Asymmetric Surface Brightness Structure of Caustic Crossing Arc in SDSS
  J1226+2152: A Case for Dark Matter Substructure
  We study the highly magnified arc SGAS J122651.3+215220 caused by a
star-forming galaxy at $z_s=2.93$ crossing the lensing caustic cast by the
galaxy cluster SDSS J1226+2152 ($z_l=0.43$), using Hubble Space Telescope
observations. We report in the arc several asymmetric surface brightness
features whose angular separations are a fraction of an arcsecond from the
lensing critical curve and appear to be highly but unequally magnified image
pairs of underlying compact sources, with one brightest pair having clear
asymmetry consistently across four filters. One explanation of unequal
magnification is microlensing by intracluster stars, which induces independent
flux variations in the images of individual or groups of source stars in the
lensed galaxy. For a second possibility, intracluster dark matter subhalos
invisible to telescopes effectively perturb lensing magnifications near the
critical curve and give rise to persistently unequal image pairs. Our modeling
suggests, at least for the most prominent identified image pair, that the
microlensing hypothesis is in tension with the absence of notable asymmetry
variation over a six-year baseline, while subhalos of $\sim
10^6$--$10^8\,M_\odot$ anticipated from structure formation with Cold Dark
Matter typically produce stationary and sizable asymmetries. We judge that
observations at additional times and more precise lens models are necessary to
stringently constrain temporal variability and robustly distinguish between the
two explanations. The arc under this study is a scheduled target of a
Director's Discretionary Early Release Science program of the James Webb Space
Telescope, which will provide deep images and a high-resolution view with
integral field spectroscopy.
