lens mergers have been found to yield enhanced lensing cross sections, we find that merely 5% of giant arcs are associated with merging lenses. Our results support not only the ΛCDM cosmology but also the long-standing anticipation that galaxy clusters, mostly formed below z = 1, are powerful gravitational telescopes for probing the high-redshift protogalaxies. We illustrate how the patchy dust regions in interacting galaxies at z = 4 may appear as filamentary structures within a giant arc when observed at the submillimeter wave band.