paper

Hamilton's Object -- a clumpy galaxy straddling the gravitational caustic of a galaxy cluster : Constraints on dark matter clumping

arXiv:2105.04562 · doi:10.1093/mnras/stab1375

Abstract

We report the discovery of a 'folded' gravitationally lensed image, 'Hamilton's Object', found in a HST image of the field near the AGN SDSS J223010.47-081017.8 (). The lensed images are sourced by a galaxy at a spectroscopic redshift of 0.8200 and form a fold configuration on a caustic caused by a foreground galaxy cluster at a photometric redshift of 0.526 seen in the corresponding Pan-STARRS PS1 image and marginally detected as a faint ROSAT All-Sky Survey X-ray source. The lensed images exhibit properties similar to those of other folds where the source galaxy falls very close to or straddles the caustic of a galaxy cluster. The folded images are stretched in a direction roughly orthogonal to the critical curve, but the configuration is that of a tangential cusp. Guided by morphological features, published simulations and similar fold observations in the literature, we identify a third or counter-image, confirmed by spectroscopy. Because the fold-configuration shows highly distinctive surface brightness features, follow-up observations of microlensing or detailed investigations of the individual surface brightness features at higher resolution can further shed light on kpc-scale dark matter properties. We determine the local lens properties at the positions of the multiple images according to the observation-based lens reconstruction of Wagner et al. (2019). The analysis is in accordance with a mass density which hardly varies on an arc-second scale (6 kpc) over the areas covered by the multiple images.

arXiv comment20 pages, 15 figures, 4 tables, accepted for publication in MNRAS

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