# Asymmetric surface brightness structure of caustic crossing arc in SDSS J1226+2152: A case for dark matter substructure

Liang Dai, Alexander A. Kaurov, Keren Sharon, Michael Florian, Jordi Miralda-Escudé, Tejaswi Venumadhav, Brenda Frye, Jane R. Rigby, Matthew Bayliss

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

## Abstract

We study the highly magnified arc SGAS J122651.3+215220 caused by a star-forming galaxy at zs = 2.93 crossing the lensing caustic cast by the galaxy cluster SDSS J1226+2152 (zl = 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 subhaloes invisible to telescopes effectively perturb lensing magnifications near the critical curve and give rise to persistently unequal image pairs. Our modelling 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 subhaloes of ∼106-$10^8\, \mathrm{ M}_\odot$ 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.

Original language English (US) 3192-3208 17 Monthly Notices of the Royal Astronomical Society 495 3 https://doi.org/10.1093/mnras/staa1355 Published - May 14 2020

## Keywords

• dark matter
• galaxies: clusters: individual: SGAS J122651.3+215220; SDSS J1226+2152
• gravitational lensing: micro
• gravitational lensing: strong

## ASJC Scopus subject areas

• Astronomy and Astrophysics
• Space and Planetary Science